Revision notes · Electricity

Current, potential difference and resistance

Standard circuit diagram symbols4.2.1.1

Circuit diagrams use standard symbols so a circuit can be drawn and understood without ambiguity.

Standard circuit diagram symbols

Component	What it does
Switch (open/closed)	Breaks or completes the circuit
Cell / battery	A cell is a single unit; a battery is two or more cells joined together — supplies potential difference
Lamp	Lights up, converting electrical energy to light (and heat)
Fuse	Contains a thin wire that melts and breaks the circuit if the current is too high, for safety
Resistor	A fixed resistance that opposes current
Variable resistor	A resistance that can be changed
Diode	Allows current to flow in one direction only
LED	A light-emitting diode — lights up when current flows through it in the allowed direction
Thermistor	A resistor whose resistance changes with temperature
LDR (light-dependent resistor)	A resistor whose resistance changes with light intensity
Voltmeter (V)	Measures potential difference, connected in parallel across a component
Ammeter (A)	Measures current, connected in series in the circuit

Common circuit symbols

⚠️ Common mistake: Connecting an ammeter in parallel or a voltmeter in series — an ammeter must always be in series (so the same current that flows through the component flows through it), and a voltmeter must always be in parallel (across the component being measured).

Electrical charge and current4.2.1.2

Definition: Current is the rate of flow of electrical charge. For charge to flow through a closed circuit, there must be a source of potential difference (e.g. a cell or battery), and the circuit must be closed (no open switches or breaks).

Equation	Units
Q = I × t	Q (charge) in C, I (current) in A, t (time) in s

Charge equation

The greater the rate of flow of charge, the greater the current. In a single closed loop (a series circuit with no branches), the current has the same value at every point around the circuit.

Current, resistance and potential difference4.2.1.3

Definition: Resistance is a measure of how much a component opposes the flow of current through it, measured in ohms (Ω).

Equation	Units
V = I × R	V (potential difference) in V, I (current) in A, R (resistance) in Ω

Potential difference equation

The current through a component depends on both its resistance and the potential difference across it. For a given potential difference, the greater the resistance of a component, the smaller the current through it.

🧠 Remember: V = IR — rearrange to find whichever quantity you need: I = V ÷ R, or R = V ÷ I.

Resistors4.2.1.4

Definition: An ohmic conductor is a resistor with a constant resistance (at a constant temperature) — current through it is directly proportional to the potential difference across it, so a current–potential difference graph is a straight line through the origin.

Some components are non-ohmic — their resistance changes with the conditions, so their current–potential difference graph is curved:

•A filament lamp: as current increases, the filament gets hotter, its resistance increases, and the graph curves (gradient decreases at higher current).
•A diode: current flows freely in one direction (low resistance), but in the reverse direction the resistance is very high and almost no current flows.
•A thermistor: resistance decreases as temperature increases.
•An LDR: resistance decreases as light intensity increases.

Investigating resistance — required practical circuit

Current–potential difference graphs

As current through a resistor increases, more energy is transferred to the resistor's atoms as the charge carriers collide with them more, making them vibrate more — this makes it harder for further charge to flow, so resistance tends to increase with current (and with temperature) for most resistors.

⚠️ Common mistake: Thinking a thermistor and a normal wire respond to temperature the same way — an ordinary wire's resistance increases with temperature, but a thermistor's resistance decreases with temperature, which is why thermistors are used in temperature sensors and thermostats.

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