A student used a simple transformer to investigate how the number of turns on the secondary coil affects the potential difference (p.d.) across the secondary coil - AQA - GCSE Physics - Question 7 - 2018 - Paper 1

You can tell from Figure 12 that the transformer changes from being a step-down to a step-up transformer when the potential difference (p.d.) across the secondary coil exceeds 2V after 20 turns.

The amount of infrared radiation emitted by the nails increases when the power supply is switched on, until the nails reach a constant temperature.

Using the equation for power:

$P = V imes I$,

we can rearrange it to find the current:

I = rac{P}{V}.

Given that the voltage from the primary coil (V_p) can be calculated as follows:

$640 = 280 imes 1.75$,

this means that $V_p = 280 ext{ V}$. Substituting into the power equation:

$I_p = \frac{336}{280} = 1.2 ext{ A}$.

Thus, the current needed from the power supply is 1.2 A.