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Questions on Electric Circuits

Q14. Gary uses the following circuit to operate the electric motor of his model crane. Look carefully at the way Gary has connected the two cells.

When he closes switch A the motor runs and the crane lifts a load.

(a) Gary opens switch A and closes switch B. Describe what happens to the motor.

The motor turns in the opposite direction - goes into reverse mode.

1 mark

(b) Gary closes both switches, A and B. Describe what happens to the motor.

It stops turning.

The two switches closed form a loop with the cells that bypasses the motor. It forms a short-circuit.

1 mark

(c) Both switches should not be closed at the same time. Explain why.

  • the batteries or cells or wires will get hot OR
  • the batteries or cells will go flat quickly

1 mark

(d) Gary puts a resistor into his circuit as shown.

What difference does the resistor make to the motor:

(i) when switch A is closed and switch B is open?

The motor turns forward just as it did before.

1 mark

(ii) when switch A is open and switch B is closed?

The motor turns in reverse mode - be more slowly than before.

The resistor in the reverse strand (the one with switch B in it) increases the resistance in the strand - therefore resucing the current flowing in it. That means the motor will turn at a slower rate.

1 mark

Maximum 5 marks