Charge and current: the charge equation Q = It

The charge equation is Q = It: charge flow = current × time. Charge Q is in coulombs (C), current I in amperes (A) and time t in seconds (s). A current of 1 A means 1 coulomb of charge flows past a point every second, so a current of 2 A flowing for 30 s moves 60 C of charge.

What are charge and current?

Charge is a property of particles such as electrons and protons. Electrons carry a negative charge, and in a metal wire it is electrons that move. Charge is measured in coulombs. One electron has a tiny charge, about 1.6 × 10⁻¹⁹ C, so a coulomb is the charge of a huge number of them.

Current is the rate of flow of charge: how much charge passes a point each second. AQA's GCSE specification puts it this way: electric current is a flow of electrical charge, and its size is the rate of flow of electrical charge. For charge to flow around a circuit, the circuit must be closed and must include a source of potential difference, such as a cell.

The charge flow equation

The second row is the definition of current: I = Q ÷ t, charge per second. So 1 A = 1 C/s, and a coulomb is the charge carried by a current of 1 A in 1 s.

Charge against time

For a steady current, charge builds up at a constant rate. The graph shows a current of 2 A. Here x is the time in seconds and y is the total charge that has flowed, in coulombs.

The gradient of the line is 2 C/s, which is the current. On any charge-time graph, the gradient is the current.

Charge equation: worked examples

Example 1: finding the charge

A current of 2 A flows for 30 s. How much charge flows?

Example 2: time in minutes

A lamp draws 0.5 A for 5 minutes. How much charge passes through it?

Convert the time first: 5 minutes = 300 s.

Example 3: finding the current

900 C of charge flows through a heater in 3 minutes. What is the current?

Example 4: finding the time

How long does it take for 120 C to flow at a current of 0.4 A?

That is 5 minutes.

Example 5: charging a phone

A phone charges at 2 A for 1.5 hours. How much charge flows into the battery?

1.5 hours = 1.5 × 3600 = 5400 s, so Q = 2 × 5400 = 10 800 C. Phone batteries are often rated in milliamp-hours instead: 2 A for 1.5 hours is 3000 mAh, which is the same 10 800 C.

Now try a charge, current and energy question from the Electricity course, then press Mark to check your answer there.

Charge and energy: E = QV

Potential difference tells you how much energy each coulomb of charge transfers. A potential difference of 1 volt means 1 joule of energy is transferred for every coulomb that flows:

Example 6: 60 C of charge flows through a 12 V lamp. The energy transferred is 60 × 12 = 720 J. Combining E = QV with Q = It gives E = VIt, and since power is energy per second, P = VI.

How many electrons in a coulomb? (A-level)

Each electron carries a charge of 1.60 × 10⁻¹⁹ C, the value AQA's A-level Physics data booklet gives. So one coulomb is 1 ÷ (1.60 × 10⁻¹⁹) = 6.25 × 10¹⁸ electrons. In Example 1, 60 C is 60 ÷ (1.60 × 10⁻¹⁹) = 3.75 × 10²⁰ electrons. At A-level, the charge equation is often written with Δ: I = ΔQ ÷ Δt.

Current in series and parallel circuits

Charge is not used up as it goes round a circuit: what flows into a component flows out again. That is why the current is the same at every point in a single closed loop, a series circuit. In a parallel circuit, the current splits between the branches, and the currents in the branches add up to the total. The Circuit Builder lets you place ammeters anywhere in a circuit to see this for yourself.

Common mistakes

• Time in minutes or hours. Q = It needs seconds. 5 minutes is 300 s; 1 hour is 3600 s.
• Current in mA. 250 mA is 0.25 A.
• Thinking current is used up by a lamp. The current is the same before and after a component in series. Energy is transferred, not charge.
• Mixing up Q = It and E = QV. Q = It links charge to current and time. E = QV links energy to charge and potential difference.
• Getting the unit of charge wrong. Charge is in coulombs, C. Current is in amps, A.

Is the charge equation on the equation sheet?

Yes. AQA's GCSE Physics equations sheet, checked in November 2026, gives charge flow = current × time, Q = It, and energy transferred = charge flow × potential difference, E = QV. AQA's specification lists Q = It as an equation students should be able to recall and apply. Other boards publish their own sheets, so check yours.

Practise electricity in a short timed paper

GCSE Physics Foundation and GCSE Physics Higher each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, including electricity. The Electricity course builds up current, charge, potential difference and resistance from the start.

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Physics · Atoms and particlesSpecific charge: formula, units and worked examplesSpecific charge is charge divided by mass, in C/kg. See the formula, the values for the electron and proton, and worked examples for nuclei and ions.