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Questions on Electric Circuits

Q17. Imran built a puzzle circuit with three identical bulbs and a 3V battery.

He covered the connections to the bulbs with a piece of card as shown below.

The bulbs could be seen through holes in the card. All the bulbs were on but their brightness was different.

Lucy removed bulbs A, B and C in turn.

Before connecting each bulb back into the circuit she observed the effect on the other two bulbs. She recorded her observations in the table below.

bulb removed

observations
Comment

A

B and C stayed on

If A can be removed without affecting B amd C we can deduce that A is in parallel with B and C.

A must be a single bulb on a trand connected to the battery.

B

C went off
A stayed on

 We would expect A to stay on as it is on its own separate strand. The fact that C went off shows C must be in series with B.

C

B went off
A stayed on

This confirms we were right!

A must be on its own strand in parallel with a strand that has B and C in series with each other.

(a) Complete the circuit diagram below to show how the three bulbs could be connected. Use your knowledge of series and parallel circuits, and the observations in the table to help you.

one mark for B and C in series

one mark for A in parallel with B and C

2 marks

(b) Imran used three identical bulbs but their brightness was different. Which bulb was the brightest (give the letter)? Give the reason for your choice.

A was the brightest.

Bulb A has the full voltage from the battery across it, whereas bulbs B and C only have half of that voltage each. Therefore a higher current flows through A - making it brighter.

OR The strand with bulb A on it has less resistance of the strand with B and C on it. Therefore the current flowing through it will be higher.

When answering questions like this give a comparison - mention BOTH things that are being compared - use words like brighter and higher... compare and contrast!

2 marks maximum

(c) Imran added a switch to the circuit so that he could turn all three bulbs on and off at the same time. Place a letter S on your circuit diagram where this switch could be placed.

See the diagram

1 mark

Total 5 marks