Figure 13 shows an LED torch - AQA - GCSE Physics - Question 9 - 2020 - Paper 1

The equation that links charge flow, current, and time is given by:

$Q = I imes t$ where Q is the charge flow in coulombs, I is the current in amperes, and t is the time in seconds.

Given:

• Time (t) = 14,400 seconds
• Current (I) = 50 mA = 0.050 A

Using the equation:

$Q = I imes t$ $Q = 0.050 imes 14,400$ $Q = 720 ext{ C}$

Thus, the total charge flow through the cells is 720 C.

The torch did not work because when the cells were inserted the wrong way around, the current could not flow through the circuit. This is due to the nature of diodes, which prevent current flow in the reverse direction. In this case, the LED acts as a diode, blocking current when the cells are incorrectly positioned.

The equation that links efficiency, total power input, and useful power output is:

$Efficiency = \frac{Useful \ power \ output}{Total \ power \ input}$

Given:

• Total power input = 0.24 W
• Efficiency = 0.75

Using the equation for efficiency:

$0.75 = \frac{Useful \ power \ output}{0.24}$ Solving for useful power output:

$Useful \ power \ output = 0.75 \times 0.24 = 0.18 \text{ W}$

Thus, the useful power output of the LED is 0.18 W.