The current through a component depends on two things: the potential difference across it and its resistance. For a given potential difference, the greater the resistance of the component, the smaller the current. Potential difference can also be called voltage, and either term gains credit in an exam.
The three quantities are linked by the equation: potential difference = current × resistance, or V = I R. Potential difference V is in volts (V), current I in amperes (A) and resistance R in ohms (Ω). Rearranged, I = V / R and R = V / I. Worked example: a current of 2 A through a 6 Ω resistor needs a potential difference of 2 × 6 = 12 V. A potential difference of 12 V that gives a current of 3 A means the resistance is 12 / 3 = 4 Ω. If the resistance is doubled and the potential difference stays the same, the current is half as large.
Required practical 3 investigates what affects resistance. To test the length of a wire at constant temperature, connect an ammeter in series with the wire and a voltmeter in parallel with it. Measure the current and potential difference for different lengths, then calculate R = V / I. A longer wire has a greater resistance. Keep the current small so the wire does not heat up, because temperature would change the result. The same circuit is used to test combinations of resistors. In series, the total resistance is the sum of the resistances, so it is greater than any one resistor. In parallel, the total resistance is smaller than the smallest resistor.