Foundations
Current from charge
- where
- current (A)
- charge (C)
- time (s)
Explain this equation
Works out the average current from how much charge passed a point and how long it took. Handy for battery and capacitor problems, and for making sense of charge ratings.
I is the rate of flow of charge. Q is the total charge that went past, in coulombs, and t is the time it took, in seconds. One ampere is one coulomb passing every second.
Current is defined as a rate, so it is simply amount divided by time. Spread the same charge over a longer time and the current falls in proportion.
Sense check: 120 C in one minute is 120 ÷ 60 = 2 A. Double the time for the same charge and the current halves.
Watch out: if the current was not steady, the answer is the average over the period, not the peak.
Imagine counting the charge passing one point in a conductor. Current tells you how quickly that charge passes. Divide the total charge by the elapsed time.
Convert minutes to seconds before calculating. For varying current this gives the average over the stated interval, not necessarily its value at every instant.
Enter your values to see the result.
Common mistake
Using minutes as though they were seconds.
Limits
Average current; elapsed time must be positive.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.