What you'll learn
Electricity is the unit of AQA GCSE Combined Science: Trilogy that explains how charge flows through circuits and how electrical energy is transferred in the home. It is the most equation-heavy unit in the physics course, but the equations are few and they connect logically, so understanding the relationships matters more than memorising them separately. By the end of this unit you should be able to draw and interpret circuit diagrams using the standard symbols, apply the relationships between charge, current, potential difference, resistance, power and energy, describe how current divides and potential difference is shared in series and parallel circuits, describe the current–potential difference characteristics of a resistor, a filament lamp and a diode, explain how the resistance of a thermistor and a light-dependent resistor changes, and describe the domestic mains supply and the function of the three wires in a plug. This unit is assessed on Physics Paper 1 and includes the required practicals on resistance and on circuit components.
Key terms and definitions
Electric current — the rate of flow of electric charge, measured in amperes, often shortened to amps
Charge — a quantity measured in coulombs; one coulomb passes when a current of one ampere flows for one second
Potential difference — the energy transferred per unit charge between two points, measured in volts
Resistance — the opposition to the flow of charge, measured in ohms
Ohmic conductor — a component whose resistance stays constant provided its temperature does not change, so current is directly proportional to potential difference
Thermistor — a component whose resistance decreases as its temperature increases
Light-dependent resistor — a component whose resistance decreases as light intensity increases
Series circuit — a circuit with a single loop, so the same current flows through every component
Parallel circuit — a circuit with more than one branch, so the current divides between the branches
Direct potential difference — a supply, such as a cell, in which the current is always in the same direction
Alternating potential difference — a supply, such as the mains, in which the direction of the current reverses repeatedly
Core concepts
Current, charge and potential difference
An electric current is the rate of flow of electric charge. For a charge to flow, the circuit must be closed and there must be a source of potential difference.
Charge flow equals current multiplied by time, with charge in coulombs, current in amperes and time in seconds. Rearranged, current equals charge divided by time.
Potential difference is the energy transferred per unit charge. The energy transferred equals charge flow multiplied by potential difference, so a component with a larger potential difference across it transfers more energy for each coulomb that passes.
Resistance and Ohm's law
The current through a component depends on both the resistance of the component and the potential difference across it. The greater the resistance, the smaller the current for a given potential difference.
The central relationship is that potential difference equals current multiplied by resistance, with potential difference in volts, current in amperes and resistance in ohms. It rearranges to give current equals potential difference divided by resistance, and resistance equals potential difference divided by current.
For an ohmic conductor at constant temperature, the current is directly proportional to the potential difference, so a graph of current against potential difference is a straight line through the origin. The resistance is constant, and it is the reciprocal of the gradient.
Current–potential difference characteristics
Three components have characteristic graphs that must be recognised.
A fixed resistor at constant temperature gives a straight line through the origin, because it is an ohmic conductor and its resistance does not change.
A filament lamp gives a curve that becomes shallower as the current increases. As the current rises, the filament heats up, the ions in the metal vibrate more and obstruct the flow of electrons more, so the resistance increases.
A diode gives a line that rises steeply in one direction and is flat in the other. A diode has a very high resistance in the reverse direction, so current flows in one direction only.
Thermistors and light-dependent resistors
The resistance of a thermistor decreases as the temperature increases. This makes thermistors useful in thermostats and temperature-sensing circuits: as the surroundings warm, the resistance falls, the current rises and a circuit can be switched.
The resistance of a light-dependent resistor decreases as light intensity increases. In darkness its resistance is very high. This makes them useful in switching on street lighting automatically and in burglar alarms.
Series circuits
In a series circuit there is only one path for the current, so the current is the same through every component. This is the single most useful fact about series circuits.
The total potential difference of the supply is shared between the components, so the individual potential differences add up to the supply potential difference. A component with a larger resistance takes a larger share.
The total resistance of two components in series is the sum of their resistances. Adding a resistor in series therefore always increases the total resistance and decreases the current.
Parallel circuits
In a parallel circuit there is more than one path, so the current from the source is divided between the branches. The currents through the separate branches add up to the total current.
The potential difference across each branch is the same, and equal to the supply potential difference. This is why household appliances are wired in parallel: each receives the full mains potential difference and can be switched independently.
The total resistance of two resistors in parallel is less than the resistance of the smaller individual resistor. This is counter-intuitive but follows from there being more paths for the charge to flow through, so the total current increases for the same potential difference.
Electrical power and energy
Power is the energy transferred per second. It can be calculated in two ways: power equals potential difference multiplied by current, or power equals current squared multiplied by resistance. Which to use depends on the quantities given.
Energy transferred equals power multiplied by time, or equivalently charge flow multiplied by potential difference. Everyday electrical appliances transfer energy from the mains or from a battery to the kinetic store of motors, or to the thermal store of heating devices.
The amount of energy an appliance transfers depends on how long it is switched on and on its power. The power of an appliance is therefore a good guide to its running cost.
Mains electricity and the plug
Mains electricity is an alternating current supply, in which the direction of the current reverses repeatedly. In the United Kingdom the domestic supply has a frequency of 50 hertz and is about 230 volts. Cells and batteries supply direct current, in which the current is always in the same direction.
A three-core cable has three wires, each with a specified colour and function. The live wire is brown and carries the alternating potential difference from the supply. The neutral wire is blue and completes the circuit, staying at close to zero volts. The earth wire is green and yellow striped, carries no current in normal operation, and is a safety wire that stops the appliance casing becoming live.
The live wire is dangerous even when a switch in the circuit is open, because there is still a potential difference of about 230 volts between it and earth. Touching it would provide a path to earth and a current would flow through the body, which is why any contact with the live wire can give an electric shock.
Worked examples
Example 1: Calculating charge and resistance (4 marks)
A current of 0.4 amperes flows through a lamp for 90 seconds, with a potential difference of 6.0 volts across it. Calculate the charge that flows and the resistance of the lamp.
Charge flow equals current multiplied by time, which is 0.4 multiplied by 90, giving 36 coulombs. Resistance equals potential difference divided by current, which is 6.0 divided by 0.4, giving 15 ohms.
Example 2: Potential difference in a series circuit (3 marks)
A 12 volt supply is connected in series to a 4 ohm resistor and an 8 ohm resistor. Calculate the current and the potential difference across the 8 ohm resistor.
In series the total resistance is the sum, which is 4 plus 8, giving 12 ohms. The current is potential difference divided by resistance, which is 12 divided by 12, giving 1.0 ampere. Since the current is the same throughout a series circuit, the potential difference across the 8 ohm resistor is current multiplied by resistance, which is 1.0 multiplied by 8, giving 8 volts.
Example 3: Explaining a filament lamp graph (4 marks)
Explain why the current–potential difference graph for a filament lamp is a curve rather than a straight line.
As the potential difference increases, the current through the filament increases, and the filament transfers more energy so its temperature rises. At the higher temperature the metal ions in the filament vibrate more and obstruct the flow of electrons more, so the resistance increases. Because the resistance is no longer constant, the current does not increase in proportion to the potential difference, and the line curves so that it becomes shallower.
Common mistakes and how to avoid them
The most frequent error in this unit is stating that current is used up as it passes through components in a series circuit. Current is the same everywhere in a series circuit; it is the potential difference that is shared.
Students often say that resistance decreases when a resistor is added in parallel because the circuit is bigger. The reason is that there are more paths for the charge to flow through, so the total current increases and the total resistance falls.
Another common slip is confusing the two power equations, or squaring the wrong quantity. In power equals current squared multiplied by resistance, only the current is squared.
In filament lamp explanations, many answers say the lamp gets hot so the resistance changes without saying it increases, or without linking it to ion vibration. Both halves are needed.
Finally, students frequently describe the earth wire as carrying current. It carries no current under normal conditions; it is a safety wire that prevents the casing becoming live.
Exam technique for "Physics: Electricity"
Label the circuit before calculating. Marking which components are in series and which are in parallel takes a few seconds and decides which rules apply.
For series and parallel questions, apply the fixed facts first: in series the current is the same and the potential difference is shared; in parallel the potential difference is the same and the current is shared. Most questions unlock immediately once the right fact is written down.
When a question gives a graph, read the gradient carefully. For a current–potential difference graph, the resistance is potential difference divided by current at a chosen point, not simply the gradient.
For the required practicals, be ready to describe how you would vary the potential difference using a variable resistor, where the ammeter goes — in series with the component — and where the voltmeter goes — in parallel across it. Swapping the two meters is a standard lost mark.
Quick revision summary
Current is the rate of flow of charge, with charge flow equal to current times time. Potential difference equals current times resistance, and energy transferred equals charge times potential difference. An ohmic conductor at constant temperature gives a straight line through the origin; a filament lamp curves because its resistance rises as it heats and the ions vibrate more; a diode conducts in one direction only. Thermistor resistance falls as temperature rises and light-dependent resistor resistance falls as light intensity rises. In series the current is the same throughout, potential differences add, and resistances add. In parallel the potential difference is the same across each branch, currents add, and total resistance is less than the smallest individual resistance. Power equals potential difference times current, or current squared times resistance, and energy equals power times time. The mains is 230 volts alternating at 50 hertz, with a brown live wire, blue neutral wire and green and yellow earth wire that carries no current in normal use.