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Current in a Series Circuit

A battery is connected by copper wires to a light bulb. Current flows through wires and bulb. The bulb has high resistance, heats up and gives out light. The wires have low resistance and remain cool.

It is easy to think that because the light bulb has high resistance, current flows more slowly in the bulb than in the copper wires which have low resistance. In fact, current in bulb and wires are the same.

If current in wire is faster, then charges will accumulate at the edge of the light bulb where current slows down. This does not happen. Current flow in the circuit is like water flow round a pipe. If it slows down at one point, it slows down everywhere.

This means that if there are more than one light bulb connected in series, then currents in all light bulbs are the same. This is true even if resistance in some bulbs are larger than in others.

Exercise. A battery is connected to two resistors 2 Ω and 4 Ω in series. The current in the 2 Ω resistor is 0.1 A. What is the current is the 4 Ω resistor?

Answer. 0.1 A, because currents in resistors that are connected in series are the same.

Exercise. A battery is connected to two resistors 2 Ω and 4 Ω in series. Potential difference across the 2 Ω resistor is 0.4 V. What is the current in the 4 Ω resistor?

Answer. Use Ohm's law V = IR to find current in the 2 Ω resistor first. Divide both sides of V = IR by R to make I the subject: I = V/R. So current in the 2 Ω resistor is I = 0.4 / 2 = 0.2 A. Since the 4 Ω resistor is connected in series, current there is also 0.2 A.