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.

There are no side-paths down which the charges can pass and the charges themselves cannot just disappear.

Please 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 is fixed by what's in the circuit: the battery and the other elements, such as bulbs. Things other than batteries reduce the flow: the flow of charge everywhere in the whole circuit decreases: the current in each element of the circuit also decreases.

Later, we'll 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 charges drift round at a speed of about 1 centimetre per minute. This is important, and very slow (try moving at 1 centimetre per minute). Think about a very big circuit – so the connections between the battery and bulb are 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 bulb is immediate, yet the charges that started in the battery have scarcely left the terminals. So any model in which charges carry energy or take energy from battery to the bulb in order to light the bulb will clash badly with this experience. If the big circuit runs from the front of the classroom 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 charges in the circuit (including those in the filament of the bulb) start moving together and the filament warms up instantly. That's why it's not a good idea to say energy is carried from battery to bulb.

The rope loop teaching model offers a convincing account of this experience, that does help you picture what's really happening.

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