Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Equation sheet: given in the exam
The equation
charge flow = current × time Q = I t
| Quantity | Symbol | Unit |
|---|---|---|
| charge flow | Q | coulombs (C) |
| current | I | amperes (A) |
| time | t | seconds (s) |
Rearranged: I = Q ÷ t | t = Q ÷ I | 1 mA = 0.001 A
Use FIFA for every answer
- F – Formula: write the equation as it appears on the sheet.
- I – Insert: put in the numbers (convert to standard units first).
- F – Fix: rearrange to make the unknown the subject.
- A – Answer: calculate and always give the unit.
Foundation: aim for Q1–8. Higher: aim for Q4–10. Most questions need rearranging.
Questions
Q1 (F) A current of 2 A flows through a torch bulb for 30 s. Calculate the charge that flows.
Show solution
F: Q = I t
I: Q = 2 × 30
F: Q is already the subject
A: 60 C
Q2 (F) 120 C of charge flows through a toaster in 60 s. Calculate the current.
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F: Q = I t
I: 120 = I × 60
F: I = 120 ÷ 60
A: 2 A
Q3 (F) A bike light draws a current of 0.5 A. How long does it take for 300 C to flow?
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F: Q = I t
I: 300 = 0.5 × t
F: t = 300 ÷ 0.5
A: 600 s
Q4 (F) A phone charges with a current of 1.5 A for 20 minutes. Calculate the charge that flows.
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Convert first: 20 minutes = 1200 s
F: Q = I t
I: Q = 1.5 × 1200
F: Q is already the subject
A: 1800 C
Q5 (F) A torch draws 250 mA. How many minutes does it take for 450 C to flow?
Show solution
Convert first: 250 mA = 0.25 A
F: Q = I t
I: 450 = 0.25 × t
F: t = 450 ÷ 0.25 = 1800 s
A: 1800 ÷ 60 = 30 minutes
Q6 (F/H) A battery can supply 3600 C. A radio draws 0.2 A from it. How many hours will the battery last?
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F: Q = I t
I: 3600 = 0.2 × t
F: t = 3600 ÷ 0.2 = 18 000 s
A: 18 000 ÷ 3600 = 5 hours
Q7 (F/H) A lightning strike transfers 60 C of charge in 2 ms. Calculate the current.
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Convert first: 2 ms = 0.002 s
F: Q = I t
I: 60 = I × 0.002
F: I = 60 ÷ 0.002
A: 30 000 A
Q8 (F/H) A calculator draws a current of 5.0 × 10−5 A. How long does it take for 1.8 C to flow? Give your answer in standard form.
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F: Q = I t
I: 1.8 = 5.0 × 10−5 × t
F: t = 1.8 ÷ 5.0 × 10−5
A: 3.6 × 104 s (10 hours)
Q9 (H) A 12 Ω heating element is connected to a 6.0 V supply for 2 minutes. Calculate the charge that flows.
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Step 1 – current
F: V = I R
I: 6.0 = I × 12
F: I = 6.0 ÷ 12 = 0.5 A
Step 2 – charge (2 minutes = 120 s)
F: Q = I t
I: Q = 0.5 × 120
A: 60 C
Q10 (H) A phone battery can supply 10 800 C. The phone draws an average current of 0.35 A. How many hours will it last? Give your answer to 2 significant figures.
Show solution
F: Q = I t
I: 10 800 = 0.35 × t
F: t = 10 800 ÷ 0.35 = 30 857 s
A: 30 857 ÷ 3600 = 8.57… = 8.6 hours (2 s.f.)
