A current of 0·6 A is drawn by an electric bulb for 10 minutes. Which one of the following is the amount of electric charge that flows through the circuit?
- (a)6 C
- (b)0·6 C
- (c)360 C
- (d)36 C
Correct — C, 360 C. Charge is current multiplied by the time for which that current flows, Q = I × t, and the two must be in matching units. Ten minutes is 600 seconds, so Q = 0.6 A × 600 s = 360 coulomb. The whole item turns on that one conversion — an ampere is one coulomb per second, so the time has to go into the formula in seconds.
- (a)6 C — This is 0.6 × 10, the time left in minutes instead of converted to seconds. It is the intended trap, and it is out by a factor of 60.
- (b)0·6 C — This is the current value copied across as though it were a charge. A current of 0.6 A means 0.6 coulomb passes every second, not 0.6 coulomb in total over ten minutes.
- (d)36 C — This is 0.6 × 60, the charge that flows in one minute. The bulb draws current for ten minutes, so it must be multiplied by ten more to give 360 C.
Electric current is defined as the rate at which charge flows past a point in a circuit, I = Q/t. Rearranged, the charge that has passed in a time t is Q = I × t. The ampere is defined so that one ampere is a flow of one coulomb per second, which is exactly why the time in this formula has to be expressed in seconds and not in minutes or hours.
The arithmetic here is trivial and the marks are lost entirely at unit conversion, which is why every wrong option is a half-finished conversion — stopping at minutes gives 6 C, converting only one minute gives 36 C, and not multiplying at all gives 0.6 C. Convert the time first, then multiply. A quick sanity check also settles it without full working: 0.6 coulomb every second, sustained for ten whole minutes, has to come to a few hundred coulomb, and only one option is in that range.
- Electric current is the rate of flow of charge, I = Q/t, so the charge that flows is Q = I × t.
- One ampere is one coulomb per second, so time must be in seconds when this formula is used.
- Ten minutes is 600 seconds, and 0.6 A × 600 s = 360 C.
- The coulomb is the SI unit of electric charge; the magnitude of the charge on a single electron is about 1.6 × 10⁻¹⁹ C.
The conversion of minutes to seconds is the whole question — skip it and you land on the trap answer of 6 C.
- Leaving the time in minutes and answering 6 C.
- Confusing the coulomb, which measures charge, with the ampere, which measures the rate of flow of charge.
- Misreading the middle-dot decimal used in the printed paper, so that 0·6 A is taken as 6 A.
As a one-step numerical on Q = I × t, or inverted, with the charge and the time given and the current asked for.
A current of 1·0 A is drawn by a filament of an electric bulb for 10 minutes. The amount of electric charge that flows through the circuit is
- (a) 0·1 C
- (b) 10 C
- (c) 600 C
- (d) 800 C
Answer(c) 600 C
The same question with a different current — NDA-I 2021 asked for the charge drawn by a bulb at 1·0 A for 10 minutes, and the answer again came from converting the ten minutes to 600 seconds before multiplying.
- practice — not a real PYQ
A current of 2 A flows through a conductor for 5 minutes. The amount of charge that passes through it is
- (a)10 C
- (b)120 C
- (c)600 C
- (d)0.4 C
Answer(c) 600 C — Q = I × t = 2 A × 300 s = 600 C.
- practice — not a real PYQ
If 180 C of charge flows through a circuit in 3 minutes, the current in the circuit is
- (a)60 A
- (b)1 A
- (c)0.6 A
- (d)540 A
Answer(b) 1 A — I = Q/t = 180 C ÷ 180 s = 1 A.