Calculate current from charge passing a point
Use I = Q/t with consistent coulombs and seconds and keep the chosen point and averaging interval explicit.
Section 4.2.2 · Electric current
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Use coulombs and time conversions.
- Distinguish current from a total transferred charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is 18 mC in coulombs?
What you will learn
- Define current as charge passing a point per unit time.
- Calculate current with consistent charge and time units.
Define the rate
Electric current is the charge passing a point per unit time: I = Q/t. With charge Q in coulombs and time t in seconds, current I is in amperes. One ampere means one coulomb per second. State the point or cross-section through which charge is counted.
For 12 C passing a marked section in 4 s, the average current over that interval is 3 A. If the current is constant, this average is also its value throughout the interval. If it varies, the total-charge calculation supplies the interval average, not every instantaneous value.
Convert both quantities
Convert prefixed charges and non-second times before dividing. For 18 mC in 6 ms, both prefixes contribute 10⁻³, so (18 × 10⁻³)/(6 × 10⁻³) = 3 A. Cancelling a prefix is justified only when the numerator and denominator use the same multiplier.
Signed charge passages must use a stated direction convention. Opposite passages can reduce the signed net total even when carriers moved. Unless such a changing-direction account is specified, introductory examples give a one-direction passed-charge magnitude. Do not infer voltage, resistance or energy from current alone.
Pause and explain
18 mC passes a point in 6 ms. What is the current?
Worked example
12 C passes a marked wire section in 4 s. Calculate the interval average current.
Show the worked solution
- Use I = Q/t.
- Both values are already in C and s.
- 12/4 = 3 A.
Answer 3 A average over the stated interval.
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.
Calculate the current when 12 C passes a point in 4 s.
Give me a hint
Divide passed charge by its associated time interval.
Compare my reasoning
- I = Q/t.
- Q = 12 C and t = 4 s.
- I = 3 A.
3 A.
Look for these in your work
- Use the same interval.
- State amperes.
Calculate average current for 30 mC in 2 s and for 18 mC in 6 ms.
Give me a hint
Convert each charge and time independently before comparing the two rates.
Compare my reasoning
- 0.030/2 = 0.015 A.
- 0.018/0.006 = 3 A.
- The different time scales produce different rates.
0.015 A (15 mA); 3 A.
Look for these in your work
- Convert both examples.
- Do not cancel unlike prefixes.
A variable current passes 20 C in 10 s. A learner claims it was exactly 2 A at every instant. Assess the claim.
Give me a hint
Ask whether the total charge determines the detailed time profile.
Compare my reasoning
- The interval average is 20/10 = 2 A.
- Many varying profiles can give the same total.
- Constancy is not established without further information.
Average 2 A; the instantaneous values are not established.
Look for these in your work
- Calculate the supported average.
- Identify the missing time-profile evidence.
Common mistakes
- Use coulombs and seconds consistently.
- An interval average need not be constant current.
- Identify the chosen section and sign convention.
What can you explain now?
What does 1 A mean?
Compare with the explanation
1 C passing a point per second.
The ampere expresses a charge-flow rate rather than a stored amount of charge.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Define and calculate charge-flow rate with consistent units and an explicit averaging interval.
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.