Charge calculator: Q = It with working

Example: a current of 2 A flows for 30 s. $Q = 2 \times 30 = 60$ C.

Time must be in seconds and current in amps.

• Minutes: 0.5 A for 5 minutes. $t = 300$ s, so $Q = 0.5 \times 300 = 150$ C.
• Milliamps: 200 mA for 1 minute. $I = 0.2$ A and $t = 60$ s, so $Q = 12$ C.
• Finding time: how long for 900 C to flow at 3 A? $t = 900 \div 3 = 300$ s (5 minutes).

Potential difference tells you the energy transferred per coulomb: $E = Q \times V$. If 60 C flows through a 12 V lamp, $E = 60 \times 12 = 720$ J. So one volt means one joule of energy per coulomb of charge. Combine with our electrical power calculator to link energy, power and time.

In a series circuit the current is the same everywhere, so the same charge passes each component each second. In a parallel circuit the current splits between branches and adds up again where they rejoin. Our series and parallel resistor calculator finds the total resistance of each arrangement.

• Leaving time in minutes.
• Forgetting to convert mA to A (divide by 1,000).
• Confusing charge (C) and current (A).

Practise with our charge and current questions.

• Write $Q = It$ first, then convert time to seconds before substituting.
• A question about energy and charge needs $E = QV$, often after finding $Q$ here.
• Current is the same everywhere in a series circuit, so the charge passing each component in a given time is the same too.
• Give the unit as C (coulombs), not A or A s, unless the question asks otherwise.

In metal wires, the charge is carried by electrons. Each electron carries a tiny charge of about $1.6 \times 10^{-19}$ C, so one coulomb is the charge of roughly $6.25 \times 10^{18}$ electrons. A current of 2 A means about $1.25 \times 10^{19}$ electrons pass a point every second. Conventional current is shown flowing from + to −, the opposite way to the electrons themselves, because it was defined before electrons were discovered.