Define current, voltage and resistance, apply Ohm's law, and describe how current and voltage behave in series and parallel circuits

What this dot point is asking

SEAB wants you to define current, voltage and resistance, to use Ohm's law in calculations, and to describe how current and voltage behave differently in series and parallel circuits. The central idea is that current is the flow of charge around a circuit, pushed by the voltage and held back by the resistance.

The answer

Current, voltage and resistance

• Current is the rate of flow of electric charge around a circuit. It is measured in amperes ($\text{A}$) with an ammeter, connected in series.
• Voltage (potential difference) is the energy given to the charge by the battery, or the energy transferred by a component. It is measured in volts ($\text{V}$) with a voltmeter, connected in parallel across the component.
• Resistance is how much a component opposes the flow of current. It is measured in ohms ($\Omega$). A higher resistance means a smaller current for the same voltage.

Ohm's law

For many components at constant temperature, the current is proportional to the voltage:

where $V$ is the voltage in volts, $I$ is the current in amperes, and $R$ is the resistance in ohms. This can be rearranged to find any one quantity from the other two.

Series circuits

In a series circuit the components are connected one after another in a single loop:

• the current is the same at every point,
• the voltage of the battery is shared between the components,
• if one component breaks, the whole circuit stops.

Parallel circuits

In a parallel circuit the components are connected on separate branches:

• the voltage across each branch is the same (equal to the battery voltage),
• the current splits between the branches, then recombines,
• if one branch breaks, the others keep working.

This is why the lights and sockets in a house are wired in parallel.

Examples in context

Example 1. Christmas lights wired in series. Old fairy lights wired in series all go out when a single bulb fails, because the broken bulb breaks the only loop. Modern sets place bulbs in parallel branches so the rest stay lit if one fails.

Example 2. A dimmer switch. A dimmer increases the resistance in the lighting circuit. By Ohm's law, a higher resistance at the same voltage means a smaller current, so the bulb glows less brightly. Reducing the resistance again brightens it.

Try this

• Cue. A $9\ \text{V}$ battery drives $3\ \text{A}$ through a resistor. Find its resistance: $R = \dfrac{9}{3} = 3\ \Omega$.
• Cue. State where an ammeter and a voltmeter are connected. The ammeter in series; the voltmeter in parallel across the component.
• Cue. Explain why home appliances are wired in parallel. Each gets the full mains voltage and can be switched on or off without affecting the others.