A student sets up a series circuit containing a battery, a switch, and two resistors. She notices that when she adds a third resistor in series, all the components become dimmer. Explain why adding a third resistor in series causes the current in the circuit to decrease, and describe what happens to the voltage across the battery as the third resistor is added.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Adding a third resistor increases the total resistance of the circuit.
Because the battery voltage is fixed, Ohm’s law (V = IR) shows that a higher resistance gives a smaller current.
In a series circuit the same current flows through every component, so all components receive less current and become dimmer.
The voltage of the battery itself does not change when the third resistor is added.
Because the battery voltage is fixed, Ohm’s law (V = IR) shows that a higher resistance gives a smaller current.
In a series circuit the same current flows through every component, so all components receive less current and become dimmer.
The voltage of the battery itself does not change when the third resistor is added.
Examiner tips
- State that total resistance rises when a resistor is added. Use V = IR to link fixed voltage to reduced current. Explain that the same current flows through all series elements. Note that the battery voltage remains unchanged.
Common mistakes
- Saying the battery voltage increases or decreases. Forgetting that the current is the same through all series components. Using the wrong equation (e.g. I = V/R without explaining the effect of R).
Mark scheme (4 marks)
- Adding a third resistor increases the total resistance of the circuit
- Because the voltage of the battery stays the same / is fixed, a higher resistance means a smaller current (reference to V = IR or equivalent reasoning)
- In a series circuit the same current flows through every component, so all components receive less current / become dimmer
- The voltage across the battery remains the same / does not change when the third resistor is added
Key terms in this question
series circuit · current · voltage
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Series and parallel circuits questions
- A student connects two identical resistors in a parallel circuit with a battery.…
- A technician is wiring the lights in a new house. She must decide whether to con…
- A string of Christmas tree lights is wired in series. When one bulb breaks, all …
- A student connects three lamps in a parallel circuit. Describe two differences b…