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Electricity and Magnetism

Electric Current

An electric current is a flow of charged particles.

A current in a metal is due to the movement of electrons. In a conducting solution, the current is due to the movement of ions.

Current is measured using an ammeter.

An ammeter measures the rate of flow of charge. For simplicity, an ammeter can be thought of as a "counter of electrons": it gives a reading which is proportional to the number of electrons which pass through it per second.

The unit of current is the Ampere, A.

An ammeter is always connected in series with other components. The resistance of an ammeter must be low compared with other components in the circuit being investigated.

Current in Series Circuits

A current of 2A corresponds to a certain (very large!) number of electrons passing per second.

So if I1= 2A, I2 and I3 must also be 2A because in a series circuit, the electrons have only one path to follow.

Conclusion

The current is the same at all points in a series circuit.

Currents in Parallel Circuits

If the three current I1, I2 and I are measured it is found that

I1 + I2 = I

This is inevitable if current readings correspond to numbers of electrons passing per second.

This result is called Kirchhoff’s current law, stated as follows.

The total current flowing towards a junction in a circuit is equal to the total current flowing away from that junction.

As an analogy, consider vehicles at a road junction.

The number of vehicles passing point 1, per minute, must be equal to the number of vehicles passing point 2 per minute plus the number of vehicles passing point 3 per minute.

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