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GCSE Standard Questions: Electric Circuits

Q16. The photo shows some hair straighteners.

Hair straighteners contain heating elements.

(a) When the hair straighteners reach normal operating temperature, an LED turns on. Draw the circuit symbol for an LED in the box below.

[1 mark]

(b) The diagram below shows the circuit diagram for the hair straighteners.

 

Each resistor represents a heating element.

The power output of the hair straighteners can be changed by closing different switches.

Why do the hair straighteners not turn on when only switch S2 is closed?

S1 creates a break/gap in the circuit - there is therefore not a complete circuit to the resistor from the power supply even if S2 is closed.

[1 mark]

(c) The hair straighteners have a maximum power output of 120 W.

The energy transferred to the hair straighteners to reach normal operating temperature is 3.6 kJ.

Calculate the time taken for the hair straighteners to reach normal operating temperature when operating at maximum power.

Power = energy/time

P = E/t

120 = 3600/t

t = 3600/120 = 30 seconds

[4 marks]

(d) The following diagram shows the hair straighteners circuit with switch S1 closed.

Switch S2 and switch S3 are then closed at the same time.

Explain what happens to the power output of the power supply.

The total resistance of the circuit decreases so the current through the power supply increases which increases the power output

OR

When switch S1 is closed current passes through the two top resistors.

Closing switch S2 causes an equal current to flow through the middle two, and switch S3 being closed causes an equal current to flow through the bottom two.

Therefore three times the current is drawn from the power supply. The potential difference supplied does not change, but three times the power output is attained.

[3 marks]

(Total 9 marks)