A student investigates a series circuit containing a battery, two resistors, and an ammeter. The student notices that the ammeter reads the same value wherever it is placed in the series circuit. Explain why the current is the same at every point in a series circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a series circuit, components are connected one after another in a single loop. A student measures the current at several different points around the loop and finds the reading is identical each time.
Model answer (4 marks)
In a series circuit there is only one path for the electrons to follow.
Because the electrons are not consumed or created by the resistors, the same electrons that enter the first resistor must leave it, then enter the second resistor, and finally return to the battery.
Thus the same charge carriers pass through every component in sequence.
The rate at which charge flows – the current – is therefore identical at all points in the circuit.
Because the electrons are not consumed or created by the resistors, the same electrons that enter the first resistor must leave it, then enter the second resistor, and finally return to the battery.
Thus the same charge carriers pass through every component in sequence.
The rate at which charge flows – the current – is therefore identical at all points in the circuit.
Examiner tips
- Use the phrase "only one path" to show understanding of series circuits. Mention that electrons are not used up, linking to the same charge carriers passing through each component. Show the logical chain: one path → same electrons → same current.
Common mistakes
- Confusing current with voltage or resistance. Failing to mention that electrons are not consumed. Using vague wording like "the same current" without explaining why.
Mark scheme (4 marks)
- There is only one path for current to flow in a series circuit
- Current is not used up as it passes through components
- The same charge carriers (electrons) pass through every component in sequence
- The rate of flow of charge (current) must therefore be the same at all points
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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