Circuits. What is current?


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	Electric circuits

	1c. Basic circuits: what is current?

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	How does current behave?

		A torch is a simple series circuit with a cell, a switch and a lamp. As soon as you close the switch, the lamp comes on. There is no delay. Let's see why this happens.

Bicycle chain analogy - interactive graphic

Picture 1.6. Illustration of electric current flowing like a bicycle chain.

		An electric current is a flow of electric charge around a circuit. The charge is already in the wires (carried by billions of tiny particles called electrons). This charge is evenly spread out through the wires. As soon as you close the switch, the cell starts to push on the charge. So all the charge starts moving at once. It's a bit like a bicycle chain. The links are like the charge, the wheel is like the lamp and your feet are like the cell. As soon as you start pedalling, the back wheel starts to move. This is because turning the pedals makes all the links move at once. It's not just the links nearest your feet that move.

Ammeter photo

Picture 1.7. A typical digital ammeter.

	Measuring current

		We use an ammeter to measure current. Using two ammeters, we can show that the same current flows all the way round the circuit (you can try this in circuit challenge 3). Electric current does not get used up as it flows round the circuit. The current is measured in amps (A).

		Circuit Challenge 3 - Try it now!

Using an ammeter graphic

Picture 1.8. Always connect an ammeter in series with other parts of the circuit.

	Using an ammeter

		An ammeter is connected in series in a circuit. In this way, we can be sure that the current flows through the ammeter. Take care! Never connect an ammeter directly to a battery or power supply. A very big current will flow and this will damage the meter.

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