These twelve questions cover charging, I = Q ÷ t, the direction of current, circuit symbols and stored charge from charge and current. All data is invented for practice and is not taken from any exam paper. They go from easier to harder.
Write your answer with its unit on paper first, then open the answer. Whether every idea here is required depends on your syllabus year, so check the Cambridge page for your exam year.
Questions
1. A plastic rod is rubbed with a cloth and becomes negatively charged. State which particles moved, in which direction, and the charge on the cloth.
Show answer
Electrons moved from the cloth to the rod. The rod gained electrons, so it is negative. The cloth lost electrons, so it is positive, with a charge equal in size to that on the rod.
2. A charge of 30 C passes a point in 10 s. Find the current.
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I = Q ÷ t = 30 ÷ 10 = 3.0 A.
3. A current of 0.40 A flows for 2.5 minutes. How much charge passes?
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t = 2.5 × 60 = 150 s. Q = I × t = 0.40 × 150 = 60 C.
4. A charge of 12 C passes through a lamp carrying a steady 0.20 A. For how long does the lamp carry it?
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t = Q ÷ I = 12 ÷ 0.20 = 60 s, which is 1.0 minute. Check: 0.20 × 60 = 12 C.
5. A current of 250 mA flows for 8.0 s. Find the charge.
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250 mA = 0.25 A. Q = 0.25 × 8.0 = 2.0 C.
6. In a wire, conventional current flows from point A to point B. In which direction do the electrons move, and which end of the wire is connected to the positive terminal of the cell?
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Electrons move from B to A. Conventional current leaves the positive terminal, so end A is nearer the positive terminal.
7. A student draws a circuit to measure the current through a resistor and the potential difference across it. Describe where the ammeter and the voltmeter must be placed.
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The ammeter goes in series, in the loop with the resistor, so the same current passes through it. The voltmeter goes in parallel, connected across both ends of the resistor.
8. Two identical metal spheres are on insulating stands. Sphere A has a charge of +6.0 µC and sphere B has −2.0 µC. They are touched together and separated. What is the charge on each sphere afterwards? (Extension: only if your teacher has covered sharing of charge.)
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Total charge is conserved: +6.0 + (−2.0) = +4.0 µC. Identical spheres share it equally, so each has +2.0 µC. Electrons moved from B to A to cancel part of A’s positive charge.
9. A charged dome holding 4.0 µC discharges through a wire in 0.050 s. Find the average current in mA.
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4.0 µC = 4.0 × 10⁻⁶ C. I = 4.0 × 10⁻⁶ ÷ 0.050 = 8.0 × 10⁻⁵ A = 0.080 mA. Check: 8.0 × 10⁻⁵ × 0.050 = 4.0 × 10⁻⁶ C.
10. A student says, “The lamp uses up current, so the ammeter after the lamp reads less than the one before it.” Is that correct? Explain.
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No. Current is the rate of flow of charge, and charge is not used up. In a series circuit the same current passes every point, so both ammeters read the same. The lamp transfers energy, not current.
11. A lamp carries 0.60 A for 3.0 minutes. (a) How much charge passes through it? (b) A second lamp is in series with it. How much charge passes through the second lamp in that time?
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(a) t = 3.0 × 60 = 180 s. Q = 0.60 × 180 = 108 C.
(b) The current is the same in series, so 108 C also passes through the second lamp.
12. A phone battery is labelled 2400 mAh (invented and treated as an ideal steady supply). (a) How much charge is this in coulombs? (b) For how many hours could it supply a steady 0.50 A?
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(a) 2400 mAh = 2.4 Ah = 2.4 A for 1 h. Q = 2.4 × 3600 = 8640 C.
(b) t = 2.4 Ah ÷ 0.50 A = 4.8 hours. Check: 0.50 × 4.8 × 3600 = 8640 C.
If you got these wrong
Match the kind of error to the lesson that fixes it.
| What went wrong | Questions | Go to |
|---|---|---|
| Named protons, or forgot the other object is charged | 1, 8 | Explain charging through electron transfer |
| Wrong answer from minutes, mA or rearranging | 2 to 5, 9, 11 | Calculate current from charge and time |
| Direction of electrons or “current is used up” | 6, 10 | Compare conventional current and electron motion |
| Meter or symbol placed wrongly | 7 | Use a circuit symbol consistently |
| Mixed up rate and amount | 9, 12 | Distinguish current from charge stored |
Add any repeated slip to the mistake log and retest queue and try a fresh question on it a few days later.
What next?
Return to the charge and current module for the study order, then move on to potential difference, resistance and circuits when these all feel secure.
If a question type keeps going wrong even after reading the worked answers, a teacher in our online one-to-one Physics tuition can ask you to explain your reasoning aloud and find the step that slips.