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Step-by-step worked examples — Electric current
Step-by-step solutions to past-paper-style questions on electric current, written exactly the way a tutor would explain them at the board.
1Charge from current (4 marks)
Extended• Adapted from 9702/22 May/Jun 2024• charge
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Question
A current of 2.5 A flows for 30 s. Find the charge transferred. (4 marks)
Step-by-step solution
Step 1
Q=It.
Q=2.5×30=75 C
Answer
Q=75 C.
2Number of electrons (5 marks)
Extended• electrons
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Question
How many electrons pass a point per second when current is 1.6 A? e=1.6×10−19 C. (5 marks)
Step-by-step solution
Step 1
Charge per second = 1.6 C.
Step 2
Number = total charge / e.
n=1.6×10−191.6=1.0×1019
Answer
1.0×1019 electrons per second.
3Drift velocity (6 marks)
Extended• drift velocity
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Question
A copper wire (n=8.5×1028 electrons/m³) carries 3.0 A with cross-section 1.0×10−6 m². Find drift velocity. (6 marks)
Step-by-step solution
Step 1
I=nAve. Rearrange.
v=nAeI=(8.5×1028)(1.0×10−6)(1.6×10−19)3.0
Step 2
Evaluate.
v≈2.2×10−4 m/s
Answer
v≈2.2×10−4 m/s (very slow!).
Key Formulae — Electric current
The formulae you need to memorise for electric current on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.
Current
I=ΔtΔQ
I
current (A)
Q
charge (C)
When to use
Rate of flow of charge. Units: ampere (A) = C/s.
Drift velocity
I=nAve
n
number density of charge carriers
A
cross-sectional area
v
drift velocity
e
charge per carrier
When to use
Current in terms of charge-carrier flow.
Key Definitions and Keywords — Electric current
Definitions to memorise and the exact keywords mark schemes credit for electric current answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.
Electric current
Examiner keyword
Rate of flow of electric charge. I=Q/t. Units: A.
Electric charge
Examiner keyword
Property of matter that causes electric force. Quantised in units of e. Units: C.
Conventional current
Examiner keyword
Direction in which POSITIVE charge would flow. Opposite to electron flow in a wire.
Drift velocity
Examiner keyword
Average velocity of charge carriers due to applied field. Slow (mm/s) despite signal propagation at near-light speed.
Common Mistakes and Misconceptions — Electric current
The traps other students keep falling into on electric current questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.
✕Saying conventional current is direction of electron flow
9702 Examiner Reports 2022-2024
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Why it happens
Historical convention.
How to avoid it
Conventional current = direction POSITIVE charge moves = OPPOSITE to electron motion.
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Electric current — frequently asked questions
The things students keep getting wrong in this sub-topic, answered.
Electric current – Study Notes & Past Paper Style Questions | Cambridge International A Level Physics 9702 | Tutopiya