AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrical charge and current: charge flow = current × time
The equation charge flow = current × time links three quantities in an electric circuit.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The equation charge flow = current × time links three quantities in an electric circuit. Charge flow is measured in coulombs (C), current in amperes (A) and time in seconds (s). If a current of 3 A flows for 4 s, the charge flow is 3 × 4 = 12 C. The equation can be rearranged: current = charge flow ÷ time, and time = charge flow ÷ current. This is useful when a question gives charge and time but asks for current, or gives charge and current but asks for time. Always convert time to seconds before substituting; for example, 5 minutes = 300 s. The equation applies when the current is steady. In calculations, write the equation, substitute the known values with units, then calculate and give the unit.
What earns the marks
- Recall the equation charge flow = current × time.
- Identify the correct units: charge flow in coulombs (C), current in amperes (A) and time in seconds (s).
- Substitute known values into the equation and calculate the unknown quantity.
- Rearrange the equation to calculate current as charge flow ÷ time or time as charge flow ÷ current.
- Convert time to seconds before using the equation, for example minutes × 60.
- Give the final answer with the correct unit and an appropriate number of significant figures.
Common mistakes and how to put them right
Multiplying current by time but forgetting to convert minutes to seconds.
Put it right: Convert all times to seconds first.
Rearranging incorrectly, for example writing current = charge flow × time.
Put it right: Current = charge flow ÷ time.
Giving the answer without a unit or with the wrong unit.
Put it right: Include C for charge, A for current or s for time.
Exam tips
- Write down the equation before substituting numbers to make rearranging clearer.
- Convert time to seconds as the first step if it is given in minutes or hours.
- Check the size and unit of your answer to see whether it is sensible.
Try the lesson
The start of the Electrical charge and current: charge flow = current × time 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. What is the charge flow?
Key words
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charge flow
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current
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time
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rearranging equations
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See every definition
- charge flow
- Measured in coulombs (C), it is calculated as current multiplied by time.
- current
- Measured in amperes (A), it can be calculated as charge flow divided by time.
- time
- Measured in seconds (s), it can be calculated as charge flow divided by current.
- rearranging equations
- The equation can be rearranged to give current as charge flow divided by time, and time as charge flow divided by current.
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