Explain why adding a second resistor in series to a circuit makes it harder for current to flow, and describe what happens to the potential difference across each resistor compared to the potential difference of the power supply.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student builds a circuit with a battery and a single resistor. She then adds a second resistor connected in series with the first.
Model answer (4 marks)
Adding a second resistor in series increases the total resistance of the circuit, which, according to Ohm’s law (I = V/R), reduces the current that can flow for a given supply voltage.
Because the resistors are in series, the same current flows through each one. The potential difference is therefore divided between the two resistors. The voltage drop across each resistor is proportional to its resistance, and the sum of the two voltage drops equals the total potential difference of the power supply.
Because the resistors are in series, the same current flows through each one. The potential difference is therefore divided between the two resistors. The voltage drop across each resistor is proportional to its resistance, and the sum of the two voltage drops equals the total potential difference of the power supply.
Examiner tips
- Show the relationship I = V/R and explain that R_total increases when a resistor is added in series.
- Explain that the same current flows through each resistor and that the voltage drops add up to the supply voltage.
Common mistakes
- Failing to mention that the same current flows through both resistors.
- Confusing series with parallel and giving the wrong voltage relationship.
- Using the wrong equation (e.g., V = IR for one resistor only without noting the total R).
Mark scheme (4 marks)
- Adding a second resistor increases the total resistance of the circuit
- Greater total resistance means the current in the circuit decreases
- The potential difference is shared between the two resistors (in series)
- The sum of the potential differences across each resistor equals the potential difference of the power supply
Key terms in this question
series · potential difference · current
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Electricity and circuits questions
- A thermistor is used in a temperature-sensing circuit. Explain how and why the c…
- A student connects two identical lamps in parallel with a battery. Explain why t…
- A student sets up a series circuit containing a battery, a fixed resistor, and a…
- A filament lamp is connected to a battery in a simple series circuit. As the lam…