Find transferred charge and time from current
Rearrange I = Q/t for constant or stated interval-average current while converting minutes and current prefixes.
Section P4.2.2 · Electric current
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Calculate I = Q/t.
- Rearrange products and convert time.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many seconds are in 2 minutes?
What you will learn
- Use Q = It and t = Q/I.
- Check units and the constant or interval-average condition.
Multiply a rate by time
For a constant current or a supplied average over the same interval, Q = It. A current of 0.50 A for 2 minutes passes 0.50 × 120 = 60 C. Convert minutes to seconds before using amperes as coulombs per second.
To find time, use t = Q/I for a non-zero stated current. If 24 C passes at 0.80 A, t = 30 s. A zero current cannot give a finite time for a non-zero passed charge under this model. The expression 0/0 does not determine a unique interval.
Match the supplied conditions
A single instantaneous reading does not generally supply the average for a whole changing interval. Use Q = It only when the current is constant or when I is explicitly the average over that same interval. If separate constant intervals are given, calculate each charge and combine the signed or one-direction amounts appropriately.
An ampere is a rate rather than a charge unit. Writing '60 A of charge' confuses the result quantity; charge is in coulombs and time in seconds after conversion. No voltage or power follows from these flow data alone.
Sources for this lesson
Pause and explain
24 C passes at a constant 0.80 A. How long does it take?
Worked example
A constant 0.50 A current flows for 2 minutes. Find the passed charge.
Show the worked solution
- Convert 2 minutes to 120 s.
- Use Q = It.
- 0.50 × 120 = 60 C.
Answer 60 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.
Calculate charge at a constant 0.50 A for 2 minutes.
Give me a hint
Convert the duration, then multiply rate by time.
Compare my reasoning
- 2 minutes = 120 s.
- Q = It = 0.50 × 120.
- Q = 60 C.
60 C.
Look for these in your work
- Convert time.
- State coulombs.
Find charge at 250 mA for 40 s and time for 24 C at 0.80 A.
Give me a hint
Use Q = It for the first and t = Q/I for the second.
Compare my reasoning
- 250 mA = 0.250 A.
- Q = 0.250 × 40 = 10 C.
- t = 24/0.80 = 30 s.
10 C; 30 s.
Look for these in your work
- Convert the current prefix.
- Use different rearrangements for the two targets.
A meter reads 2 A once during a variable 10 s interval. Can 20 C be concluded?
Give me a hint
Check whether the reading is a constant value or the interval average.
Compare my reasoning
- A single reading gives one observation.
- The rest of the current profile is not specified.
- Q = 2 × 10 is justified only for a matching constant or average current.
No; 20 C requires 2 A to be constant or the average over those 10 s.
Look for these in your work
- State the required condition.
- Do not treat one reading as a full interval account.
Common mistakes
- Convert minutes to seconds.
- Use the current average for the same interval.
- Do not divide by zero or label charge in amperes.
What can you explain now?
What unit results from ampere multiplied by second?
Compare with the explanation
Coulomb.
The charge-flow rate multiplied by time gives the passed charge.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Find passed charge or duration with matching current and interval conditions.
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.