recall and use Q = It

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?

  1. Identify the given values: \$I = 2\,\text{A}\$, \$t = 30\,\text{s}\$.

  2. Insert into \$Q = I\,t\$:

  3. \$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! 🚀