Atomic Structure and Density

Atomic Structure

• JJ Thompson discovered atoms contain positive and negative charges, proposing the plum pudding model.
• Ernest Rutherford found the positive part of the atom is a tiny nucleus, with electrons orbiting far away.
• Niels Bohr discovered electrons exist in shells or orbitals.
• James Chadwick discovered neutrons in the nucleus, which are neutral charges; protons are positive charges.
• Atomic number (bottom number in element symbol) represents the number of protons.
• Mass number (top number in element symbol) indicates total protons and neutrons.
• Isotopes are atoms of the same element with differing numbers of neutrons.

Density

• Density measures how compact the mass of a material is.
• Density = Mass / Volume
• Symbol for density is the Greek letter rho (ρ).
• Density depends on the type of particles and how tightly packed they are.
• Water vapor is less dense than liquid water because its molecules are more spread out.

Measuring Density

• The volume of a cuboid is calculated by multiplying the length of its three sides.
• Use a balance to measure mass.
• Vernier calipers have a resolution of 0.1 mm.
• Micrometers have a resolution of 0.01 mm, used for very thin objects like wires.
• Eureka can is used to find volume of irregular object by measuring water displacement.

States of Matter

• Solid (ice): particles vibrate around fixed positions.
• Liquid (water): molecules touch but can move past each other.
• Gas (water vapor): particles are far apart and move randomly, compressible.
• Melting a solid or evaporating a liquid requires energy to overcome electrostatic forces.

Specific Heat Capacity

• Specific heat capacity is found by heating a material and measuring the temperature change.
• Experiment: use electric heater in metal cylinders, measure PD (voltmeter) and current (ammeter) to get power.
• A balance measures the mass of the block.
• Use a timer and thermometer to measure the temperature increase of block during heating process.
• Energy = power x time.
• Energy is transferred to surroundings during heating, temperature change is lower, resulting in a higher calculated SHC

Internal Energy and Latent Heat

• During melting, a substance's temperature remains constant as energy goes into potential energy,not kinetic.
• Internal energy is the sum of the kinetic and potential energy of particles.
• Increased temperature indicates increased kinetic energy of particles.
• Change of state, when heating, indicates increased potential energy.
• E = mcΔT: increase change in thermal energy = mass x SHC x change in temp (increase kinetic energy).
• Specific latent heat (SLH) indicates energy to change the state of 1 kg of a substance.
• E = mL: energy needed to change state = mass x specific latent heat.

Gas Pressure

• Heating a gas increases particle kinetic energy and speed,.
• Increased speed causes more frequent collisions with container walls, increasing pressure.
• Pressure can be increased by compressing a gas, which exerts an inward force.
• If a gas is at a constant temperature, pressure x volume is constant P1V1=P2V2.
• Pressure is measured in Newtons per meter squared (N/m²) or pascals (Pa).
• Higher altitude means less dense atmosphere, hence lower pressure.

Pressure in Fluids

• Pressure is how concentrated a force is: Pressure = Force / Area.
• Unit for pressure is N/m² or pascals (Pa).
• Water pressure increases with depth due to the weight of water above.
• Pressure in liquid is calculated with p = hρg (height x density x field strength).
• Density of water is 1,000 kg/m³.

Forces

• Forces are pushes or pulls, either contact or non-contact.
• Contact forces: pushing a door (normal contact force), friction, air resistance, tension.
• Non-contact forces: magnetism, electrostatic forces, gravity.
• Silly definition: contact forces are electrostatic repulsion between electrons.
• Forces are represented with vectors i.e. arrows (direction and magnitude).
• Magnitude: The size of the force and that's indicated by the length of the arrow

Resultant Force

• Resultant force is found technically by adding the force vectors together.
• If forces are in opposite directions, one is negative.
• If vectors are right angles to each other, use Pythagoras to find the resultant force.
• Use trigonometry (SOH CAH TOA) to find angles of force vectors.
• If forces are balanced, the resultant force is zero and no acceleration occurs..
• Balanced forces do not mean the object isn't moving, just that it moves at a constant velocty.
• Balanced forces are Newton's first law: object doesn't accelerate if net force is zero.
• Scalar: measurement only has magnitude (size), not direction.
• Vector: measurement with both magnitude and direction.

Weight and Work Done

• Weight is the force due to gravity calculated as mass x gravitational field strength (g = 9.8 N/kg or 10 N/kg).
• Holding an object requires an upward force equal to its weight.
• Lifting at constant speed means force equals weight.
• Work done (energy transferred) is calculated using Work Done = Force x Distance.
• Gain in gravitational potential energy (GPE) is calculated using GPE = mgh.

Hooke's Law

• Hooke's Law states F = ke (Force = spring constant x extension).
• Spring constant (k) measured in Newtons per meter (N/m).
• Hooke's law works for objects stretched elastically i.e. returns to shape without force when removed.
• Force and extension are directly proportional if K is constant.
• Systematic error is introduced if ruler isn't lined up with bottom of the spring
• Random error is introduced if the measurer doesn't align their eye level with the measurement
• Energy stored in a spring follows the equation: E = ½ke².

Moments

• Moment is a turning force, equals force x perpendicular distance to the pivot (Nm).
• If clockwise moments equal anticlockwise moments, object does not turn (principle of moments).
• Gears: small gear turns a large gear to increase the moment produced

Speed, Velocity and Acceleration

• Speed and velocity are measured in meters per second (m/s).
• Velocity has direction.
• Calculated by distance/displacement divided by time.
• Gradient of a distance-time graph gives the speed/velocity.
• Gradient of a velocity-time graph gives the acceleration.
• Unit of acceleration is m/s².
• Negative gradient on velocity-time graph indicates deceleration.
• For falling objects, acceleration is 9.8 m/s².

Suvat Equations

• Suvat equations predict object behavior during acceleration.
• S: displacement, U: initial velocity, V: final velocity, A: acceleration, T: time.
• U = 0 if starts at rest, V = 0 if decelerates to a standstill.
• For falling objects, a = g = 9.8 m/s².
• Write down variables, identify knowns, choose the right equation and rearrange.

Newton's Laws of Motion

• First Law: an object's motion stays constant without a net force (inertia) i.e. no change in velocity.
• Second Law: F = ma i.e. unbalanced forces cause acceleration.
• Prove Newton's 2nd law using a trolley on a track being pulled by weight, use light gates to measure acceleration.
• Graph force against acceleration proves the relationship, gradient gives total mass.
• Third Law: for every action force, there's an equal and opposite reaction force.

Momentum

• Momentum measures how hard it is to stop something.
• Momentum = mass x velocity (kg m/s), is a vector.
• Total momentum is conserved in collisions.
• M1U1+M2U2=M1V1+M2V2 (momentum calculations, account for directions).
• Recoil: Cannon and cannon ball have equal and opposite momentum.

Force and Momentum

• Force is equal to the change in momentum over time (rate of change of momentum).
• Shorter time equals bigger force (seatbelts, airbags increase the time).
• The bigger the force, the faster the acceleration i.e. fast change in momentum.

Electricity Basics

• Electricity: flow of charge (electrons) carrying energy from a source to a component.
• Battery: stores chemical potential energy transformed to electrons that flow through wires, and light up a bulb.
• Current: movement of electric charge from positive to negative terminal where it is transferred into light and heat.
• Electrons don't disappear, recycled back to the battery.
• Charge is measured in coulombs (C).
• Potential difference (PD) or voltage measures energy transferred per coulomb.
• If a battery says 1 volt, it means one joule of energy is given to every coulomb of electrons that pass through it
• PD measured with a voltmeter in parallel.
• V = E / Q
• Current is the rate of flow of charge
• Current in measured in amps (A), measured with an ammeter in series.
• I = Q / T

Resistance

• Resistance resists current, enabling components to work.
• Resistance is transferred into emitted light when energy flows through a bulb.
• Increasing PD results in greater current, so PD and current are directly proportional.
• Resistor has constant resistance and is ohmic: PD and current are directly proportional.
• V = IR (PD = Current x Resistance): Ohm's Law.
• Find resistance of IV graph from ohms by picking an appropriate point on the line.
• Metals have changing resistance with increased PD and current i.e. non-ohmic.
• Metals contain a lattice of ions, surrounded by delocalized electrons.
• Collisions heat the metal and increase resistance for flow of electrons increases.
• Diode allows current in one direction only.
• LED emits light like a diode when current flows.

Resistance Measurement

• Measure V and I for a metal wire with crocodile clips, find resistance using Ohm's law.
• Moving clips changes wire length, resistance is directly proportional to length.

Series Circuits

• In series: total PD is shared, current is the same, total resistance is summed.
• Potential divider circuit shares total PD.
• The greater the resistance, the greater the share of the total PD it gets.
• If two resistors have the same resistance, they each take up 1/2 the total PD (etc).
• Can use Ohm's law for whole circuit.

Parallel Circuits

• In parallel: PD is the same, current is shared, more resistors lower total resistance.

Thermistors and LDRs

• Thermistor resistance decreases if temperature increases, basis of a temperature sensor.
• LDR (light-dependent resistor) resistance decreases with increased light intensity, used on a street lamp for example.

Magnets

• Permanent magnet molecules are aligned, producing a magnetic field.
• Magnets have North and South Poles.
• Magnetic field lines are complete loops (never intersect) from North to South.
• An induced magnet temporarily aligns, making its field ex. iron nail on any pole of a permanent magnet.
• Opposite poles attract, like poles repel.

Electromagnetism

• Current through a wire makes concentric circles, use right hand to find direction.
• B: is the shorthand for a magnetic field (magnetic flux density), also tesla.
• Motor effect: wire in different magnetic field experiences a force.
• F = BIL (force = magnetic flux density x current x length) that are perpendicular.
• Fleming's left-hand rule indicates force direction (thumb-force, first finger-field, middle finger-current).
• Measure force practically with a magnet on a balance.

Electric Motors and Loudspeakers

• Electric motors are coils that experience opposite forces on each side, causing it to turn.
• Commutator reverses current every half turn otherwise it will stop.
• To turn faster: increase current, use stronger magnet, add more turns.
• Loudspeaker: vibration due to varying current produces sound waves attaches to speaker cone.

Dynamo Effect

• Moving a wire through a magnetic field induces a current or potential (voltage), called the dynamo effect.
• Generator: coil turned creates a potential (basic power generation principle).
• Doesn't need commulator since it's AC.
• To increase output: turn faster or similar to a motor, add turns, use a stronger magnet.
• Induced current creates opposing magnetic field.
• Dynamos produce DC instead of AC.

Microphones and Transformers

• Microphone: Sound moves the diagraphm which then moves a coil around a magnet to induce current.
• Transformers change voltage in the National Grid.
• Electrical power = voltage x current (V x I). Primary coil connected to power source. Secondary connected to overhead cables. Soft iron core acts an magnetic guide.
• Step-up transformer increases voltage, reducing current (less energy loss).
• In Ideal World Power for primary coil = Power for secondary i.e. power is 100%.
• Electrical power = voltage x current (V x I).
• If secondary coil has double the turns = doubles the voltage = halves the current.
• Np / Ns = Vp / Vs: The ratio of turns in coil is equal to the ratio of voltages (rearrange as appropiate)
• No electricity in core: electricity is wirelessly transmitted from one coil to other.
• AC induces voltage because wire in magnetic field needs AC to experience a change in B.
• Step-down reduces voltage (fewer turns on secondary).