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Questions on Electric Circuits

Q13.

A circuit is shown below.

(a) The switch is open. Steven connects point A to point B with a piece of copper wire. Which bulbs, if any, light up?

Bulb X

 

1 mark

(b) Steven removes the copper wire and uses it to connect point C to point D. The switch is still open. Which bulbs, if any, light up?

Neither light up.

1 mark

 

(c) Steven removes the copper wire and closes the switch. Both bulbs light up, but not very brightly. He then uses the copper wire to connect point B to point C.

(i) What happens to bulb X?

It goes out

The wire formed a short circuit across bulb X. Bulb X was in parallel with the wire and the current through a parallel system flows primarily through the lowest resistance branch - in this case the shorting wire. Virtually no current flows through the bulb so it goes out.

1 mark

(ii) What happens to bulb Y?

It gets brighter.

The voltage across the two bulbs before the wire was added was equally shared. As soon as bulb X was part of the low resistance parallel arrangement the voltage share changed - bulb Y gets virtually all of the voltage as it has a much higher resistance than the parallel arrangement does. The bigger voltage across it makes a higher current flow through it... it thereforegets brighter.

1 mark

(d) Steven removes the copper wire. The switch is still closed. Both bulbs light up, but not very brightly. He then uses the copper wire to connect point A to point B.

(i) What happens to bulb X?

It gets brighter.

The voltage across the two bulbs before the wire was added was equally shared (therefore they were not very bright). As soon as bulb Y became part of a low resistance parallel arrangement with the shorting wire, the voltage share changed - bulb X gets virtually all of the voltage as it has a much higher resistance than the parallel arrangement does. The bigger voltage across it makes a higher current flow through it... it thereforegets brighter.

1 mark

(ii) What happens to bulb Y?

It goes out.

The wire formed a short circuit across bulb Y. Bulb Y is parallel with the wire and the current through a parallel system flows primarily through the lowest resistance branch - in this case the shorting wire. Virtually no current flows through the bulb so it goes out.

1 mark

Maximum 6 marks