A Level Physics Notes PDF

AS Physics Unit 1 21 The Oscilloscope Particles, Quantum Phenomena and Unit 2 Electricity Mechanics, Materials and Waves 1 Constituents of the Atom 1 Scal...

AS Physics Unit 1 21 The Oscilloscope Particles, Quantum Phenomena and Unit 2 Electricity Mechanics, Materials and Waves 1 Constituents of the Atom 1 Scalars and Vectors 2 Particles and Antiparticles 2 Resolving Vectors 3 Quarks 3 Moments 4 Hadrons 4 Velocity and Acceleration 5 Leptons 5 Motion Graphs 6 Forces and Exchange Particles 6 Equations of Motion 7 The Strong Interaction 7 Terminal Velocity and Projectiles 8 The Weak Interaction 8 Newton’s Laws 9 Feynman Diagrams 9 Work, Energy and Power 10 The Photoelectric Effect 10 Conservation of Energy 11 Excitation, Ionisation and Energy Levels 11 Hooke’s Law 12 Wave Particle Duality 12 Stress and Strain 13 QVIRt 13 Bulk Properties of Solids 14 Ohm’s Law and I-V Graphs 14 Young’s Modulus 15 Resistivity and Superconductivity 15 Progressive Waves 16 Series and Parallel Circuits 16 Longitudinal and Transverse Waves 17 Energy and Power 17 Superposition and Standing Waves 18 EMF and Internal Resistance 18 Refraction 19 Kirchhoff and Potential Dividers 19 Total Internal Reflection 20 Alternating Current 20 Interference 21 Diffraction Constituents of the Atom Unit 1 Lesson 1 To be know the constituents of the atom with their masses and charges Learning To be able to calculate the specific charge of the constituents Outcomes To be able to explain what isotopes and ions are N. DWYER The Nuclear Model (Also seen in GCSE Constitue Charge Mass Physics 1 and 2) nt (C) (kg) We know from Rutherford’s experiment that the Proton 1.6 x 10-19 1.673 x structure of an atom consists of positively 10-27 charged protons and neutral neutrons in one Neutron 0 1.675 x place called the nucleus. The nucleus sits in the 10-27 middle of the atom and has negatively charged Electron - 1.6 x 10 - 9.1 x 10-31 electrons orbiting it. At GCSE we used charges 19 and masses for the constituents relative to each other, the table above shows the actual charges and masses. Almost all of the mass of the atom is in the tiny nucleus which takes up practically no space when compared to the size of the atom. If we shrunk the Solar System so that the Sun was the size of a gold nucleus the furthest electron would be twice the distance to Pluto. If the nucleus was a full stop it would be 25 m to the first electron shell, 100 to the second and 225 to the third. Notation (Also seen in GCSE Physics 2) We can represent an atom of element X in the following way: Z is the proton number. This is the number of protons in the nucleus. In an uncharged atom the number of electrons orbiting the nucleus is equal to the number of protons. In Chemistry it is called the atomic number A is the nucleon number. This is the total number of nucleons in the nucleus (protons + neutrons) which can be written as A = Z + N. In Chemistry it is called the atomic mass number N is the neutron number. This is the number of neutrons in the nucleus. Isotopes (Also seen in GCSE Physics 1 and 2) Isotopes are different forms of an element. They always have the same number of protons but have a different number of neutrons. Since they have the same number of protons (and electrons) they behave in the same way chemically. Chlorine If we look at Chlorine in the periodic table we see that it is represented by. How can it have 18.5 neutrons? It can’t! There are two stable isotopes of Chlorine, which accounts for ~75% and which accounts for ~25%. So the average of a large amount of Chlorine atoms is. Specific Charge Specific charge is another title for the charge-mass ratio. This is a measure of the charge per unit mass and is simply worked out by worked out by dividing the charge of a particle by its mass. You can think of it as a how much charge (in Coulombs) you get per kilogram of the ‘stuff’. Constituent Charge (C) Mass (kg) Charge-Mass Ratio (C kg-1) or (C/kg)  Proton 1.6 x 10-19 1.673 x 10-27 1.6 x 10-19 ÷ 1.673 x 9.58 x 107 -27 10 Neutron 0 1.675 x 10-27 0 ÷ 1.675 0 -27 x 10 Electron (-) 1.6 x 10- 9.1 x 10-31 1.6 x 10-19 ÷ 9.11 x (-) 1.76 x 19 10-31 1011 We can see that the electron has the highest charge-mass ratio and the neutron has the lowest. Ions (Also seen in GCSE Physics 2) An atom may gain or lose electrons. When this happens the atoms becomes electrically charged (positively or negatively). We call this an ion. If the atom gains an electron there are more negative charges than positive, so the atom is a negative ion. Gaining one electron would mean it has an overall charge of -1, which actually means - 1.6 x 10-19C. Gaining two electrons would mean it has an overall charge of -2, which actually means - 3.2 x 10-19C. If the atom loses an electron there are more positive charges than negative, so the atom is a positive ion. Losing one electron would mean it has an overall charge of +1, which actually means +1.6 x 10-19C. Losing two electrons would mean it has an overall charge of +2, which actually means +3.2 x 10-19C. Particles and Antiparticles Unit 1 Lesson 2 To know what is the difference between particles and antiparticles Learning To be able to explain what annihilation is Outcomes To be able to explain what pair production is N. DWYER Antimatter British Physicist Paul Dirac predicted a particle of equal mass to an electron but of opposite charge (positive). This particle is called a positron and is the electron’s antiparticle. Every particles has its own antiparticle. An antiparticle has the same mass as the particle version but has opposite charge. An antiproton has a negative charge, an antielectron has a positive charge but an antineutron is also uncharged like the particle version. American Physicist Carl Anderson observed the positron in a cloud chamber, backing up Dirac’s theory. Anti particles have opposite Charge, Baryon Number, Lepton Number and Strangeness. If they are made from quarks the antiparticle is made from antiquarks Annihilation Whenever a particle and its antiparticle meet they annihilate each other. Annihilation is the process by which mass is converted into energy, particle and antiparticle are transformed into two photons of energy. Mass and energy are interchangeable and can be converted from one to the other. Einstein linked energy and mass with the equation: You can think of it like money; whether you have dollars or pounds you would still have the same amount of money. So whether you have mass or energy you still have the same amount. The law of conservation of energy can now be referred to as the conservation of mass- energy. The total mass-energy before is equal to the total mass-energy after. Photon Max Planck had the idea that light could be released in ‘chunks’ or packets of energy. Einstein named these wave-packets photons. The energy carried by a photon is given by the equation:  Since we can also write this as: How is there anything at all? When the Big Bang happened matter and antimatter was produced and sent out expanding in all directions. A short time after this there was an imbalance in the amount of matter and antimatter. Since there was more matter all the antimatter was annihilated leaving matter to form protons, atoms and everything around us. Pair Production Pair production is the opposite process to annihilation, energy is converted into mass. A single photon of energy is converted into a particle-antiparticle pair. (This happens to obey the conservation laws) This can only happen if the photon has enough mass-energy to “pay for the mass”. Let us image mass and energy as the same thing, if two particles needed 10 “bits” and the photon had 8 bits there is not enough for pair production to occur. If two particles needed 10 bits to make and the photon had 16 bits the particle-antiparticle pair is made and the left over is converted into their kinetic energy. If pair production occurs in a magnetic field the particle and antiparticle will move in circles of opposite direction but only if they are charged. (The deflection of charges in magnetic fields will be covered in Unit 4: Force on a Charged Particle) Pair production can occur spontaneously but must occur near a nucleus which recoils to help conserve momentum. It can also be made to happen by colliding particles. At CERN protons are accelerated and fired into each other. If they have enough kinetic energy when they collide particle-antiparticle pair may be created from the energy. The following are examples of the reactions that have occurred: In all we can see that the conservation laws of particle physics are obeyed. Quarks Unit 1 Lesson 3 To know what quarks are and where they are found Learning To be able to explain how they were discovered Outcomes To know the properties of each type of quark N. DWYER Rutherford Also seen in GCSE Physics 2 Rutherford fired a beam of alpha particles at a thin gold foil. If the atom had no inner structure the alpha particles would only be deflected by very small angles. Some of the alpha particles were scattered at large angles by the nuclei of the atoms. From this Rutherford deduced that the atom was mostly empty space with the majority of the mass situated in the centre. Atoms were made from smaller particles. Smaller Scattering In 1968 Physicists conducted a similar experiment to Rutherford’s but they fired a beam of high energy electrons at nucleons (protons and neutrons). The results they obtained were very similar to Rutherford’s; some of the electrons were deflected by large angles. If the nucleons had no inner structure the electrons would only be deflected by small angles. These results showed that protons and neutrons were made of three smaller particles, each with a fractional charge. Quarks These smaller particles were named quarks and are thought to be fundamental particles (not made of anything smaller). There are six different quarks and each one has its own antiparticle.  We need to know about the three below as we will be looking at how larger particles are made from different combinations of quarks and antiquarks. Baryon Strangen Baryon Strangene Anti Quark Charge Number ess Charge Number ss Quark (Q) (B) (S) (Q) (B) (S) d -⅓ +⅓ 0 d̄ +⅓ -⅓ 0 u +⅔ +⅓ 0 ū -⅔ -⅓ 0 s -⅓ +⅓ -1 s̄ +⅓ -⅓ +1 The other three are Charm, Bottom and Top. You will not be asked about these three Baryon Strangene Charmnes Quark Charge Bottomness Topness No. ss s d -⅓ +⅓ 0 0 0 0 u +⅔ +⅓ 0 0 0 0 s -⅓ +⅓ -1 0 0 0 c +⅔ +⅓ 0 +1 0 0 b -⅓ +⅓ 0 0 -1 0 t +⅔ +⅓ 0 0 0 +1 The Lone Quark? Never! Quarks never appear on their own. The energy required to pull two quarks apart is so massive that it is enough to make two new particles. A quark and an antiquark are created, another example of pair production. A particle called a neutral pion is made from an up quark and an antiup quark. Moving these apart creates another up quark and an antiup quark. We now have two pairs of quarks. Trying to separate two quarks made two more quarks. Particle Classification Now that we know that quarks are the smallest building blocks we can separate all other particles into two groups, those made from quarks and those that aren’t made from quarks. Hadrons – Heavy and made from smaller particles Leptons – Light and not made from smaller particles Hadrons Unit 1 Lesson 4 To know what a hadron is and the difference between the two types Learning To know the properties common to all hadrons Outcomes To know the structure of the common hadrons and which is the most N. stable DWYER Made from Smaller Stuff Hadrons, the Greek for ‘heavy’ are not fundamental particles they are all made from smaller particles, quarks. The properties of a hadron are due to the combined properties of the quarks that it is made from. There are two categories of Hadrons: Baryons and Mesons. Baryons Made from three quarks Baryon Strangen Baryon Strangene Proto Neutr Charge Number ess Charge Number ss n on (Q) (B) (S) (Q) (B) (S) u +⅔ +⅓ 0 d -⅓ +⅓ 0 u +⅔ +⅓ 0 u +⅔ +⅓ 0 d -⅓ +⅓ 0 d -⅓ +⅓ 0 p +1 +1 0 n 0 +1 0  The proton is the only stable hadron, all others eventually decay into a proton. Mesons Made from a quark and an antiquark Baryon Strangen Baryon Strangene Pion Charge Number ess Pion Charge Number ss Plus (Q) (B) (S) Minus (Q) (B) (S) u +⅔ +⅓ 0 ū -⅔ -⅓ 0 d̄ +⅓ -⅓ 0 d -⅓ +⅓ 0 + - π +1 0 0 π -1 0 0 Baryon Strangen Baryon Strangene Pion Charge Number ess Pion Charge Number ss Zero (Q) (B) (S) Zero (Q) (B) (S) u +⅔ +⅓ 0 d -⅓ +⅓ 0 ū -⅔ -⅓ 0 d̄ +⅓ -⅓ 0 π0 0 0 0 π0 0 0 0 Baryon Strangen Baryon Strangene Kaon Charge Number ess Kaon Charge Number ss Plus (Q) (B) (S) Minus (Q) (B) (S) u +⅔ +⅓ 0 ū -⅔ -⅓ 0 s̄ +⅓ -⅓ +1 s -⅓ +⅓ -1 K+ +1 0 +1 K- -1 0 -1 Baryon Strangen AntiKao Baryon Strangene Kaon Charge Number ess n Charge Number ss Zero (Q) (B) (S) Zero (Q) (B) (S) d -⅓ +⅓ 0 d̄ +⅓ -⅓ 0 s̄ +⅓ -⅓ +1 s -⅓ +⅓ -1 K0 0 0 +1 K̄ 0 0 0 -1 Anti Hadrons Anti hadrons are made from the opposite quarks as their Hadron counterparts, for example a proton is made from the quark combination uud and an antiproton is made from the combination ūūd̄ We can see that a π+ and a π- are particle and antiparticle of each other. Anti Baryon Strangen Anti Baryon Strangene Proto Charge Number ess Neutr Charge Number ss n (Q) (B) (S) on (Q) (B) (S) ū -⅔ -⅓ 0 d̄ +⅓ -⅓ 0 ū -⅔ -⅓ 0 ū -⅔ -⅓ 0 d̄ +⅓ -⅓ 0 d̄ +⅓ -⅓ 0 p̄̄ -1 -1 0 n̄ 0 -1 0 You need to know all the quark combination shown on this page as they may ask you to recite any of them. Leptons Unit 1 Lesson 5 To be able to explain what a lepton is Learning To know the properties common to all leptons Outcomes To be able to explain the conservation laws and be able to use them N. DWYER Fundamental Particles A fundamental particle is a particle which is not made of anything smaller. Baryons and Mesons are made from quarks so they are not fundamental, but quarks themselves are. The only other known fundamental particles are Bosons (see Lesson 6: Forces and Exchange Particles) and Leptons. Leptons Leptons are a family of particles that are much lighter than Baryons and Mesons and are not subject to the strong interaction. There are six leptons in total, three of them are charged and three are uncharged. The charged particles are electrons, muons and tauons. The muon and tauon are similar to the electron but bigger. The muon is roughly 200 times bigger and the tauon is 3500 times bigger (twice the size of a proton). Each of the charged leptons has its own neutrino. If a decay involves a neutrino and a muon, it will be a muon neutrino, not a tauon neutrino or electron neutrino. The neutrino is a chargeless, almost massless particle. It isn’t affected by the strong interaction or EM force and barely by gravity. It is almost impossible to detect. Charg Lepton Lepton Charge Lepton e Number Anti Lepton Number (Q) (Q) (L) (L) Electron e- -1 +1 Anti Electron e+ +1 -1 Electron Anti Electron Neutrino νe 0 +1 Neutrino ν̄ e 0 -1 Muon μ- -1 +1 Anti Muon μ+ +1 -1 Muon Neutrino νμ 0 +1 Anti Muon Neutrino ν̄ μ 0 -1 Tauon τ- -1 +1 Anti Tauon τ+ +1 -1 Tauon Anti Tauon Neutrino ντ 0 +1 Neutrino ν̄ τ 0 -1 Conservation Laws For a particle interaction to occur the following laws must be obeyed, if either is violated the reaction will never be observed (will never happen): Charge: Must be conserved (same total value before as the total value after) Baryon Number: Must be conserved Lepton Number: Must be conserved Strangeness: Conserved in EM and Strong Interaction. Doesn’t have to be conserved in Weak Interaction Examples In pair production a photon of energy is converted into a particle and its antiparticle γ → e- + e+ Q 0 → -1 + +1 0 → 0 Conserved B 0 → 0 + 0 0 → 0 Conserved L 0 → +1 + -1 0 → 0 Conserved S 0 → 0 + 0 0 → 0 Conserved Let us look at beta plus decay as we knew it at GCSE. A neutron decays into a proton and releases an electron. n → p + e- Q 0 → +1 + -1 0 → 0 Conserved B +1 → +1 + 0 +1 → +1 Conserved Not L 0 → 0 + +1 0 → +1 Conserved S 0 → 0 + 0 0 → 0 Conserved This contributed to the search for and discovery of the neutrino. Number Reminders There may be a clue to the charge of a particle; π +, K+ and e+ have a positive charge. It will only have a baryon number if it IS a baryon. Mesons and Leptons have a Baryon Number of zero. It will only have a lepton number if it IS a lepton. Baryons and Mesons have a Lepton Number of zero. It will only have a strangeness if it is made from a strange quark. Leptons have a strangeness of zero. Unit 1  Lesson 6 Forces and Exchange Particles To know the four fundamental forces, their ranges and relative strengths To know what each force does and what it acts on To be able to explain what exchange particles are N. DWYER The Four Interactions There are four forces in the universe, some you will have come across already and some will be new: The electromagnetic interaction causes an attractive or repulsive force between charges. The gravitational interaction causes an attractive force between masses. The strong nuclear interaction causes an attractive (or repulsive) force between quarks (and so hadrons). The weak nuclear interaction does not cause a physical force, it makes particles decay. ‘Weak’ means there is a low probability that it will happen. Interaction/Force Range Relative Strength -15 Strong Nuclear ~10 m 1 (1) Electromagnetic ∞ ~10–2 (0.01) -18 –7 Weak Nuclear ~10 m ~10 (0.0000001) (0.000000000000000000000000000000 Gravitational ∞ ~10–36 000001) Exchange Particles In 1935 Japanese physicist Hideki Yukawa put forward the idea that the interactions/forces between two particles were caused by ‘virtual particles’ being exchanged between the two particles. He was working on the strong nuclear force which keeps protons and neutrons together and theorised that they were exchanging a particle back and forth that ‘carried’ the force and kept them together. This is true of all the fundamental interactions. The general term for exchange particles is bosons and they are fundamental particles like quarks and leptons. Ice Skating Analogy Imagine two people on ice skates that will represent the two bodies experiencing a force. If A throws a bowling ball to B, A slides back when they release it and B moves back when they catch it. Repeatedly throwing the ball back and forth moves A and B away from each other, the force causes repulsion. The analogy falls a little short when thinking of attraction, but bear with it. Now imagine that A and B are exchanging a boomerang (bear with it), throwing it behind them pushes A towards B, B catches it from behind and moves towards A. The force causes attraction. Which Particle for What Force Each of the interactions/forces has its own exchange particles. Interaction/ Exchange Particle What is acts upon Force Gluons between Pions between Strong Nuclear Nucleons (Hadrons) quarks Baryons Electromagnetic Virtual Photon Charged particles Weak Nuclear W+ W– Z0 All particles Gravitational Graviton Particles with masses Borrowing Energy to Make Particles The exchange particles are made from ‘borrowed’ energy, borrowed from where? From nowhere! Yukawa used the Heisenberg Uncertainty Principle to establish that a particle of mass-energy ΔE could exist for a time Δt as long as where h is Planck’s constant. This means that a heavy particle can only exist for a short time while a lighter particle may exist for longer. h is Planck’s Constant, h = 6.63 x 10-34 J s. In 1947 the exchange particle of the strong nuclear interaction were observed in a cloud chamber. Lending Money Analogy Think of making exchange particles in terms of lending somebody some money. If you lend somebody £50 you would want it paid back fairly soon. If you lend somebody 50p you would let them have it for longer before paying you back. The Strong Interaction Unit 1 Lesson 7 To know why a nucleus doesn’t tear itself apart Learning To know why a nucleus doesn’t collapse in on itself Outcomes To know why the neutron exists in the nucleus N. DWYER The Strong Interaction The strong nuclear force acts between quarks. Since Hadrons are the only particles made of quarks only they experience the strong nuclear force. In both Baryons and Mesons the quarks are attracted to each other by exchanging virtual particles called ‘gluons’. On a larger scale the strong nuclear force acts between the Hadrons themselves, keeping them together. A pi-meson or pion (π) is exchanged between the hadrons. This is called the residual strong nuclear force. Force Graphs Neutron-Neutron or Neutron-Proton Here is the graph of how the force varies between two neutrons or a proton and a neutron as the distance between them is increased. We can see that the force is very strongly repulsive at separations of less than 0.7 fm ( x 10 –15 m). This prevents all the nucleons from crushing into each other. Above this separation the force is strongly attractive with a peak around 1.3 fm. When the nucleons are separated by more than 5 fm they no longer experience the SNF. Proton-Proton The force-separation graphs for two protons is different. They both attract each other due to the SNF but they also repel each other due to the electromagnetic force which causes two like charges to repel.  Graph A Graph B Graph C Graph A shows how the strong nuclear force varies with the separation of the protons Graph B shows how the electromagnetic force varies with the separation of the protons Graph C shows the resultant of these two forces: repulsive at separations less than 0.7 fm, attractive up to 2 fm when the force becomes repulsive again. Neutrons – Nuclear Cement In the lighter elements the number of protons and neutrons in the nucleus is the same. As the nucleus gets bigger more neutrons are needed to keep it together. Adding another proton means that all the other nucleons feel the SNF attraction. It also means that all the other protons feel the EM repulsion. Adding another neutron adds to the SNF attraction between the nucleons but, since it is uncharged, it does not contribute to the EM repulsion. The Weak Interaction Unit 1 Lesson 8 To be able to write the equation for alpha and beta decay Learning To know what a neutrino is and why is must exist Outcomes To be able to state the changes in quarks during beta plus and beta N. minus decay DWYER Alpha Decay When a nucleus decays in this way an alpha particle (a helium nucleus) is ejected from the nucleus. or All the emitted alpha particles travelled at the same speed, meaning they had the same amount of energy. The law of conservation of mass-energy is met, the energy of the nucleus before the decay is the same as the energy of the nucleus and alpha particle after the decay. Alpha decay is NOT due to the weak interaction but Beta decay IS Beta Decay and the Neutrino In beta decay a neutron in the nucleus changes to a proton and releases a beta particle (an electron). The problem with beta decay was that the electrons had a range of energies so the law of conservation of mass-energy is violated, energy disappears. There must be another particle being made with zero mass but variable speeds, the neutrino. We can also see from the particle conservation laws that this is a forbidden interaction: Charge Q: 0 +1–1 0 0 Charge is conserved Baryon Number B: +1 +1+0 1 1 Baryon number is conserved Lepton Number L: 0 0+1 0 1 Lepton number is NOT conserved Beta Minus (β–) Decay In neutron rich nuclei a neutron may decay into a proton, electron and an anti electron neutrino. Charge Q: 0 +1–1+0 0 0 Charge is conserved Baryon Number B: +1 +1+0+0 1 1 Baryon number is conserved Lepton Number L: 0 0+1–1 0 0 Lepton number is conserved In terms of quarks beta minus decay looks like this: which simplifies to: Charge Q: – ⅓ +⅔–1+0 – ⅓ – ⅓ Charge is conserved Baryon Number B: +⅓ +⅓+0+0 ⅓ ⅓ Baryon number is conserved Lepton Number L: 0 0+1–1 0 0 Lepton number is conserved Beta Plus (β+) Decay In proton rich nuclei a proton may decay into a neutron, positron and an electron neutrino. Charge Q: +1 0+1+0 1 1 Charge is conserved Baryon Number B: +1 +1+0+0 1 1 Baryon number is conserved Lepton Number L: 0 0–1+1 0 0 Lepton number is conserved In terms of quarks beta plus decay looks like this: which simplifies to: Charge Q: +⅔ –⅓+1+0 ⅔ ⅔ Charge is conserved Baryon Number B: +⅓ +⅓+0+0 ⅓ ⅓ Baryon number is conserved Lepton Number L: 0 0–1+1 0 0 Lepton number is conserved Strangeness The weak interaction is the only interaction that causes a quark to change into a different type of quark. In beta decay up quarks and down quarks are changed into one another. In some reactions an up or down quark can change into a strange quark meaning strangeness is not conserved. During the weak interaction there can be a change in strangeness of ±1. Feynman Diagrams Unit 1 Lesson 9 To know what a Feynman diagram shows us Learning To be able to draw Feynman diagrams to represent interactions and decays Outcomes To be able to state the correct exchange particle N. DWYER Feynman Diagrams An American Physicist called Richard Feynman came up with a way of visualising forces and exchange particles. Below are some examples of how Feynman diagrams can represent particle interactions.  The most important things to note when dealing with Feynman diagrams are the arrows and the exchange particles, the lines do not show us the path that the particles take only which come in and which go out. The arrows tell us which particles are present before the interaction and which are present after the interaction. The wave represents the interaction taking place with the appropriate exchange particle labelled. Examples Diagram 1 represents the strong interaction. A proton and neutron are attracted together by the exchange of a neutral pion. Diagram 2 represents the electromagnetic interaction. Two electrons repel each other by the exchange of a virtual photon. Diagram 3 represents beta minus decay. A neutron decays due to the weak interaction into a proton, an electron and an anti electron neutrino Diagram 4 represents beta plus decay. A proton decays into a neutron, a positron and an electron neutrino. Diagram 5 represents electron capture. A proton captures an electron and becomes a neutron and an electron neutrino. Diagram 6 represents a neutrino-neutron collision. A neutron absorbs a neutrino and forms a proton and an electron. Diagram 7 represents an antineutrino-proton collision. A proton absorbs an antineutrino and emits a neutron and an electron. Diagram 8 represents an electron-proton collision. They collide and emit a neutron and an electron neutrino. Getting the Exchange Particle The aspect of Feynman diagrams that students often struggle with is labelling the exchange particle and the direction to draw it. Look at what you start with: If it is positive and becomes neutral you can think of it as throwing away its positive charge so the boson will be positive. This is the case in electron capture. If it is positive and becomes neutral you can think of it as gaining negative to neutralise it so the boson will be negative. This is the case in electron-proton collisions. If it is neutral and becomes positive we can think of it either as gaining positive (W+ boson) or losing negative (W– boson in the opposite direction). Work out where the charge is going and label it. The Photoelectric Effect Unit 1 Lesson 10 Learning To know what the photoelectric effect is and how frequency and intensity affect Outcomes it To be able to explain what photon, photoelectron, work function and threshold frequency are  To be able to calculate the kinetic energy of a photoelectron N. DWYER Observations When light fell onto a metal plate it released electrons from the surface straight away. Increasing the intensity increased the number of electrons emitted. If the frequency of the light was lowered, no electrons were emitted at all. Increasing the intensity and giving it more time did nothing, no electrons were emitted. If Light was a Wave… Increasing the intensity would increase the energy of the light. The energy from the light would be evenly spread over the metal and each electron would be given a small amount of energy. Eventually the electron would have enough energy to be removed from the metal. Photon Max Planck had the idea that light could be released in ‘chunks’ or packets of energy. Einstein named these wave-packets photons. The energy carried by a photon is given by the equation: Since we can also write this as: Explaining the Photoelectric Effect Einstein suggested that one photon collides with one electron in the metal, giving it enough energy to be removed from the metal and then fly off somewhere. Some of the energy of the photon is used to break the bonds holding the electron in the metal and the rest of the energy is used by the electron to move away (kinetic energy). He represented this with the equation: hf represents the energy of the photon,  is the work function and EK is the kinetic energy. Work Function,  The work function is the amount of energy the electron requires to be completely removed from the surface of the metal. This is the energy just to remove it, not to move away. Threshold Frequency, f0 The threshold frequency is the minimum frequency that would release an electron from the surface of a metal, any less and nothing will happen. Since , the minimum frequency releases an electron that is not moving, so EK = 0 which can be rearranged to give: Increasing the intensity increases the number of photons the light sources gives out each second. If the photon has less energy than the work function an electron can not be removed. Increasing the intensity just sends out more photons, all of which would still not have enough energy to release an electron. Graph If we plot a graph of the kinetic energy of the electrons against frequency we get a graph that looks like this: Start with and transform into. EK is the y-axis and f is the x- axis. This makes the equation become: So the gradient represents Planck’s constant and the y-intercept represents (–) the work function. Nightclub Analogy We can think of the photoelectric effect in terms of a full nightclub; let the people going into the club represent the photons, the people leaving the club represent the electrons and money represent the energy. The club is full so it is one in and one out. The work function equals the entrance fee and is £5: If you have £3 you don’t have enough to get in so noone is kicked out. If 50 people arrive with £3 no one has enough, so one gets in and noone is kicked out.  If you have £5 you have enough to get in so someone is kicked out, but you have no money for booze. If 50 people arrive with £5 you all get in so 50 people are kicked out, but you have no money for booze. If you have £20 you have enough to get in so someone is kicked out and you have £15 to spend on booze. If 50 people arrive with £20 you all get in so 50 people are kicked out and you have £15 each to spend on booze. Excitation, Ionisation and Unit 1 Lesson 11 Energy Levels To know how Bohr solved the falling electron problem Learning To be able to explain what excitation, de-excitation and ionisation are Outcomes To be able to calculate the frequency needed for excitation to a N. certain level DWYER The Electronvolt, eV The Joule is too big use on an atomic and nuclear scale so we will now use the electronvolt, represented by eV. One electronvolt is equal to the energy gained by an electron of charge e, when it is accelerated through a potential difference of 1 volt. 1eV = 1.6 x 10-19J 1J = 6.25 x 1018eV eV  J multiply by e J  eV divide by e The Problem with Atoms Rutherford’s nuclear model of the atom leaves us with a problem: a charged particle emits radiation when it accelerates. This would mean that the electrons would fall into the nucleus. Bohr to the Rescue Niels Bohr solved this problem by suggesting that the electrons could only orbit the nucleus in certain ‘allowed’ energy levels. He suggested that an electron may only transfer energy when it moves from one energy level to another. A change from one level to another is called a ‘transition’. To move up and energy level the electron must gain the exact amount of energy to make the transition. It can do this by another electron colliding with it or by absorbing a photon of the exact energy. When moving down a level the electron must lose the exact amount of energy when making the transition. It releases this energy as a photon of energy equal to the energy it loses. E1 is the energy of the level the electron starts at and E2 is the energy of the level the electron ends at Excitation When an electron gains the exact amount of energy to move up one or more energy levels De-excitation When an electron gives out the exact amount of energy to move back down to its original energy level Ionisation An electron can gain enough energy to be completely removed from the atom.  The ground state and the energy levels leading up to ionisation have negative values of energy, this is because they are compared to the ionisation level. Remember that energy must be given to the electrons to move up a level and is lost (or given out) when it moves down a level. Line Spectra Atoms of the same element have same energy levels. Each transition releases a photon with a set amount of energy meaning the frequency and wavelength are also set. The wavelength of light is responsible for colour it is. We can analyse the light by using a diffraction grating to separate light into the colours that makes it up, called its line spectra. Each element has its own line spectra like a barcode. To the above right are the line spectra of Hydrogen and Helium. We can calculate the energy difference that created the colour. If we know the energy differences for each element we can work out which element is responsible for the light and hence deduce which elements are present. We can see that there are 6 possible transitions in the diagram to the left, A to F. D has an energy difference of 1.9 eV or 3.04 x 10 -19 J which corresponds to a frequency of 4.59 x 1014 Hz and a wavelength of 654 nm – red. Wave-Particle Duality Unit 1 Lesson 12 To know how to calculate the de Broglie wavelength and what is it Learning To be able to explain what electron diffraction shows us Outcomes To know what wave-particle duality is N. DWYER De Broglie In 1923 Louis de Broglie put forward the idea that ‘all particles have a wave nature’ meaning that particles can behave like waves. This doesn’t sound too far fetched after Einstein proved that a wave can behave like a particle. De Broglie said that all particles could have a wavelength. A particle of mass, m, that is travelling at velocity, v, would have a wavelength given by: which is sometime written as where p is momentum This wavelength is called the de Broglie wavelength. The modern view is that the de Broglie wavelength is linked to the probability of finding the particle at a certain point in space. De Broglie wavelength is measured in metres, m Electron Diffraction Two years after de Broglie came up with his particle wavelengths and idea that electrons could diffract, Davisson and Germer proved this to happen. They fired electrons into a crystal structure which acted as a diffraction grating. This produced areas of electrons and no electrons on the screen behind it, just like the pattern you get when light diffracts. Electron Wavelength We can calculate the de Broglie wavelength of an electron from the potential difference, V, that accelerated it. Change in electric potential energy gained = eV  This is equal to the kinetic energy of the electron The velocity is therefore given by: We can substitute this into to get: Sand Analogy If we compare a double slit electron diffraction to sand falling from containers we can see how crazy electron diffraction is. Imagine two holes about 30cm apart that sand is dropping from. We would expect to find a maximum amount of sand under each hole, right? This is not what we find! We find a maximum in between the two holes. The electrons are acting like a wave. Wave-Particle Duality Wave-particle duality means that waves sometimes behave like particles and particles sometimes behave like waves. Some examples of these are shown below: Light as a Wave Diffraction, interference, polarisation and refraction all prove that light is a wave and will be covered in Unit 2. Light as a Particle We have seen that the photoelectric effect shows that light can behave as a particle called a photon. Electron as a Particle The deflection by an electromagnetic field and collisions with other particles show its particle nature. Electron as a Wave Electron diffraction proves that a particle can show wave behaviour. QVIRt Unit 1 Lesson 13 To be able to explain what current, charge, voltage/potential difference and resistance are Learning To know the equations that link these Outcomes To know the correct units to be use in each N. DWYER Definitions (Also seen in GCSE Physics 2) Current, I Electrical current is the rate of flow of charge in a circuit. Electrons are charged particles that move around the circuit. So we can think of the electrical current is the rate of the flow of electrons, not so much the speed but the number of electrons moving in the circuit. If we imagine that electrons are Year 7 students and a wire of a circuit is a corridor, the current is how many students passing in a set time. Current is measured in Amperes (or Amps), A Charge, Q The amount of electrical charge is a fundamental unit, similar to mass and length and time. From the data sheet we can see that the charge on one electron is actually -1.60 x 10-19 C. This means that it takes 6.25 x 1018 electrons to transfer 1C of charge. Charge is measured in Coulombs, C Voltage/Potential Difference, V Voltage, or potential difference, is the work done per unit charge. 1 unit of charge is 6.25 x 1018 electrons, so we can think of potential difference as the energy given to each of the electrons, or the pushing force on the electrons. It is the p.d. that causes a current to flow and we can think of it like water flowing in a pipe. If we make one end higher than the other end, water will flow down in, if we increase the height (increase the p.d.) we get more flowing. If we think of current as Year 7s walking down a corridor, the harder we push them down the corridor the more we get flowing. Voltage and p.d. are measured in Volts, V Resistance, R The resistance of a material tells us how easy or difficult it is to make a current flow through it. If we think of current as Year 7s walking down a corridor, it would be harder to make the Year 7s flow if we added some Year 11 rugby players into the corridor. Increasing resistance lowers the current. Resistance is measured in Ohms, Ω Time, t You know, time! How long stuff takes and that. Time is measured in seconds, s Equations There are three equations that we need to be able to explain and substitute numbers into. 1 This says that the current is the rate of change of charge per second and backs up or idea of current as the rate at which electrons (and charge) flow. This can be rearranged into which means that the charge is equal to how much is flowing multiplied by how long it flows for. 2 This says that the voltage/p.d. is equal to the energy per charge. The ‘push’ of the electrons is equal to the energy given to each charge (electron). 3 This says that increasing the p.d. increases the current. Increasing the ‘push’ of the electrons makes more flow. It also shows us that for constant V, if R increases I gets smaller. Pushing the same strength, if there is more blocking force less current will flow. Ohm’s Laws and I-V Graphs Unit 1 Lesson 14 To be able to sketch and explain the I-V graphs of a diode, filament lamp and resistor Learning To be able to describe the experimental set up and measurements required to Outcomes obtain these graphs To know how the resistance of an LDR and Thermistor varies N. DWYER Ohm’s Law (Also seen in GCSE Physics 2) After the last lesson we knew that a voltage (or potential difference) causes a current to flow and that the size of the current depends on the size of the p.d. For something to obey Ohm’s law the current flowing is proportional to the p.d. pushing it. V=IR so this means the resistance is constant. On a graph of current against p.d. this appears as a straight line. Taking Measurements To find how the current through a component varies with the potential difference across it we must take readings. To measure the potential difference we use a voltmeter connected in parallel and to measure the current we use an ammeter connected in series.  If we connect the component to a battery we would now have one reading for the p.d. and one for the current. But what we require is a range of readings. One way around this would be to use a range of batteries to give different p.d.s. A better way is to add a variable resistor to the circuit, this allows us to use one battery and get a range of readings for current and p.d. To obtain values for current in the negative direction we can reverse either the battery or the component. I-V Graphs (Also seen in GCSE Physics 2) Resistor This shows that when p.d. is zero so is the current. When we increase the p.d. in one direction the current increases in that direction. If we apply a p.d. in the reverse direction a current flows in the reverse direction. The straight line shows that current is proportional to p.d. and it obeys Ohm’s law. Graph a has a lower resistance than graph b because for the same p.d. less current flows through b. Filament Lamp At low values the current is proportional to p.d. and so, obeys Ohm’s law. As the potential difference and current increase so does the temperature. This increases the resistance and the graph curves, since resistance changes it no longer obeys Ohm’s law. Diode This shows us that in one direction increasing the p.d. increases the current but in the reverse direction the p.d. does not make a current flow. We say that it is forward biased. Since resistance changes it does not obey Ohm’s law. Three Special Resistors (Also seen in GCSE Physics 2) Variable Resistor A variable resistor is a resistor whose value can be changed. Thermistor The resistance of a thermistor varied with temperature. At low temperatures the resistance is high, at high temperatures the resistance is low. Light Dependant Resistor (L.D.R) The resistance of a thermistor varied with light intensity. In dim light the resistance is high and in bright light the resistance is low. Resistivity and Unit 1 Lesson 15 Superconductivity To be able to state what affects resistance of a wire and explain how they affect it Learning To be able to describe the experimental set up required to calculate resistivity Outcomes and define it To be able to explain superconductivity and state its uses N. DWYER Resistance The resistance of a wire is caused by free electrons colliding with the positive ions that make up the structure of the metal. The resistance depends upon several factors: Length, l Length increases – resistance increases The longer the piece of wire the more collisions the electrons will have. Area, A Area increases – resistance decreases The wider the piece of wire the more gaps there are between the ions.  Temperature Temperature increases – resistance increases As temperature increases the ions are given more energy and vibrate more, the electrons are more likely to collide with the ions. Material The structure of any two metals is similar but not the same, some metal ions are closer together, others have bigger ions. Resistivity, ρ The resistance of a material can be calculate using where ρ is the resistivity of the material. Resistivity is a factor that accounts for the structure of the metal and the temperature. Each metal has its own value of resisitivity for each temperature. For example, the resistivity of copper is 1.7x10-8 Ωm and carbon is 3x10-5 Ωm at room temperature. When both are heated to 100°C their resistivities increase. Resistivity is measured in Ohm metres , Ωm Measuring Resistivity In order to measure resistivity of a wire we need to measure the length, cross-sectional area (using Area = πr2) and resistance. Remember, to measure the resistance we need to measure values of current and potential difference using the set up shown on the right We then rearrange the equation to and substitute values in Superconductivity The resistivity (and so resistance) of metals increases with the temperature. The reverse is also true that, lowering the temperature lowers the resistivity. When certain metals are cooled below a critical temperature their resistivity drops to zero. The metal now has zero resistance and allows massive currents to flow without losing any energy as heat. These metals are called superconductors. When a superconductor is heated above it’s critical temperature it loses its superconductivity and behaves like other metals. The highest recorded temperature to date is – 196°C, large amounts of energy are required to cool the metal to below this temperature. Uses of Superconductors High-power electromagnets Power cables Magnetic Resonance Imaging (MRI) scanners Series and Parallel Circuits Unit 1 Lesson 16 To be able to calculate total current in series and parallel circuits Learning To be able to calculate total potential difference in series and parallel circuits Outcomes To be able to calculate total resistance in series and parallel circuits N. DWYER Series Circuits (Also seen in GCSE Physics 2) In a series circuit all the components are in one circuit or loop. If resistor 1 in the diagram was removed this would break the whole circuit. The total current of the circuit is the same at each point in the circuit. The total voltage of the circuit is equal to the sum of the p.d.s across each resistor. The total resistance of the circuit is equal to the sum of the resistance of each resistor. Parallel Circuits (Also seen in GCSE Physics 2) Components in parallel have their own separate circuit or loop. If resistor 1 in the diagram was removed this would only break that circuit, a current would still flow through resistors 2 and 3. The total current is equal to the sum of the currents through each resistor. The total potential difference is equal to the p.d.s across each resistor. The total resistance can be calculated using the equation: Water Slide Analogy Imagine instead of getting a potential difference we get a height difference by reaching the top of a slide. This series circuit has three connected slides and the parallel circuit below has three separate slides that reach the bottom. Voltages/P.D.s In series we can see that the total height loss is equal to how much you fall on slide 1, slide 2 and slide 3 added together. This means that the total p.d. lost must be the p.d. given by the battery. If the resistors have equal values this drop in potential difference will be equal. In parallel we see each slide will drop by the same height meaning the potential difference is equal to the total potential difference of the battery. Currents If we imagine 100 people on the water slide, in series we can see that 100 people get to the top. All 100 must go down slide 1 then slide 2 and final slide 3, there is no other option. So the current in a series circuit is the same everywhere. In parallel we see there is a choice in the slide we take. 100 people get to the top of the slide but some may go down slide 1, some down slide 2 and some down slide 3. The total number of people is equal to the number of people going down each slide added together, and the total current is equal to the currents in each circuit/loop. Energy and Power Unit 1 Lesson 17  To know what power is and how to calculate the power of an electrical circuit Learning To know how to calculate the energy transferred in an electrical circuit Outcomes To be able to derive further equations or use a series of equations to N. find the answer DWYER Power (Also seen in GCSE Physics 1) Power is a measure of how quickly something can transfer energy. Power is linked to energy by the equation: Energy is measured in Joules, J Time is measured in seconds, s Pow er is measured in Watts, W New Equations If we look at the equations from the QVIRt lesson we can derive some new equations for energy and power. Energy can be rearranged into and we know that so combining these equations we get a new one to calculate the energy in an electric circuit: critical mass: more neutrons are produced than are escaping. Meltdown Nuclear Fusion (Also see GCSE Physics 2) Fusion occurs when two nuclei join to form a bigger nucleus The two nuclei must have very high energies to be moving fast enough to overcome the electrostatic repulsion of the protons then, when close enough, the strong nuclear force will pull the two nuclei together. Here is an example of the fusing of two hydrogen isotopes: Which Will Happen? Looking at the graph we can see the Iron 56 has the highest binding energy per nucleon, the most energy required to remove one proton or neutron from the nucleus. This makes it the most stable. Nuclei lighter than Iron will undergo fusion. Protons and neutrons feel the attraction of the strong nuclear force but only protons feel the repulsion of the electrostatic force. For light nuclei, adding an extra proton increases the strong nuclear force to pull the nucleon together. This is because at this range the s.n.f. force is stronger than the other three fundamental forces. The nucleons move closer together  potential energy is lost  energy is given out Nuclei heavier than Iron will undergo fission. Beyond Iron, each proton that is added to the nuclei adds to the electrostatic repulsion. The bigger the nucleus become the less the outer protons feel the strong nuclear force from the other side. We can see the binding energy per nucleon decrease for heavier nuclei. A big nucleus will break into two smaller nuclei, each being stronger bonded together due to the smaller size. The nucleons move closer together  potential energy is lost  energy is given out.  Nuclear Reactors Unit 5 Lesson 8 To be able to explain how a nuclear reactor produces electricity To be able to explain the roles of the fuel rods, moderator, coolant and control Learning rods Outcomes To be able to give examples of the materials use for each of the N. above DWYER Making Electricity This is a typical nuclear fission reactor. A nuclear power station is similar to a power station powered by the combustion of fossil fuels or biomass. In such a station the fuel is burnt in a boiler, the heat this produces it uses to heat water into steam in the pipes that cover the roof and walls of the boiler. This steam is used to turn a turbine which is connected to a generator that produces electricity (see GCSE Physics 3 and A2 Unit 4). Steam enters the cooling towers where is it condensed into water to be used again. In a nuclear fission reactor the heat is produced in a different way. Components of a Nuclear Reactor Fuel Rods This is where nuclear fission reactions happen. They are made or Uranium and there are hundreds of them spread out in a grid like pattern. Natural Uranium is a mixture of different isotope. The most common are U 238 which accounts for 99.28% and U235 which accounts for only 0.72% of it. 238 will only undergo fission when exposed to very high-energy neutrons whilst 235 will undergo fission much more easily. The Uranium that is used in fuel rods has a higher percentage of 235 and is said to be enriched. This is so more fission reactions may take place. Moderator Role: The neutrons that are given out from nuclear fission are travelling too fast to cause another fission process. They are released at 1 x 10 7 m/s and must be slowed to 2 x 10 3 m/s, losing 99.99975% of their kinetic energy. The neutrons collide with the atoms of the moderator which turns the kinetic energy into heat. Neutrons that are travelling slow enough to cause a fission process are called thermal neutrons, this is because they have the same amount of kinetic energy as the atoms of the moderator (about 0.025 eV at 20°C). Factors affecting the choice of materials: Must have a low mass number to absorb more kinetic energy with each collision and a low tendency to absorb neutrons so it doesn’t hinder the chain reaction. Typical materials: graphite and water. Coolant Role: Heat is carried from the moderator to the heat exchanger by the coolant. The pressuriser and the pump move the hot coolant to the heat exchanger, here hot coolant touches pipes carrying cold water. Heat flows from hot coolant to cold water turning the water into steam and cooling the coolant. The steam then leaves the reactor (and will turn a turbine) as the coolant return to the reactor. Factors affecting the choice of materials: Must be able to carry large amounts of heat (L11 The Specifics), must be gas or liquid, non-corrosive, non-flammable and a poor neutron absorber (less likely to become radioactive). Typical materials: carbon dioxide and water. Control rods  Role: For the reactor to transfer energy at a constant rate each nuclear fission reaction must lead to one more fission reaction. Since each reaction gives out two or more we must remove some of the extra neutrons. The control rods absorb neutrons, reducing the amount of nuclear fission processes occurring and making the power output constant. They can be lowered further into the fuel rods to absorb more neutrons and further reduce the amount of fission occurring. Some neutrons leave the reactor without interacting, some travel too fast while other are absorbed by U 238 nuclei. If we need more neutrons we can raise the control rods. Factors affecting the choice of materials: Ability to absorb neutrons and a high melting point. Typical materials: boron and cadmium. Nuclear Safety Aspects Unit 5 Lesson 9 To be able to list and explain the safety features of a nuclear reactor Learning To be able to explain how an emergency shut-down happens in a nuclear reactor Outcomes To be able to state and explain the methods of nuclear waste N. disposal DWYER Nuclear Reactor Safety There are many safety features and controls in place designed to minimise the risk of harm to humans and the surrounding environment. Fuel Used Using solids rather than liquids avoids the danger of leaks or spillages. They are inserted and removed from the reactor by remote controlled handling devices. Shielding The reactor core (containing the fuel, moderator and control rods) is made from steel and designed to withstand high temperatures and pressures. The core itself is inside a thick, leak proof concrete box which absorbs escaping neutrons and gamma radiation. Around the concrete box is a safety area, not to be entered by humans. Emergency Shut-down There are several systems in place to make it impossible for a nuclear disaster to take place: If the reactor needs stopping immediately the control rods are inserted fully into the core, they absorb any neutrons present and stop any further reactions from happening. Some reactors have a secondary set of control rods held up by an electromagnet, so if a power cut happens the control rods fall into the core. If there is a loss of coolant and the temperature of the core rises beyond the safe working limits an emergency cooling system floods the core (with nitrogen gas or water) to cool it and absorb any spare neutrons. Nuclear Waste Disposal There are three levels of waste, each is produced, handled and disposed of in different ways: High-level Radioactive Waste What it is? Spent fuel rods from the reactor and unwanted, highly radioactive material separated from the spent fuel rods. How do we get rid? The spent fuel rods are taken from the reactor and stored in cooling ponds with in the power station to allow most of the short-term radioactivity to die away. It is then transported to a processing plant. Here it is encased in steel containers and kept under water. The cladding is eventually removed and the fuel rods are separated into unused uranium and plutonium and highly radioactive waste. The uranium and plutonium is kept in sealed container for possible future use. The waste is converted into powder, fused into glass blocks, sealed in air-cooled containers for around 50 years before being stored deep underground in a stable rock formation. Time scale? Up to a year in the cooling ponds. Radioactive waste can remain at dangerous levels for thousands of years. Intermediate-level Radioactive Waste What it is? Fuel element cladding, sludge from treatment processes, contaminated equipment, hospital radioisotopes and containers of radioactive materials. How do we get rid? Sealed in steel drums that are encased in concrete and stored in buildings with reinforced concrete. Also stored deep underground in a suitable location that has a stable rock formation and low water flow. Time scale? Thousands of years. Low-level Radioactive Waste What is it? Worn-out laboratory equipment, used protective clothing, wrapping material and cooling pond water. How do we get rid? Sealed in metal drums and buried deep underground in a supervised repository. Treated cooling pond water is released into the environment. Time scale? A few months. Heat, Temperature and Internal Unit 5 Lesson 10 Energy To know what internal energy is To be able to explain the difference between heat, temperature and internal Learning energy Outcomes To be able to explain what absolute zero is and how it was found N. DWYER Internal Energy The internal energy of a substance is due to the vibrations/movement energy of the particles (kinetic) and the energy due to the bonds holding them together (potential). Solids: In a solid the particles are arranged in a regular fixed structure, they cannot move from their position in the structure but can vibrate. The internal energy of a solid is due to the kinetic energy of the vibrating particles and the potential energy from the bonds between them. Liquids: In a liquid the particles vibrate and are free to move around but are still in contact with each other. The forces between them are less than when in solid form. The internal energy of a liquid is due to the kinetic and potential energies of the particles but since they are free to slide past each other the potential energy is less than that of it in solid form. Gases: In a gas particles are free to move in all directions with high speeds. There are almost no forces of attraction between them. The internal energy of a gas is almost entirely due to the kinetic energy of the particles. Temperature Temperature is a measure of the kinetic energies of the particles in the substance. As we can see from the graph something with a high temperature means the particles are vibrating/moving with higher average speeds that a substance at a lower temperature. It is possible for two objects/substances to be at the same temperature but have different internal energies. We will go into this further in the next lesson: The Specifics. Heat Heat is the flow of thermal energy and it flows from a high temperature to a low temperature. If two objects are at the same temperature we say that they are in thermal equilibrium and no heat flows. If object A is in thermal equilibrium with object B and object B is in thermal equilibrium with object C then A and C must be in thermal equilibrium with each other. Get into a hot or cold bath and energy is transferred:  In a cold bath thermal energy is transferred from your body to the water. In a hot bath thermal energy is transferred from the water to your body. As the energy is transferred you and the water become the same temperature. When this happens there is no longer a flow of energy  so no more heat. You both still have a temperature due to the vibrations of your particles but there is no longer a temperature difference so there is no longer a flow of energy. Temperature Scale The Celsius scale was established by giving the temperature at which water becomes ice a value of 0 and the temperature at which it boils a value of 100. Using these fixed points a scale was created. Absolute Zero and Kelvins In 1848 William Thomson came up with the Kelvin scale for temperature. He measured the pressure caused by gases at known temperatures (in °C) and plotted the results. He found a graph like this one. By extrapolating his results he found the temperature at which a gas would exert zero pressure. Since pressure is caused by the collisions of the gas particles with the container, zero pressure means the particles are not moving and have a minimum internal energy. At this point the particle stops moving completely and we call this temperature absolute zero, it is not possible to get any colder. This temperature is -273°C. 1 Kelvin is the same size as 1 degree Celsius but the Kelvin scale starts at absolute zero. °C = K – 273 K = °C + 273 The Specifics Unit 5 Lesson 11 To be able to explain and calculate specific heat capacity Learning To be able to explain and calculate specific latent heat Outcomes To know the correct units to use and the assumptions we make in N. energy transfer DWYER Specific Heat Capacity We know that when we heat a substance the temperature will increase. The equation that links heat (energy) and temperature is: c is the specific heat capacity which is the energy required to raise the temperature of 1 kg of a substance by 1 degree. It can be thought of as the heat energy 1 kg of the substance can hold before the temperature will increase by 1 degree. Specific Heat Capacity is measured in Joules per kilogram per Kelvin, J/kg K or J kg-1 K-1 Water Analogy We can think of the energy being transferred as volume of water. Consider two substances: one with a high heat capacity represented by 250 ml beakers and one with a low heat capacity represented by 100 ml beakers. When a beaker is full the temperature of the substance will increase by 1 degree. We can see that 2 litres of water will fill 8 of the 250 ml beakers or 20 of the 100ml beakers meaning the same amount of energy can raise the temperature of the first substance by 8 degrees or the second by 20 degrees. Changes of State When a substance changes state there is no change in temperature. When a solid is heated energy is transferred to the particles making them vibrate more which means the temperature increases. The potential energy of the solid remains constant but the kinetic energy increases. At melting point the particles do not vibrate any faster, meaning the kinetic energy and temperature are constant. The bonds that keep the particles in a rigid shape are broken and the potential energy increases. In liquid form the particles are still in contact with each other but can slide past each other. As more energy is transferred the particles vibrate more. The kinetic energy increases but the potential energy is constant. At boiling point the particles do not vibrate any faster, meaning the kinetic energy and temperature are constant. The bonds holding the particles together are all broken, this takes much more energy than when melting since all the bonds need to be broken. When a gas is heated the particles move faster, meaning the kinetic energy and temperature increases. The potential energy stays constant. Specific Latent Heat Different substances require different amounts of energy to change them from solid to liquid and from liquid to gas. The energy required is given by the equation: l represents the specific latent heat which is the energy required to change 1 kg of a substance from solid to liquid or liquid to gas without a change in temperature. Specific Latent Heat is measured in Joules per kilogram, J/kg or J kg-1 The specific latent heat of fusion is the energy required to change 1 kg of solid into liquid The specific latent heat of vaporisation is the energy required to change 1 kg of liquid into gas. As we have just discussed, changing from a liquid to a gas takes more energy than changing a solid into a gas, so the specific latent heat of vaporisation is higher than the specific latent heat of fusion. Gas Laws Unit 5 Lesson 12 To know and be able to use the correct units for volume, temperature and pressure Learning To be able to state Boyle’s Charles’ and the Pressure law for gases Outcomes To be able to sketch the graphs that show these laws N. DWYER Gas Properties Volume, V: This is the space occupied by the particles that make up the gas. Volume is measured in metres cubed, m3 Temperature, T: This is a measure of the internal energy of the gas and this is equal to the average kinetic energy of its particles. Temperature is measured in Kelvin, K Pressure, p: When a gas particle collides with the walls of its container it causes a pressure. Pressure is given by the equation pressure = Force/Area or ‘force per unit area’. Pressure is measured in pascals, Pa 1 pascal is equal to a pressure of 1 newton per square metre. Understanding the Gas Laws We are about to look at the three different laws that all gases obey. To help us understand them let us apply each one to a simple model. Image one ball in a box; the pressure is a measure of how many collisions between the ball and the box happen in a certain time, the volume is the area of the box and the temperature is the average speed of the ball. To simply thing further let us assume it is only moving back and forth in the x direction. Boyle’s Law The pressure of a fixed mass of gas is inversely proportional to its volume when kept at a constant temperature. for constant T Think about it… If temperature is constant this means that the ball is travelling at a fixed, constant speed. If we increase the size of the box it makes fewer collisions in the same time because it has to travel further before it collides with the side. If we make the box smaller the ball will collide with the box more often since it has less distance to travel. Charles’ Law All gases expand at the same rate when heated. The volume of a fixed mass of gas is proportional to its temperature when kept at a constant pressure.  for constant p Think about it… If pressure is constant that means that the same number of collisions with the box are taking place. So if the box was made bigger the ball would have to move faster to make sure there were the same amount of collisions per unit time. The Pressure Law The pressure of a fixed mass of gas is proportional to its temperature when kept at a constant volume. for constant V Think about it… If the volume in constant it means the box has a fixed size. If we increase the speed at which the ball is moving it will hit the sides of the box more often. If we slow the ball down it will hit the sides less often. Ideal Gases Unit 5 Lesson 13 To be able to calculate the pressure, volume or temperature of a gas Learning To know and be able to use the ideal gas equation Outcomes To know the significance of Avogadro’s constant, Boltzmann’s N. constant and moles DWYER Messing with Gases The three gas laws can be combined to give us the equation: We can rearrange this to give: constant We can use this to derive a very useful equation to compare the pressure, volume and temperature of a gas that is changed from one state (p1, V1, T1) to another (p2, V2, T2). Temperatures must be in Kelvin, K Avogadro and the Mole One mole of a material has a mass of M grams, where M is the molecular mass in atomic mass units, u. Oxygen has a molecular mass of 16, so 1 mole of Oxygen atoms has a mass of 16g, 2 moles has a mass of 32g and so on. An Oxygen molecule is made of two atoms so it has a molecular mass of 32g. This means 16g would be half a mole of Oxygen molecules. where n is the number of moles, m is the mass and M is the molecular mass. Avogadro suggested that one mole of any substance contains the same number of particles, he found this to be 6.02 x 10 23. This gives us a second way of calculating the number of moles where N is the number of particles and NA is the Avogadro constant. NA is the Avogadro Constant, NA = 6.02 x 1023 mol-1 Ideal Gases We know from the three gas laws that constant Ideal gases all behave in the same way so we can assign a letter to the constant. The equation becomes: If the volume and temperature of a gas are kept constant then the pressure depends on R and the number of particles in the container. We must take account of this by bringing the number of moles, n, into the equation:   R is the Molar Gas Constant, R = 8.31 J K-1 mol-1 This is called the equation of state for an ideal gas. The concept of ideal gases is used to approximate the behaviour of real gases. Real gases can become liquids at low temperatures and high pressures. Using the Avogadro’s equation for n we can derive a new equation for an ideal gas:   Boltzmann Constant – cheeky! Boltzmann noticed that R and NA in the above equation are constants, so dividing one by the other will always give the same answer. The Boltzmann constant is represented by k and is given as k is the Boltzmann Constant, k = 1.38 x 10-23 J K-1 can become which can also be written as Molecular Kinetic Theory Unit 5 Lesson 14 Model To be able to list the assumptions needed to derive an equation for the pressure of a gas Learning To be able to derive an equation for the pressure of a gas Outcomes To be able to calculate the mean kinetic energy of a gas molecule N. DWYER Assumptions 1. There are a very large number of molecules (N) 2. Molecules have negligible volume compared to the container 3. The molecules show random motion (ranges of speeds and directions) 4. Newton’s Laws of Motion can be applied to the molecules 5. Collisions are elastic and happen quickly compared to the time between collisions 6. There are no intermolecular forces acting other than when they collide The Big, Bad Derivation The molecules move in all directions. Let us start with one molecule of mass m travelling with velocity vx. It collides with the walls of the container, each wall has a length of L. Calculate the change in momentum: before it moves with velocity vx and after the collision it move with –vx.  Equation 1 The time can be given by using distance/speed: the speed is vx and the distance is twice the length of the box (the distance to collide and then collide again with the same wall) Equation 2 Force can be calculated by: Substitute in Equation 1 and 2    Equation 3, gives the force of one molecule acting on the side of the container. We can now calculate the pressure this one molecule causes in the x direction: Substituting Equation 3    Equation 4  (If we assume that the box is a cube, we can replace L3 with V, both units are m3) All the molecules of the gas have difference speeds in the x direction. We can find the pressure in the x direction due to them all by first using the mean value of vx and then multiplying it by N, the total number of molecules:  Equation 5 Equation 5 gives us the pressure in the x direction. But since the  The mean speed in all directions is given  by: average velocities in all We can substitute this into the Equation 5 for pressure above: directions are    Equation 6 Kinetic Energy of a Gas From the equation we have just derived we can find an equation for the mean kinetic energy of a gas: Since and combine these to get Equation 7 Kinetic energy is given by so we need to make the above equation look the same.      Don’t forget that cheeky chap Boltzmann  Equation 8