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?

Correct — A, 0.015 C. Current is charge per unit time, so the charge that passes in a given interval is Q = I × t. The only care needed is with the unit of time: 10 milliseconds is 10 × 10⁻³ s, that is 0.010 s. Then Q = 1.5 × 0.010 = 0.015 C. Reading the four options shows they are the same digits shifted by powers of ten, which is the examiner announcing that the whole item is about the millisecond.

• (b)0.15 C — 0.15 C is what 100 ms would give. A single place has slipped in the conversion, turning 10 milliseconds into a tenth of a second.
• (c)1.5 C — 1.5 C is the charge in one full second — the current value copied straight down without multiplying by the time at all.
• (d)15 C — 15 C corresponds to 10 seconds. Here the milli has been dropped completely and the 10 read as whole seconds.

One ampere is one coulomb per second, so Q = It links the three quantities. Charge is also quantised, in units of the elementary charge 1.6 × 10⁻¹⁹ C, so a coulomb represents about 6.24 × 10¹⁸ electrons. Both relations turn up constantly in electricity questions, one for sums like this and the other when a question asks how many electrons passed.

The physics here is one line; the marks are in the prefix. Milli means 10⁻³, micro 10⁻⁶ and nano 10⁻⁹, and a candidate who converts before substituting cannot go wrong. It also pays to notice that all four options are the same significant figures at different magnitudes — whenever an option list looks like that, the question is testing units rather than method, and the safest move is to write the time in seconds explicitly before touching the multiplication.

• Q = I × t, with current in amperes, time in seconds and charge in coulombs.
• One ampere is one coulomb per second.
• 10 ms = 10 × 10⁻³ s = 0.010 s.
• The elementary charge is about 1.6 × 10⁻¹⁹ C, so one coulomb is roughly 6.24 × 10¹⁸ electrons.
• The prefixes to keep straight are milli 10⁻³, micro 10⁻⁶ and nano 10⁻⁹.

• Substituting the time in milliseconds directly into Q = It.
• Reading 10 ms as 10 s and answering 15 C.
• Confusing the coulomb with the ampere, and quoting the current as the answer.

A one-line formula item whose real content is a unit conversion; the NDA versions use minutes instead of milliseconds so that the conversion runs the other way.

• practice — not a real PYQ

  A current of 2 A flows for 5 ms. The charge that passes is

  • (a)0.01 C
  • (b)0.1 C
  • (c)1 C
  • (d)10 C

  Answer(a) 0.01 C — 2 × 5 × 10⁻³ = 0.01 C.

• practice — not a real PYQ

  A charge of 60 C passes through a bulb in 2 minutes. The current is

  • (a)0.5 A
  • (b)2 A
  • (c)30 A
  • (d)120 A

  Answer(a) 0.5 A — 2 minutes is 120 s, so I = 60 ÷ 120 = 0.5 A.