Series and parallel circuits — GCSE Physics practice questions

Question 1

The diagram shows part of the control circuit of a rice cooker.

What is component X?

Accepted Answer

C: thermistor. A rectangle crossed by a diagonal line that ends in a short horizontal foot is the symbol for a thermistor, a resistor whose resistance changes with temperature. It lets the rice cooker sense how hot the pan is.

Question 2

A smart radiator valve senses the room temperature with a thermistor in series with a fixed resistor. A voltmeter is connected across the fixed resistor.
The room warms up from 15 °C to 22 °C.

What happens to the current in the thermistor and to the reading on the voltmeter?

Accepted Answer

D: increases · increases. Warming lowers the thermistor's resistance, so the total resistance falls and the current rises. The fixed resistor's p.d. is I × 1.5 kΩ, which rises: it now takes a larger share of the 5.0 V, while the thermistor takes a smaller share.

Question 3

In a night light, an LED is connected in parallel with an LDR. This pair is in series with a 1.0 kΩ resistor and a 6.0 V battery. The LED lights only when the p.d. across it is 2.0 V or more.
The resistance of the LDR is 200 Ω in daylight and 20 kΩ in darkness.

Which row shows whether the LED is lit?

Accepted Answer

C: not lit · lit. While the LED is off, the 6.0 V is shared between the LDR and the 1.0 kΩ resistor. In daylight the LDR gets 6.0 × 200 / 1200 = 1.0 V, below 2.0 V, so the LED stays off. In darkness the LDR would get 6.0 × 20 / 21 ≈ 5.7 V, well above 2.0 V, so the LED conducts and lights. The LDR acts as a bypass that steals the p.d. from the LED in daylight.

	current	voltmeter reading
A	decreases	decreases
B	decreases	increases
C	increases	decreases
D	increases	increases