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AS · Practice questions · Electric current

Count the charge.

Six original Cambridge-style questions on Q = It, I = Anvq and drift speed.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
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Charge per second.

01
Analysis
[2 marks]

Define electric current and state its SI unit.

  • Current is the rate of flow of charge, I = Q/t ✓
  • The SI unit is the ampere (A) ✓
02
Analysis
[2 marks]

A steady current of 0.50 A flows for 2.0 minutes. Find the charge transferred.

  • Q = It = 0.50 × (2.0 × 60) ✓
  • Q = 0.50 × 120 = 60 C ✓
03
Analysis
[3 marks]

A wire of cross-sectional area 1.0 × 10⁻⁶ m² carries charge carriers of density 1.0 × 10²⁹ m⁻³ drifting at 1.0 × 10⁻⁴ m s⁻¹ (q = 1.6 × 10⁻¹⁹ C). Find the current.

  • I = Anvq = (1.0 × 10⁻⁶)(1.0 × 10²⁹)(1.0 × 10⁻⁴)(1.6 × 10⁻¹⁹) ✓
  • I = 1.6 A ✓
04
Analysis
[2 marks]

How many electrons make up a charge of 4.0 C? (e = 1.6 × 10⁻¹⁹ C)

  • N = Q/e = 4.0 / (1.6 × 10⁻¹⁹) ✓
  • N = 2.5 × 10¹⁹ electrons ✓
05
Analysis
[3 marks]

A current of 3.2 A flows in a wire of area 2.0 × 10⁻⁶ m² with carrier density 1.0 × 10²⁹ m⁻³ (q = 1.6 × 10⁻¹⁹ C). Find the drift speed.

  • v = I/(Anq) = 3.2 / ((2.0 × 10⁻⁶)(1.0 × 10²⁹)(1.6 × 10⁻¹⁹)) ✓
  • v = 3.2 / (3.2 × 10⁴) = 1.0 × 10⁻⁴ m s⁻¹ ✓
06
Analysis
[2 marks]

The drift speed of electrons in a wire is less than a millimetre per second. Explain why a lamp lights almost instantly when the switch is closed.

  • The electric field is set up along the wire at nearly the speed of light ✓
  • so charge carriers everywhere begin to drift almost at once, even though each moves slowly ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.