Electric current, potential difference and resistance, the equation V = IR, and the current-voltage graphs for a resistor, a filament lamp and a diode.

What this dot point is asking

WJEC Double Award Unit 3 wants you to define current, potential difference and resistance, use the equation $V = IR$, and describe the current-voltage graphs for a resistor, a filament lamp and a diode.

Current, potential difference and resistance

A bigger potential difference pushes a bigger current through a component; a bigger resistance reduces the current for a given potential difference.

The equation V = IR

To use it, pick the form you need, substitute the values in the right units, and work out the answer.

Current-voltage graphs

The way the current changes with voltage shows how a component behaves:

• Fixed resistor (at constant temperature): a straight line through the origin. The current is proportional to the voltage, so the resistance is constant.
• Filament lamp: a curve that becomes less steep at higher voltages. As more current flows, the filament gets hotter, which increases its resistance, so the current rises more slowly.
• Diode: current flows in one direction only. In the forward direction it conducts (above a small voltage); in the reverse direction it has a very high resistance and blocks the current.

Investigating resistance

A standard practical measures the current through a component for different voltages, using a variable resistor to change the voltage. Plotting current against voltage gives the characteristic graph, and the resistance at any point is found from $R = \dfrac{V}{I}$. For a wire, the resistance increases if the wire is longer and decreases if it is thicker, which can also be investigated.

What charge and current mean

Electric current is a flow of charge, carried by electrons moving through the wires. The more charge that flows each second, the bigger the current. Charge is measured in coulombs (C) and is linked to current and time by $Q = It$, where $Q$ is the charge in coulombs, $I$ the current in amperes and $t$ the time in seconds. So a current of 2 amperes flowing for 10 seconds moves $2 \times 10 = 20$ coulombs of charge. This explains why current is described as the "rate of flow of charge": one ampere is one coulomb passing a point each second.

Resistance in everyday components

Different components are chosen for their resistance. A variable resistor (rheostat) is used to change the current in a circuit, for example to dim a light or change the volume. A component with a high resistance allows only a small current, while one with a low resistance allows a large current for the same voltage. Heating elements (in kettles and heaters) use a resistance wire so that the current does work against the resistance and transfers energy as heat. Knowing that resistance controls the size of the current helps you predict how a circuit behaves when a component is changed.

Try this

Q1. State the unit of resistance. [1 mark]

• Cue. The ohm ($\Omega$).

Q2. A current of $2\,\text{A}$ flows through a $5\,\Omega$ resistor. Calculate the potential difference. [2 marks]

• Cue. $V = IR = 2 \times 5 = 10\,\text{V}$.