Relate current to charge flow
Model electric current as charge passing a cross-section each second and use Q = It with quantised charge.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 9. Circuit calculations and component sketches are original study practice; full timed assessment, supervised practical competence and human subject-teacher review remain outstanding.
Before you begin
- Use charge, time, energy, power, area and consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a current of 1 A mean?
What you will learn
- Understand current as flow of charge carriers
- Understand that charge is quantised
- Recall and use Q = It
Current measures charge flow rate
Electric current is the rate at which charge passes a cross-section. A current of one ampere means one coulomb of charge passes each second. Conventional current direction is the direction positive charge would move; electrons in a metal drift in the opposite direction.
For steady current, Q = It, where Q is charge in coulombs, I is current in amperes and t is time in seconds. Rearrange only after identifying which quantity the question asks for, and convert minutes or hours into seconds when using SI units.
Charge is carried in discrete units
Charge is quantised: the net charge on an isolated object is an integer multiple of the elementary charge magnitude e. An object with N excess electrons has charge −Ne; positive ions or deficits of electrons have positive charge. The sign records the type or imbalance of carriers, while Q = It often uses the magnitude of charge transferred.
This does not mean current flows as a smooth stream of fractional electrons. In a macroscopic conductor many discrete carriers cross a section, giving a measurable average current. Keep the signed charge convention and the current magnitude clear in a calculation.
Pause and explain
How much charge flows in 4.0 s at 2.5 A?
Worked example
A current of 0.40 A flows for 3.0 min. Find charge transferred.
Show the worked solution
- Convert 3.0 min to 180 s.
- Use Q = It = 0.40 × 180.
- State charge in coulombs.
Answer Q = 72 C.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 0.80 A current flows for 25 s. Find charge transferred.
Give me a hint
Multiply current in amperes by time in seconds.
Compare my reasoning
- Q = It.
- Q = 0.80 × 25.
- Give charge in coulombs.
20 C.
Look for these in your work
- Correct relation selected
- Product calculated
- Coulomb unit stated
A charge of 540 C passes in 3.0 min. Find the current.
Give me a hint
Convert minutes to seconds, then rearrange Q = It.
Compare my reasoning
- t = 3.0 × 60 = 180 s.
- I = Q/t = 540/180.
- State amperes.
3.0 A.
Look for these in your work
- Time converted
- Equation rearranged
- Current unit stated
A metal wire carries 0.50 A for 8.0 s. Estimate the number of electrons passing a cross-section, using e = 1.60 × 10⁻¹⁹ C.
Give me a hint
First find total charge, then divide its magnitude by charge per electron.
Compare my reasoning
- Q = It = 0.50 × 8.0 = 4.0 C.
- N = Q/e = 4.0/(1.60 × 10⁻¹⁹).
- Give a number of carriers, not coulombs.
2.5 × 10¹⁹ electrons pass the cross-section.
Look for these in your work
- Transferred charge found
- Elementary charge used
- Carrier count calculated
Common mistakes
- Using time in minutes without converting to seconds
- Treating current as the amount of charge rather than charge per second
- Ignoring the negative sign when identifying an electron's charge
What can you explain now?
What does a current of 1 A mean?
Compare with the explanation
1 C of charge passes each second
Ampere is coulomb per second.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Why is current described as a rate rather than an amount of charge?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.