A delivery driver notices that when two identical electric vehicles (EVs) are being charged in parallel from the same charging station, the charging station's total current is greater than when only one EV is being charged. The station's output voltage remains constant. Explain why connecting the two EVs in parallel increases the total current drawn from the charging station, and describe what happens to the total resistance of the circuit as more EVs are added in parallel.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two identical electric vehicles are connected in parallel to a charging station. The potential difference supplied by the charging station remains constant at all times.
Model answer (5 marks)
In a parallel circuit each EV is connected directly across the supply, so every EV experiences the same potential difference as the charging station.
When a second EV is added in parallel an extra branch is created. Current can now flow through both branches, so the total current drawn from the station is the sum of the currents through each EV.
Because the supply voltage is constant and each EV has the same resistance, the current through each EV is the same. Adding another EV therefore doubles the total current.
Adding more EVs in parallel keeps the voltage the same but creates more parallel paths. The equivalent resistance of the circuit is given by 1/R_eq = 1/R + 1/R + …, so R_eq decreases as more EVs are added.
With V constant and R_eq decreasing, Ohm’s law (V = I R) shows that the total current I must increase.
When a second EV is added in parallel an extra branch is created. Current can now flow through both branches, so the total current drawn from the station is the sum of the currents through each EV.
Because the supply voltage is constant and each EV has the same resistance, the current through each EV is the same. Adding another EV therefore doubles the total current.
Adding more EVs in parallel keeps the voltage the same but creates more parallel paths. The equivalent resistance of the circuit is given by 1/R_eq = 1/R + 1/R + …, so R_eq decreases as more EVs are added.
With V constant and R_eq decreasing, Ohm’s law (V = I R) shows that the total current I must increase.
Examiner tips
- State that each EV sees the same V in parallel
- Explain that current adds across branches
- Show the relationship V = I R to link lower R to higher I
- Mention that adding branches reduces R_eq
Common mistakes
- Confusing series and parallel relationships
- Claiming voltage changes when it is constant
- Ignoring that each EV has the same resistance
Mark scheme (5 marks)
- In a parallel circuit, each EV (component) has the same potential difference across it as the supply
- Adding a second EV in parallel provides an additional path/branch for current to flow through
- The total current drawn from the station increases because the total current equals the sum of the currents through each branch
- The total resistance of the circuit decreases as more EVs are added in parallel
- Because voltage is constant and resistance decreases, by V = IR the total current must increase (linking resistance decrease to current increase)
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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