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:

$Q = I imes t$

To calculate the total charge flow, we use the equation derived earlier. Given that:

• The current, $I = 50 ext{ mA} = 0.050 ext{ A}$
• The time, $t = 14400 ext{ s}$, we substitute these values:

$Q = 0.050 imes 14400 = 720 ext{ C}$. Thus, the total charge flow is 720 C.

The torch did not work because the cells were installed the wrong way around. In a circuit involving a diode (such as an LED), current only flows in one direction. When the cells are reversed, they put the diode in the reverse direction, causing it to block current flow. This results in the entire circuit being incomplete.

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

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

Given the total power input to the LED is 0.24 W and the efficiency is 0.75, we can calculate the useful power output as follows:

$Useful\ power\ output = Efficiency \times Total\ power\ input$

Substituting the values:

$Useful\ power\ output = 0.75 \times 0.24 = 0.18 \text{ W}$. Thus, the useful power output of the LED is 0.18 W.