Cambridge Syllabus Notes

Electric Current

Electric current is the flow of electric charge through a conductor. Think of it like a river of tiny charged particles (electrons) moving from one place to another. 🌊

What is Electric Current?

Current is measured in amperes (A). One ampere means one coulomb of charge passes a point in one second.

Key Formula: $Q = I\,t$

This simple equation links the amount of charge $Q$ that flows, the current $I$ that flows, and the time $t$ it flows for.

$Q = I\,t$

Units and Conversion

$Q$ is in coulombs (C)
$I$ is in amperes (A)
$t$ is in seconds (s)
1 A = 1 C/s, so $Q$ (C) = $I$ (A) × $t$ (s)

Practical Example

A battery supplies a current of 2 A to a light bulb for 30 seconds. How much charge passes through the bulb?

Identify the given values: $I = 2\,\text{A}$, $t = 30\,\text{s}$.
Insert into $Q = I\,t$:
$Q = 2\,\text{A} \times 30\,\text{s} = 60\,\text{C}$.

Result
$Q = 60\,\text{C}$

Exam Tips

Remember the Units!

• Always check that current is in amperes and time in seconds.
• If time is given in minutes, convert to seconds (1 min = 60 s).
• If charge is required in coulombs, no further conversion is needed.
• Show the equation $Q = I\,t$ clearly in your answer.
• Use a calculator for large numbers and round to the correct significant figures.
• Include units in every step to avoid marks lost for missing units.
• If the question asks for current, rearrange the formula: $I = \dfrac{Q}{t}$.
• If the question asks for time, rearrange: $t = \dfrac{Q}{I}$.
• Check for any given charge in coulombs and current in milliamps (mA) – convert to amperes first.
• Practice quick mental maths: e.g., 5 A × 4 s = 20 C.
• Use the colour-coded boxes above to summarise your answer.
• Keep your answer neat and label each step.
• Remember: $Q$ is the total charge that has moved, not the instantaneous current.
• Good luck! 🚀

recall and use Q = It