KinexiaLEARN PHYSICS

Capacitance — Camera Flash (Worked Example)

Easy
Problem

A camera flash uses a 100 μF capacitor charged to 300 V. How much charge does it store?

Given

  • C = 100 μF = 1.0 × 10⁻⁴ F

  • V = 300 V

Find

The charge Q.

Principle

Q = CV: capacitance times voltage is the stored charge — the flash's reservoir.

Formula

Solution

  1. Convert: C=1.0×10−4 FC = 1.0 \times 10^{-4}\ \mathrm{F}

  2. Substitute: Q=(1.0×10−4)(300)Q = (1.0 \times 10^{-4})(300)

  3. Multiply: Q=0.030 CQ = 0.030\ \mathrm{C}

0.030 C

Unit check

F × V = (C/V) × V = C — correct.

What this means physically

0.030 C dumped through the flash tube in a millisecond makes a ~30 A pulse — that is why the flash is so blindingly bright for an instant.

Common mistake

Using 100 F instead of 100 μF. The micro prefix (10⁻⁶) is the difference between a camera and a power station.