OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and use the relationship between quantity of charge, current and time
Full specification wording
Static and charge: recall and use the relationship between quantity of charge, current and time
Charge, current and time are linked by Q = I × t, where Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s).
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 8 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Charge, current and time are linked by Q = I × t, where Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). Current is the rate of flow of charge, so one ampere means one coulomb passing a point each second. To use the relationship, identify the two known quantities, convert time to seconds (for example 3 minutes = 180 s) and multiply current by time to find charge, or divide charge by time to find current. A current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged forms are I = Q ÷ t and t = Q ÷ I. In a multiple-choice question, work the calculation before looking at the options, then check the unit matches coulombs, amperes or seconds as asked.
What earns the marks
- Selects the option that applies Q = I × t with current in amperes and time in seconds.
- Converts a time given in minutes into seconds before multiplying by the current.
- Rearranges the relationship correctly to find current as charge divided by time.
- Identifies the coulomb as the unit of charge corresponding to one ampere for one second.
Common mistakes and how to put them right
Uses time in minutes without converting to seconds.
Put it right: Convert every time to seconds before calculating.
Multiplies charge by time when finding current.
Put it right: Divide charge by time instead.
Confuses the unit of charge with the unit of current.
Put it right: Label charge in coulombs and current in amperes.
Exam tips
- Write down Q = I × t and substitute values before scanning the answer options.
- Convert all times to seconds first, since the equation assumes seconds.
- Check the unit of your answer against the unit named in the question stem.
Try the lesson
The start of the Static and charge: recall and use the relationship between quantity of charge, current and time lesson, made from checked teaching notes for this specification point.
Have a go first
A charge of 20 C passes a point in a circuit in 4 s. Using current = charge ÷ time, what is the current?
Listen
Charge, current and time are linked by Q = I × t, where Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). Current is the rate of flow of charge, so one ampere means one coulomb passing a point each second. To use the relationship, identify the two known quantities, convert time to seconds (for example 3 minutes = 180 s) and multiply current by time to find charge, or divide charge by time to find current. A current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged forms are I = Q ÷ t and t = Q ÷ I. In a multiple-choice question, work the calculation before looking at the options, then check the unit matches coulombs, amperes or seconds as asked. A strong answer earns marks for these points. Selects the option that applies Q = I × t with current in amperes and time in seconds. Converts a time given in minutes into seconds before multiplying by the current. Rearranges the relationship correctly to find current as charge divided by time. Identifies the coulomb as the unit of charge corresponding to one ampere for one second. Exam tip: Write down Q = I × t and substitute values before scanning the answer options.
See it
Key words
Say what each word means before you turn the card.
Charge flow and current
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Rate of flow of charge
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Using and rearranging equations
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Unit conversion of time
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See every definition
- Charge flow and current
- Charge, current and time are linked by Q = I × t, where Q is charge in coulombs, I is current in amperes and t is time in seconds.
- Rate of flow of charge
- One ampere means one coulomb passing a point each second.
- Using and rearranging equations
- Identify the two known quantities, then multiply current by time to find charge, or divide charge by time to find current.
- Unit conversion of time
- Convert time to seconds before calculating, for example 3 minutes = 180 s.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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