Electric charge and current, the equation Q = I t, potential difference, resistance, and Ohm's law V = I R.

What this dot point is asking

CCEA Double Award wants you to define charge and current, use Q = I t, define potential difference and resistance, and use Ohm's law, V = I R. These quantities and equations underpin every circuit calculation in P2.

Charge and current

Current is measured with an ammeter placed in series in the circuit.

Potential difference (voltage)

Voltage is measured with a voltmeter placed in parallel across a component.

Resistance and Ohm's law

For a fixed resistor at constant temperature, current is directly proportional to voltage, so a graph of current against voltage is a straight line through the origin.

A model for circuits

It helps to picture a circuit as a model. The voltage is like the push from a pump, the current is the flow of charge around the loop, and the resistance is anything that opposes that flow. The three are tied together by Ohm's law, $V = IR$, so changing any one affects the others. Practising the rearrangements $I = V/R$ and $R = V/I$ until they are automatic makes circuit questions much faster, because most are just a substitution into one of these forms.

Examples in context

Example 1. A dimmer switch

Turning a dimmer increases the resistance in the circuit, so by Ohm's law the current falls and the bulb glows less brightly, showing how resistance controls current.

Example 2. A car battery

A $12\ \text{V}$ battery provides the potential difference that pushes a large current through the starter motor, transferring energy quickly to turn the engine over.

Example 3. A torch bulb

The cell provides the voltage (the push), the bulb provides the resistance, and the current is the rate at which charge flows. A higher-voltage cell increases the current, so the bulb glows more brightly, a direct demonstration of Ohm's law.

Example 4. A thin versus a thick wire

A long thin wire has a higher resistance than a short thick one, so for the same voltage it carries a smaller current. This is why connecting leads are made thick (low resistance) but a heating element is made long and thin (high resistance, so it gets hot).

Try this

Q1. State the equation linking charge, current and time. [1 mark]

• Cue. $Q = I t$.

Q2. A $9.0\ \text{V}$ supply drives $0.50\ \text{A}$ through a resistor. Find the resistance. [2 marks]

• Cue. $R = V/I = 9.0/0.50 = 18\ \Omega$.

Q3. How should an ammeter be connected in a circuit? [1 mark]

• Cue. In series with the component.