Electrical Circuit
Electricity and Magnetism

A simple loop: current the same everywhere

Physics Narrative for 5-11 11-14 Supporting Physics Teaching

Electric currents do not get used up

Electric currents do not get used up. In a simple circuit with one battery and one bulb, the size of the electric current is the same wherever you measure it.

If it is 0.75 ampere in the wire before the bulb, it is 0.75 ampere in the wire after the bulb, and 0.75 ampere in the battery and bulb.

In other words, 0.75 coulomb of charge pass each point in the circuit every second. There are no side-paths down which the charged particles can pass and the charged particles themselves cannot just disappear.

You should note that we're showing conventional charge flow in the diagrams.

Current as flow of charge

You can picture a steady and continuous flow of charge around the whole circuit. The rate of flow of charge (the current) for the whole of the circuit with a given battery is fixed by the size of the bulb's resistance. If that resistance is reduced somehow, then the flow of charge everywhere in the whole circuit increases and the current in each element of the circuit also increases.

We can build on these ideas by considering what happens when changes are made to our simple circuit, and how we can use the electric circuit model to both predict and explain what happens.

Consequences of a slow drift

In a complete circuit charged particles drift round at a speed of about 1 centimetre per minute. This has implications if the circuit is very big – so the connections between the elements made with very long wires. When the big circuit is completed, the bulb appears to light immediately. Think about this for a while.

The effect of the current in the lamp is immediate, yet the charged particles that started in the battery have scarcely left the terminals. So any model in which charged particles carry or take energy from battery to the lamp in order to light the lamp will clash badly with this observation. If the big circuit runs from the front of the lab to the back, such a model predicts that the bulb might then take 10 hours to light up! What happens, of course, is that as the circuit is completed, all of the charged particles in the circuit (including those in the filament of the bulb) start moving together and the filament warms up instantly. Energy is not carried from battery to bulb.

The rope loop teaching model offers a convincing view of this effect.

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