Measuring current and potential difference

You can measure current and potential difference in circuits. They are different things and so are measured in different ways.

Current

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Current is a measure of how much electric charge flows through a circuit. The more charge that flows, the bigger the current.

Current is measured in amperes. The symbol for ampere is A. For example, 20 A is a bigger current than 5 A. The word ‘ampere’ is often abbreviated to ‘amp’, so people talk about how many amps flow.

Measuring current

A device called an ammeter is used to measure current. Some types of ammeter have a pointer on a dial, but most have a digital display. To measure the current flowing through a component in a circuit, you must connect the ammeter in series with it.

Two circuits, both with a cell, an ammeter, and a lamp. In the first, the ammeter is connected before the lamp. In the second, the ammeter is connected after the lampA circuit with an ammeter connected in two different places, both in series with the cell and lamp

When two components are connected in series, you can follow the path through both components without lifting your finger or going back over the path you have already taken.

Potential difference

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Potential difference is a measure of the difference in energy between two parts of a circuit. The bigger the difference in energy, the bigger the potential difference.

Potential difference is measured in volts. The symbol for volts is V. For example, 230 V is a bigger potential difference than 12 V. Instead of talking about potential difference, people often talk about voltage, so you may hear or see ‘voltage’ instead of ‘potential difference’.

Measuring potential difference

Potential difference is measured using a device called a voltmeter. Just like ammeters, some types have a pointer on a dial, but most have a digital display. However, unlike an ammeter, you must connect the voltmeter in parallel to measure the potential difference across a component in a circuit.

Shows a circuit with a cell, a lamp, and a voltmeter. The voltmeter is connected in parallel to the lampA circuit diagram showing a voltmeter in parallel with a lamp

When two components are connected in parallel, you cannot follow the circuit through both components from one side to the other without lifting your finger or going back over the path you have already taken.

Cells

You can measure the potential difference across a cell or battery. If the two or more cells point in the same direction, the more cells, the bigger the potential difference.

One circuit has a voltage of 1.5V. A lamp and a voltmeter are connected. The other circuit has a voltage of 4.5V. It has a lamp and a voltmeter connectedEach cell has a potential difference of 1.5 V, so three cells give 4.5 V

Checkpoint

Table showing differences between current and potential difference. Current is measured in amps (A) using ammeter in series. Potential difference is measured in volts (V) using voltmeter in parallel.