CAPF AC 2021 Q21: Science & Technology – Physics
Let an electric current of 1.5 A flow through an incandescent lamp in a circuit. What is the amount of charge that flows through it in 10 ms?
- (a) 0.015 C
- (b) 0.15 C
- (c) 1.5 C
- (d) 15 C
Answer: (a) 0.015 C
Explanation
Charge flowing through a lamp
Given: electric current of 1.5 A flows through an incandescent lamp for 10 ms.
How to think: current is charge per second.
So charge = current × time.
The only trap is the unit: ms means millisecond.
| Step | Working |
|---|---|
| Formula | Q = I × t |
| Convert time | 10 ms = 10 × 0.001 s = 0.01 s |
| Substitute | Q = 1.5 A × 0.01 s |
| Result | Q = 0.015 C |
Unit check: 1 ampere = 1 coulomb per second.
So ampere × second gives coulomb.
Answer: (a) 0.015 C.
Why each option is right or wrong
- (a) CORRECT. Q = 1.5 A × 0.01 s = 0.015 C.
- (b) Incorrect. 0.15 C would need 0.1 s, which is 100 ms, not 10 ms.
- (c) Incorrect. 1.5 C is the charge that flows in one full second.
- (d) Incorrect. 15 C comes from using 10 as seconds and ignoring the milli.
Key point
Q = I × t. Always convert time to seconds: 1 ms = 0.001 s. So 1.5 A for 10 ms carries 0.015 C.
Remember
Charge = current × time (in seconds).
Common mistakes
- Reading 10 ms as 10 seconds and getting 15 C.
- Converting milliseconds wrongly: 10 ms is 0.01 s, not 0.1 s.
- Dividing current by time instead of multiplying them.
Exam tip
Educated guess: in 1 second only 1.5 C flows. The time here is a tiny fraction of a second, so the answer must be far smaller than 1.5 C. Only (a) and (b) qualify; one hundredth of a second gives (a).
Sources
- NCERT Class X Science — Electricity (electric current and circuit)
- NCERT Class XII Physics — Current Electricity