An LED bicycle safety light is powered by cells connected in series.
Write down the equation which links charge flow (QQQ), current (III), and time (ttt).
The safety light worked for 18 000 s18\,000\text{ s}18000 s before the cells needed replacing. The current in the LED was 40 mA40\text{ mA}40 mA. Calculate the total charge flow through the LED.
When the cells were replaced, they were put into the safety light the wrong way around. Explain why the light did not work.
Write down the equation which links efficiency, total power input, and useful power output.
The total power input to the LED was 0.15 W0.15\text{ W}0.15 W. The efficiency of the LED was 0.800.800.80. Calculate the useful power output of the LED.
39 exam-style questions on AQA GCSE Physics 2.1 Current, potential difference and resistance, covering 2.1.1 Standard circuit diagram symbols, 2.1.2 Electrical charge and current, 2.1.3 Current, resistance and potential difference, and 2.1.4 Resistors. Each one has a worked solution and a mark scheme showing where the marks go.