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Questions on Electric Circuits

Q8. John connects up the circuit shown below.

The bulb is not bright enough. His friend suggests four circuits which could be used to make the bulb brighter.

(a) Which is the correct circuit to use: A, B, C or D?

Circuit A

1 mark

Next John sets up circuit E and notes the reading on the ammeter.

He then places another bulb in the circuit, to make circuit F.

He notes the ammeter reading in circuit F.

(b) How will the ammeter reading in circuit F compare with that in circuit E? Explain your answer.

The reading in circuit F will be lower than that of circuit E. The voltage from the power supply will be shared by the bulbs in circuit F. The current flow will thererfore be lower.OR the resistance in the strand has increased - therefore the current will be less.

The current would only 'halve' if the resistance of a bulb was constant. The filament in a lamp gets hotter as the current through it increases. This means that as you increase the voltage across a bulb the current will increase but not by as much as you might expect - this means it has an increased resistance. A 'dim' bulb has a lower resistance than a brightly lit one. See here for more detail.

2 marks

(c) Draw a circuit diagram in which two bulbs are lit as brightly as the bulb in circuit E, and the ammeter reading is the same as in circuit E.

To light two brightly in series you need an additional power supply as they will share the voltage from the battery.

In parallel an additional strand will have the full voltage from the power supply. Make sure you put the ammeter in a position that measure the current through one bulb - do the combinattion current as shown below.

1 mark

Maximum 4 marks