Charge Flow Calculator

GCSE PhysicsAQAEdexcelOCRFoundation & Higher

Find charge flow, current or time using Q = I × t. Enter milliamps, minutes or hours if the question uses them and the solver converts for you.

Quick answer

Charge flow = current × time. Use current in amps and time in seconds to get coulombs (C). For example, 0.5 A for 2 minutes (120 s) gives Q = 0.5 × 120 = 60 C.

By Mehmood Kakar · Updated

Q = I × t
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How to calculate charge flow

Current is the rate of flow of electric charge. A current of 1 amp means 1 coulomb of charge passes a point every second. The equation links the three:

charge flow = current × time

  • Q is charge flow in coulombs (C)
  • I is current in amperes (A)
  • t is time in seconds (s)

A current of 0.5 A flowing for 2 minutes moves 0.5 × 120 = 60 C of charge. Notice the time had to become seconds first.

Two conversions catch students out here. Currents in small circuits are often given in milliamps, so divide by 1000 to get amps. Times are often given in minutes or hours, so multiply by 60 or 3600.

This equation often appears as the first step of a longer question. Once you know the charge, you can use E = QV to find the energy transferred.

Common mistakes that cost marks

  1. Leaving time in minutes. 2 minutes is 120 s. Multiply minutes by 60 and hours by 3600.
  2. Not converting milliamps. 250 mA is 0.25 A. Divide by 1000.
  3. Mixing up Q and C. Q is the symbol for charge; C (coulomb) is its unit.
  4. Dividing the wrong way. Current is charge ÷ time, not time ÷ charge.

Practice questions

Try each one on paper first, then reveal the worked answer.

Q1A current of 0.5 A flows through a lamp for 2 minutes. Calculate the charge that flows.[3 marks]
  1. Q = I × t
  2. Convert: t = 2 × 60 = 120 s
  3. Q = 0.5 × 120
  4. Q = 60 C
Q21800 C of charge flows through a heater in 15 minutes. Calculate the current.[3 marks]
  1. Rearrange: I = Q ÷ t
  2. Convert: t = 15 × 60 = 900 s
  3. I = 1800 ÷ 900
  4. I = 2.0 A
Q3A current of 250 mA flows in a circuit. Calculate how long it takes for 30 C of charge to flow.[3 marks]
  1. Rearrange: t = Q ÷ I
  2. Convert: I = 250 ÷ 1000 = 0.25 A
  3. t = 30 ÷ 0.25
  4. t = 120 s

Frequently asked questions

What is the equation for charge flow?

Charge flow = current × time (Q = It). Charge is in coulombs (C), current in amperes (A) and time in seconds (s).

What is a coulomb?

A coulomb is the unit of electric charge. One coulomb flows past a point each second when the current is 1 amp.

How do I convert mA to A?

Divide by 1000. For example, 250 mA = 0.25 A.

What is current?

Current is the rate of flow of electric charge. In a single closed loop, the current is the same at every point.