The potential difference (p.d.) across a component is the push that drives charge through it. It tells you how much energy each coulomb of charge transfers: 1 volt is 1 joule per coulomb.

You measure it with a voltmeter connected across the component, in parallel:

Here the ammeter is in series with the lamp and the voltmeter is across the lamp. The bigger the p.d. across a component, the bigger the current through it.

Resistance is how much a component opposes the current. For the same p.d., a bigger resistance means a smaller current.

potential difference = current × resistance
V = I R

p.d. in volts (V), current in amperes (A), resistance in ohms (Ω). Recall it for AQA, Edexcel and OCR: it is not on the sheet.

1. A heater element has 6.0 V across it and a current of 0.25 A. Resistance?
2. V = I R, so R = V ÷ I
3. R = 6.0 ÷ 0.25 = 24 Ω

For a fixed resistor at constant temperature the current is directly proportional to the p.d.: double the p.d., double the current. The resistance stays the same. That is an ohmic conductor.

Real resistors are often a few thousand ohms, so currents come out in milliamps.

1. A 4.7 kΩ resistor is on a 9.4 V supply. Current in mA?
2. 4.7 kΩ = 4.7 × 1000 = 4700 Ω
3. I = V ÷ R = 9.4 ÷ 4700 = 0.002 A
4. 0.002 A × 1000 = 2 mA

k means × 1000 and m means ÷ 1000. Convert to Ω and A, calculate, then convert the answer to the unit the question asks for.