Explain what happens to the current and voltage in a series circuit when an extra resistor is added to the circuit.

WJEC GCSE Physics (Wales) — 1.1 Electric circuits · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student has a series circuit containing a battery, a bulb, and a switch. The student adds an extra resistor in series with the bulb.

Model answer (4 marks)

Adding a resistor in series increases the total resistance of the circuit. According to Ohm’s law, the current supplied by the battery therefore decreases.

The battery voltage is now divided over more components. The extra resistor takes a proportion of the supply voltage, so the voltage drop across the bulb is reduced.

Thus the current falls and the bulb receives a lower voltage than before.

Examiner tips

Use the phrase ‘total resistance increases’ to show understanding of series circuits.
State that the current decreases by citing Ohm’s law (I=V/R).
Explain that the battery voltage is shared, leading to a lower voltage across the bulb.
Mention that the voltage drop across the added resistor is equal to I×R added to the bulb’s drop.

Common mistakes

Saying the current increases or stays the same.
Claiming the bulb’s voltage stays unchanged.
Using the wrong sign for voltage drop (e.g., ‘voltage increases’).

Mark scheme (4 marks)

The total resistance in the circuit increases
The current in the circuit decreases
The voltage of the battery/supply is shared between more components
The voltage across the bulb decreases (because it shares the supply voltage with the added resistor)

Key terms in this question

series circuit · current · voltage