Extended · Practice questions · Charge flow

Current, charge, time.

Six original Cambridge-style questions on I = Q ÷ t: defining the coulomb, finding charge from current and time, rearranging for time, and converting minutes to seconds.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

How to handle charge-flow questions

Q = It, with t in seconds.

Three forms: I = Q ÷ t, Q = It, t = Q ÷ I.
Units: amperes, coulombs, seconds. One ampere is one coulomb per second.
Convert first: change any minutes or hours into seconds before using the equation.

Recall

[2 marks]

Write the equation linking current, charge and time, naming each quantity and its unit, and state what one coulomb is.

I = Q ÷ t, where I is current (A), Q is charge (C) and t is time (s). ✓
One coulomb is the charge that passes when a current of 1 A flows for 1 s. ✓

Calculation

[2 marks]

A current of 0.50 A flows for 60 s. Calculate the charge that passes.

Q = It = 0.50 × 60

Q = 30 C

Calculation

[2 marks]

A charge of 90 C passes through a lamp in a time of 60 s. Calculate the current.

I = Q ÷ t = 90 ÷ 60

I = 1.5 A

Calculation

[2 marks]

A current of 3.0 A is needed to deliver a charge of 60 C. Calculate the time for which the current must flow.

t = Q ÷ I = 60 ÷ 3.0

t = 20 s

Calculation

[3 marks]

A steady current of 0.20 A flows for 5.0 minutes. Calculate the charge that passes, giving your answer in coulombs.

t = 5.0 × 60 = 300 s Q = It = 0.20 × 300

Q = 60 C

convert minutes to seconds first

Analysis

[3 marks]

Two wires carry the same charge of 24 C past a point. In wire A it takes 12 s; in wire B it takes 4.0 s.

Calculate the current in each wire, and state which wire carries the larger current.

Wire A: I = 24 ÷ 12 = 2.0 A Wire B: I = 24 ÷ 4.0 = 6.0 A

Wire B carries the larger current. ✓

same charge in less time means a larger current

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.