Ohm's law explained: V = IR, I–V graphs and components

1. A current of 0.5 A flows through a 20 Ω resistor. $V = 0.5 \times 20 = 10$ V.
2. A 12 V supply is connected across a 4 Ω resistor. $I = 12 \div 4 = 3$ A.
3. A lamp draws 250 mA from a 6 V supply. $I = 0.25$ A, so $R = 6 \div 0.25 = 24$ Ω.

A component that follows Ohm's law is called ohmic.

• A thermistor's resistance falls as its temperature rises. It's used in thermostats and temperature sensors.
• A light-dependent resistor (LDR)'s resistance falls as light intensity increases. It's used in automatic night lights.

Exam questions often put these in a circuit with a fixed resistor and ask what happens to the potential difference across each as conditions change.

A common practical investigates how the resistance of a wire depends on its length:

1. Connect a length of wire in series with an ammeter, with a voltmeter across the wire.
2. Measure the current and potential difference for several lengths.
3. Calculate $R = V \div I$ for each and plot resistance against length.

The graph is a straight line: resistance is directly proportional to length. Keep the current low so the wire doesn't heat up, which would change its resistance.

Electricity questions combine $V = IR$ with:

• Charge: $Q = I t$ (coulombs)
• Power: $P = I V$ and $P = I^2 R$ (watts)
• Energy: $E = P t$ and $E = Q V$ (joules)

For circuits with more than one component, see series and parallel circuits.

• Using milliamps without converting to amps.
• Assuming every component is ohmic.
• Drawing the voltmeter in series; it always goes in parallel across the component.

1. A current of 0.2 A flows through a 50 Ω resistor. What is the potential difference across it?
2. An appliance draws 10 A from a 230 V supply. What is its resistance?
3. A 1.5 V cell is connected across a 30 Ω resistor. What current flows, in milliamps?

Answers: 10 V; 23 Ω; 0.05 A, which is 50 mA.