AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrical charge and current: time, t, in seconds, s
Time, symbol t, is measured in seconds, symbol s, when calculating electrical charge.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Time, symbol t, is measured in seconds, symbol s, when calculating electrical charge. The equation Q = I t links charge in coulombs to current in amperes and time in seconds, so time must be in seconds before substitution. If a time is given in minutes, multiply by 60; if given in hours, multiply by 3600; if given in milliseconds, divide by 1000. For example, a current of 0.5 A flowing for 3 minutes transfers charge Q = 0.5 × 180 = 90 C. Time is measured in the laboratory with a stopwatch or timer, and in circuit questions it is often the duration for which a current flows. Rearranging Q = I t gives t = Q ÷ I, which allows the time to be found when charge and current are known.
What earns the marks
- States that time is measured in seconds, with the unit symbol s, in the equation Q = I t.
- Uses the symbol t for time and substitutes time in seconds into charge calculations.
- Converts minutes to seconds by multiplying by 60 and hours to seconds by multiplying by 3600.
- Converts milliseconds to seconds by dividing by 1000 before substitution.
- Rearranges Q = I t to t = Q ÷ I and calculates time when charge and current are known.
- Measures or records the duration of a current with a stopwatch or timer, reading to an appropriate precision.
Common mistakes and how to put them right
Substituting minutes directly into Q = I t.
Put it right: Convert minutes to seconds by multiplying by 60 first.
Treating milliseconds as seconds.
Put it right: Divide milliseconds by 1000 to obtain seconds.
Rearranging Q = I t incorrectly, for example writing t = Q × I.
Put it right: Divide charge by current, t = Q ÷ I.
Exam tips
- Write down the time in seconds before starting the calculation, showing the conversion if the question gives minutes or hours.
- Include the unit s with a calculated time, and check that the value is sensible for the current and charge involved.
- When timing an experiment, start and stop the stopwatch at the same point in the cycle to reduce timing error.
Try the lesson
The start of the Electrical charge and current: time, t, in seconds, s lesson, made from checked teaching notes for this specification point.
Have a go first
A current of 0.40 A transfers a charge of 120 C. Calculate the time the current flows for.
Key words
Say what each word means before you turn the card.
Unit conversion
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Rearranging the charge equation
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Practical timing and precision
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See every definition
- Unit conversion
- Changing a time given in minutes, hours or milliseconds into seconds before substitution, by multiplying or dividing as required.
- Rearranging the charge equation
- Changing Q = I t into t = Q ÷ I so that time can be found when charge and current are known.
- Practical timing and precision
- Measuring or recording the duration of a current with a stopwatch or timer, reading to an appropriate precision.
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