Describe current, voltage and resistance, state their units, and use the relationship voltage equals current times resistance

What this dot point is asking

This dot point wants you to describe three key electrical quantities: current, voltage and resistance. You should know what each one means, its unit, and the instrument used to measure it, and you should be able to use the formula that links them. The big idea is that voltage is the push that drives a current around a circuit, resistance is anything that slows the current down, and the three are linked by one simple formula.

The answer

Current: the flow of electricity

Current is the flow of electricity around a circuit. The bigger the current, the more electricity is flowing each second.

Current is measured in amperes (A), often just called amps. It is measured with an ammeter, which is connected in series in the circuit (in the main loop, so all the current flows through it).

Voltage: the push

Voltage is the push that drives the current around the circuit. It is supplied by the cell or battery (or the mains). A bigger voltage gives a bigger push, which drives a bigger current.

Voltage is measured in volts (V). It is measured with a voltmeter, which is connected in parallel across the component you are measuring (alongside it).

Resistance: the slowing-down

Resistance is anything that slows down or opposes the current. Every component has some resistance. A component with a high resistance is hard for the current to flow through, so it lets less current through.

Resistance is measured in ohms (the symbol is the Greek letter omega). Long, thin wires have more resistance than short, thick ones, and a resistor is a component made to have a set resistance.

How they are linked

Current, voltage and resistance are linked by a single formula:

Here $V$ is the voltage in volts, $I$ is the current in amperes, and $R$ is the resistance in ohms. The formula can be rearranged:

• To find current: $I = \dfrac{V}{R}$ (current = voltage $\div$ resistance).
• To find resistance: $R = \dfrac{V}{I}$ (resistance = voltage $\div$ current).

What the formula tells you

The formula shows the pattern. For a fixed voltage, a higher resistance gives a smaller current, because it is harder for the current to flow. For a fixed resistance, a higher voltage gives a bigger current, because the push is stronger. This matches everyday experience: a stronger battery makes a bulb brighter (more current), and adding resistance dims it (less current).

Examples in context

Example 1. A dimmer switch on a lamp. A dimmer switch works by changing the resistance in the lamp's circuit. Turn the dimmer to add more resistance and the current falls, so the bulb glows less brightly. Turn it the other way to lower the resistance and the current rises, so the bulb gets brighter. The link between resistance and current is right there on the wall.

Example 2. Why thick cables carry big currents. Heavy appliances like an oven need a large current, so they are wired with thick cables. A thick wire has a low resistance, so it lets a large current flow without getting too hot. A thin wire has more resistance and would heat up dangerously if a big current were forced through it.

Try this

• Cue. A circuit has a current of $3\ \text{A}$ through a resistance of $4\ \text{ohms}$. Work out the voltage. Voltage = current $\times$ resistance = $3 \times 4 = 12\ \text{V}$.

• Cue. State the unit and the measuring instrument for current. Current is measured in amperes (A), using an ammeter connected in series.

• Cue. Explain what happens to the current in a circuit if the resistance is increased while the voltage stays the same. The current decreases, because a higher resistance makes it harder for current to flow for the same voltage push.