AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrical charge and current: Q = I t
This equation links charge flow to current and time.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation links charge flow to current and time. Current is the rate of flow of electrical charge: one ampere means one coulomb of charge passes a point each second. Rearranged, Q = I × t, I = Q ÷ t and t = Q ÷ I. For example, a current of 2 A flowing for 30 s transfers Q = 2 × 30 = 60 C. If 120 C passes in 40 s, I = 120 ÷ 40 = 3 A. Always convert time to seconds: 5 minutes is 300 s. The equation applies to steady currents in series circuits and to any component, including filament lamps and resistors, where charge carriers pass through. It also underpins Q = ne, linking total charge to the number of electrons, each of charge 1.6 × 10⁻¹⁹ C.
What earns the marks
- State the equation as Q = I t, identifying Q as charge flow in coulombs, I as current in amperes and t as time in seconds.
- Substitute values correctly, converting minutes to seconds by multiplying by 60 before calculating.
- Rearrange the equation to find current or time when charge and the other quantity are given.
- Calculate charge flow for a stated current and time, for example 0.5 A for 120 s gives 60 C.
- Explain that one coulomb is the charge transferred when one ampere flows for one second.
- Use the equation with series circuits, recognising that current is the same at every point in a series loop.
Common mistakes and how to put them right
Using minutes directly instead of converting to seconds.
Put it right by multiplying minutes by 60 before substitution.
Confusing current with charge.
Put it right by remembering current is charge per second, measured in amperes, while charge is measured in coulombs.
Rearranging incorrectly, for example writing I = Q t.
Put it right by dividing both sides by t to give I = Q ÷ t.
Exam tips
- Write the equation, then substitute numbers with units before calculating so method marks are visible.
- Check the unit of time in the question and convert to seconds immediately if it is given in minutes.
- Give the unit C with your final charge answer and check the size is sensible for the current and time stated.
Try the lesson
The start of the Electrical charge and current: Q = I t lesson, made from checked teaching notes for this specification point.
Have a go first
A current of 2 A flows through a lamp for 5 s. Using Q = I t, what is the charge that flows through the lamp?
Key words
Say what each word means before you turn the card.
Charge flow
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Current
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Quantitative circuit calculations
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Unit conversion and standard units
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See every definition
- Charge flow
- The quantity Q in the equation Q = I t, measured in coulombs.
- Current
- The rate of flow of electrical charge, where one ampere means one coulomb of charge passes a point each second.
- Quantitative circuit calculations
- Substituting values into Q = I t, and its rearrangements, to calculate charge flow, current or time.
- Unit conversion and standard units
- Converting time to seconds, for example multiplying minutes by 60, before substituting into the equation.
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