OCR A Level Physics Charge & Current PDF

Head to savemyexams.com for more awesome resources OCR A Level Physics 4.1 Charge & Current Contents 4.1.1 Electric Current & Charge 4.1.2 Electric Current & Electron Flow 4.1.3 Kirchhoff's First Law 4.1.4 Current in a Current Carrying Conductor 4.1.5 Conductors, Semiconductors & Insulators 4.1.6 Ci...

Head to savemyexams.com for more awesome resources OCR A Level Physics 4.1 Charge & Current Contents 4.1.1 Electric Current & Charge 4.1.2 Electric Current & Electron Flow 4.1.3 Kirchhoff's First Law 4.1.4 Current in a Current Carrying Conductor 4.1.5 Conductors, Semiconductors & Insulators 4.1.6 Circuit Symbols & Diagrams Page 1 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.1 Electric Current & Charge Your notes Electric Current Electric Current Electric current is defined as the rate of flow of positive charge carriers It is measured in units of amperes (A) or amps The symbol for current is I The charge, current and time are related by the equation: Where: I = current (A) ΔQ = change in charge (Q) Δt = time interval (s) When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative There are several examples of electric currents, including in household wiring and electrical appliances Current is measured using an ammeter Ammeters should always be connected in series with the part of the circuit you wish to measure the current through Page 2 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Your notes An ammeter can be used to measure the current around a circuit and always connected in series Worked example When will 8 mA of current pass through an electrical circuit? A. When 1 J of energy is used by 1 C of charge B. When a charge of 4 C passes in 500 s C. When a charge of 8 C passes in 100 s D. When a charge of 1 C passes in 8 s ANSWER: B Step 1: Write out the equation relating current, charge and time, rearranging for charge Q Q = It Step 2: Rule out any obviously incorrect options Option A does not contain charge or time, so can be ruled out Step 3: Try the rest of the options to determine the correct answer Consider option B: I = 4 / 500 = 8 × 10–3 = 8 mA Consider option C: Page 3 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources I = 8 / 100 = 80 × 10–3 = 80 mA Consider option D: Your notes I = 1 / 8 = 125 × 10–3 = 125 mA Therefore, the correct answer is B Exam Tip Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers. Page 4 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Electric Charge Charge is a property certain particles have. It can either be: A positive charge (+) (eg. proton) A negative charge (–) (eg. electron) A neutral (no) charge (eg. neutron) An atom is neutral. This is because it has an equal number of protons (positive charge) and electrons (negative) charge However, just the nucleus which is made up of protons and neutrons is positively charged In physics, the charge is represented by the symbol Q or q The Coulomb The unit of charge is the Coulomb (C) This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 A s Definition of the Coulomb Quantisation of Charge The charge on charge carriers is quantised This means the charge comes in definite, finite quantities In this way, the quantity of charge can be quantised depending on how many protons or electrons are present Positive and negative charge has a definite minimum magnitude and comes in multiples of that magnitude This magnitude is the elementary charge, e = 1.60 × 10-19 C Page 5 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources The magnitude of the charge just refers to its value, rather than whether it is positive or negative The net charge on a particle can be quantised, meaning it is always a multiple of the charge of an electron by convention The charge of an electron, e is -1.60 × 10-19 C The charge of a proton, +e is +1.60 × 10-19 C Worked example Determine the charge of an ion with charge 3 e. State an appropriate unit for your answer. Step 1: Calculate the charge 1e = –1.60 × 10–19 3 e = 3 × (–1.60 × 10–19) = –4.8 × 10 –19 Step 2: Include the unit for charge The units of charge is coulombs (C) Therefore 3 e = –4.8 × 10–19 C Exam Tip Although the charge of the electron is given on your data sheet, you will be expected to remember that the charge of the proton has the same magnitude Page 6 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.2 Electric Current & Electron Flow Your notes Defining Current Electric current is the rate of flow of charge carriers This is defined as the movement of Electrons in metals Ions in electrolytes Electrons in Metals In metals, such as copper, mercury and titanium, the flow of charge is made up of electrons The metal ions are closely packed and arranged in a crystal lattice structure The atoms have many free (delocalised) electrons that are free to move randomly These are sometimes known as conduction electrons These are what makes metals good conductors of electricity When conducting electricity, meaning one side of the metal is attaching to a negative terminal and the other to a positive, the conduction electrons drift slowly through the metal creating a current Free electrons in metal create current flow Ions in Electrolytes Ions are atoms that have lost or gained an electron. This means they are either: An anion - a negative ion (gained an electron) A cation - a positive ion (lost an electron) Page 7 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources An electrolyte is a substance that produces an electrically conducting solution The charge carriers are not electrons, but cations and anions An example of an electrolyte is copper sulfate dissolved in water Anions are attracted to the anode and cations to the cathode Page 8 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources Current & Electron Flow In electrical wires, the current is a flow of electrons Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood By definition, conventional current always goes from positive to negative (even through electrons go the other way) Page 9 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.3 Kirchhoff's First Law Your notes Kirchhoff's First Law Kirchhoff’s first law states that: The sum of the currents entering a junction always equal the sum of the currents out of the junction This is a consequence of conservation of charge Charges cannot be created nor destroyed Therefore, the current shouldn’t decrease or increase in a circuit when it splits In a circuit: A junction is a point where at least three circuit paths meet A branch is a path connecting two junctions If a circuit splits into two branches, then the current before the circuit splits should be equal to the current after it has split In the circuit below, I = I1 + I2 + I3, where I represents the current in the circuit before it branches, and I1, I2 and I3 represent the current in the respective three branches: The current I into the junction is equal to the sum of the currents out of the junction The charge is conserved on both sides of the junction In a series circuit, the current is the same at any point Page 10 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Your notes The current is the same at all points in a series circuit In a parallel circuit, the current divides at the junctions and each branch has a different value Kirchhoff’s first law applies at each junction The current divides at each junction in a parallel circuit Page 11 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Exam Tip Junctions only appear in parallel circuits and as circuits become more complex, it can be confusing as to which currents are into the junction and which are out.Drawing arrows on the diagram for the current flow (making sure it’s from positive to negative) at each junction like in the worked example will help with this. Page 12 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.4 Current in a Current Carrying Conductor Mean Drift Velocity of Charge Carriers In a conductor, the current is due to the movement of charge carriers The charge carriers can be negative or positive, however, the current is always taken to be in the same direction In conductors, the charge carrier is usually free electrons However, these electrons only travel a small difference before colliding with a metal ion This means they have a relatively slow drift velocity, v In the diagram below, the current in each conductor is from right to left but the charge carriers move in opposite directions shown by the direction of the drift velocity v In diagram A (positive charge carriers), the drift velocity is in the same direction as the current In diagram B (negative charge carriers), the drift velocity is in the opposite direction to the current Conduction in a current-carrying conductor The drift velocity is the average velocity of the charge carriers travelling through the conductor You will find this value is relatively slow (∼ 10-3 m s -1) However, since the number density of charge carriers is so large, the current flow still seems to happen instantaneously Page 13 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources Calculating Current in a Current Carrying Conductor The current can be expressed in terms of the number density (number of charge carriers per unit volume) n, the cross-sectional area A, the drift velocity v and the charge of the charge carriers q Current in a conductor equation The same equation is used whether the charge carriers are positive or negative The minus sign will indicate current in the opposite direction to the charge carriers The charge q will be e for electrons (-1.60 × 10-19 C) The number density n represents the number of free charge carriers (electrons) per unit volume Conductors, such as metals, have a high value of n Insulators, such as plastics, have a small value of n The cross-sectional area A of a wire is the area of a circle A = πr 2 Where: r = radius of the wire (m) This equation shows: v is inversely proportional to n meaning more charge carriers per unit volume will slow down their speed through the conductor I is directly proportional to n since greater n means greater charge is flowing and therefore a larger current I. When the value of n is lower, the charge carriers must travel faster to carry the same current Page 14 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources Worked example A copper wire has 9.2 × 1028 free electrons m-3. The wire has a current of 3.5 A and a crosssectional area of 1.5 mm2.Calculate the average drift speed of the electrons. Exam Tip Remember that the cross-sectional area is in m2 , the drift velocity is in m s -1 and the number density is in m-3. Therefore, sometimes unit conversions from cm or mm may be required, so make sure you're comfortable with these. Page 15 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.5 Conductors, Semiconductors & Insulators Conductors, Semiconductors & Insulators The number density of charge carriers, n of a material determines how well the material conducts electricity Value of n For Different Materials Table Conductors Conductors have a very large value of n This is often in the range of 1028 m-3 This means there are many free electrons per unit volume moving through the material Since current is the rate of flow of electrons, this makes them very good electrical conductors Examples of good electrical conductors are copper, aluminium and calcium This is why electric wires are often made from copper and aluminium Semiconductors Semiconductors are materials that have a conductivity between conductors and insulators Their conductivity depends on their temperature Page 16 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources At low temperatures, their resistivity rises, therefore their conductivity falls At high temperatures, their resistivity falls, therefore their conductivity rises Metal conductors behave in the opposite way Examples of semiconductor materials are silicon and germanium They are often used to make electronic devices such as diodes and transistors Semiconductors have very intermediate conduction properties This means they are neither very good nor very poor in terms of their conducting properties Therefore, semiconductor materials undergo doping This is when impurities are added to the material This helps improve their conducting properties, by increasing the value of n Insulators Insulators have a very small value of n This is close to 0 This means there are next to no free electrons per unit volume moving through the material With no current, this makes them very poor electrical conductors Examples of electrical insulators include plastic, rubber and glass These are useful for making parts of an object in which heat or electricity should not to flow for safety reasons eg. the casing of a plug Page 17 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources 4.1.6 Circuit Symbols & Diagrams Your notes Circuit Symbols The diagrams below show the various circuit symbols that could be used in circuit diagrams The most common symbols are: Page 18 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Your notes The function of the most common components are: Cell / battery / D.C supply: Provides the circuit with a source of e.m.f. A battery is two or more cells Switch: Turn the circuit on (closed), or off (open) Fixed resistor: A resistor limits the flow of current. A fixed resistor has a resistance it cannot change Variable resistor: A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls Thermistor: The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa Light-dependent resistor (LDR): The resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa Diode: A diode allows current to flow in one direction only. They are used to convert AC to DC current Light-emitting diode (LED): This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays (TVs, road signs) Page 19 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Head to savemyexams.com for more awesome resources Ammeter: Used to measure the current in a circuit. Connected in series with other components Voltmeter: Use to measure the potential difference of an electrical component. Connected in parallel with the relevant component Each of these components have an electrical resistance that may impact the current in the circuit However, the resistance of the ammeter and voltmeter are taken as negligible in exam questions Exam Tip You must be able to recognise and draw all of these circuit symbols for the exam, as you will rarely be told within the exam question itself. Page 20 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources Circuit Diagrams Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic Electric circuit diagrams require the following to work effectively: An energy source – This is a source of e.m.f so a current can flow. This can be a cell, battery, or a power supply (D.C, A.C or variable) A closed path or a complete circuit – Electrons need to flow in a complete loop for a current to flow. A circuit can be open and closed using a switch Electrical components – These could act as sensors that respond to the environment (LDR, thermistor), or measure a value (ammeter, voltmeter), or transfer electrical energy to other forms of energy (LED, lamp). These must be drawn with the correct circuit symbol The key rules to remember are: An ammeter is always connected in series A voltmeter is always connected in parallel to the component the voltage is being measured The direction of current flow is always from the positive to the negative terminal of the power supply Worked example Which circuit diagram correctly represents a circuit with current flowing through? Page 21 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers Your notes Head to savemyexams.com for more awesome resources Your notes For a circuit to be connected, the switch must be closed This is either circuit B or D The other circuit symbol is a diode Diodes only allow current to flow in one direction Since current flow is from positive to negative, a forward-biased diode must point in this direction in order for the current to flow This is seen in circuit B Exam Tip When asked to draw a circuit diagram, make sure to draw the wires as straight lines with a straight edge or a ruler and make it as neat as possible, especially the circuit symbols.If the diagram is too small or there is ambiguity as to what a symbol represents, the examiner may not award you full marks! Page 22 of 22 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers