Current is the charge that passes a point each second, so I = Q ÷ t. It is measured in amperes (A), with charge Q in coulombs (C) and time t in seconds (s).
This equation appears in most electricity papers, often as the first step of a longer question. It builds on how objects become charged, because the charge that flows is carried by electrons.
How is the equation built?
If 5 C of charge passes a point in 1 s, the current is 5 A. If the same 5 C takes 2 s, the current is only 2.5 A, because it is spread over more time. The equation I = Q ÷ t expresses that idea.
It has three forms, all from the same relationship:
| To find | Use | Units |
|---|---|---|
| Current | I = Q ÷ t | A |
| Charge | Q = I × t | C |
| Time | t = Q ÷ I | s |
Which unit habits matter?
Convert before you calculate. Minutes and hours become seconds (× 60, × 3600). Milliamperes become amperes (÷ 1000).
If the question gives a value in a different unit and you leave it, the answer will not come out in coulombs or amperes.
Worked example
A charge of 240 C passes through a heater element in 2.0 minutes. Calculate the current. (Invented example data.)
Step 1, choose the relationship: I = Q ÷ t.
Step 2, convert the time: 2.0 minutes = 2.0 × 60 = 120 s.
Step 3, substitute: I = 240 ÷ 120.
Step 4, calculate: I = 2.0 A.
Step 5, check: Q = I × t = 2.0 × 120 = 240 C, which matches the charge given.
The mistake to watch for
The most common slip is to use the time exactly as written.
Mistaken answer: I = 240 ÷ 2.0 = 120 A.
The student used 2.0 minutes as if it were seconds. A household lamp does not carry 120 A, so the size of the answer should have been a warning.
The correction is to write the unit next to every number on the substitution line. If the line reads “240 C ÷ 2.0 min”, the mismatch is visible at once, because the equation needs seconds.
Check yourself
Try these, then open each answer.
1. A charge of 18 C passes a point in 6.0 s. Find the current.
Show answer
I = 18 ÷ 6.0 = 3.0 A.
2. A current of 0.50 A flows for 10 minutes. How much charge passes?
Show answer
t = 10 × 60 = 600 s. Q = 0.50 × 600 = 300 C.
3. A current of 45 mA carries 27 C through a component. How long does this take, in minutes?
Show answer
I = 45 ÷ 1000 = 0.045 A. t = 27 ÷ 0.045 = 600 s. In minutes, 600 ÷ 60 = 10 minutes. Check: 0.045 × 600 = 27 C.
Where this leads next
The next idea is direction: comparing conventional current with electron motion. To separate the flow from the amount stored, see distinguishing current from charge stored. The charge and current practice set mixes all of these.
If your answers are right only when a question is fully laid out, a teacher can give you fresh questions with units hidden in awkward places. That is one thing our online one-to-one Physics tuition can do.