Charge equation: Q = It
Charge flow = current × time. In symbols, Q = I t. Q is the charge in coulombs (C), I is the current in amperes (A) and t is the time in seconds (s). AQA, Edexcel and both OCR courses print it the same way, and it is one of the equations you find on the exam sheet.
Equation
Q = I t
| Symbol | Quantity | Unit |
|---|---|---|
Q | charge | C |
I | current | A |
t | time | s |
What each specification says
| Exam board | Status | Equation |
|---|---|---|
| AQA Physics (8463) | Recall | Q = I t |
| AQA Combined Science (8464) | Recall | Q = I t |
| Edexcel Physics (1PH0) | Recall | Q = I t |
| Edexcel Combined Science (1SC0) | Recall | Q = I t |
| OCR Gateway Physics A (J249) | Recall | Q = I t |
| OCR Twenty First Century Physics B (J259) | Recall | Q = I t |
Calculator: Q = I t
What charge and current are
In a metal wire, tiny charged particles called electrons drift along. Charge is the amount of this electricity that moves. We measure it in coulombs.
Current is how fast the charge flows. One ampere, or amp, means one coulomb passing a point every second. So Q = I t just adds up the flow: the rate multiplied by how long it lasts.
Think of water in a pipe. Current is like litres per second. Charge is like the total litres that have gone through.
Where it sits on each board's list
AQA words it as charge flow = current × time. Edexcel and OCR say charge = current × time. The symbols are Q = I t for all of them.
All six specifications, including AQA 8464 and Edexcel 1SC0 for Combined Science, place it on the recall list. That is their own label for the equations you use most. It makes no difference in the exam room, because from 2025 the sheet prints every one. See the OCR Gateway list or the physics equation sheet overview for the full set.
Using it with other electricity equations
- I = Q ÷ t, to find the current
- t = Q ÷ I, to find the time
Once you know the charge, you can find the energy it carries with energy = charge × potential difference. Current also links to resistance through V = IR.
Batteries are often rated in milliamp-hours. That is a charge too: a current in mA multiplied by a time in hours. To turn it into coulombs, change the current to amps and the time to seconds, then use Q = I t.
Worked examples
Q
- Change the time to seconds: 5 × 60 = 300 s
- Q = I t
- Q = 2 × 300
- Q = 600 C
600 C
Q
- Change the current to amps: 250 mA = 0.25 A
- Q = I t
- Q = 0.25 × 40
- Q = 10 C
10 C
I
- Change the time to seconds: 3 ms = 0.003 s
- I = Q ÷ t
- I = 15 ÷ 0.003
- I = 5000 A
5000 A
Common mistakes
- Using milliamps as amps. 250 mA is a quarter of an amp, and 250 in the equation gives a charge 1000 times too large.
- Mixing up ms and minutes. Milliseconds are thousandths of a second, while a minute is 60 seconds.
- Calling charge and current the same thing. Current is a rate in A, and charge is an amount in C.
- Writing the unit as Q. The letter Q is the quantity, and the coulomb, written C, is its unit.
Build speed at making I or t the subject of Q = It in the Equation Rearranger.
Questions people ask
How big is one coulomb?
It is the charge carried by a very large number of electrons, more than six million million million. A current of 1 A moves one coulomb each second.
Why is the current so large in the third example?
A short spark can move a lot of charge in a tiny time. Dividing by a very small time gives a very large current.