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Question
The circuit shown in figure, contains a battery, a rheostat and two identical lamps. What will happen to the brightness of the lamps if the resistance of the rheostat is increased?
The circuit shown in figure, contains a battery, a rheostat and two identical lamps. What will happen to the brightness of the lamps if the resistance of the rheostat is increased?
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Solution
The correct option is A
Lamp P
Lamp Q
(a)
Less brighter
Brighter
Consider two extreme cases.
∙ When the resistance of the rheostat is zero whole current will pass through the rheostat such that the current through Q is zero or Q will be short circuited. The circuit is then essentially a battery in series with lamp P and lamp P will glow brightly.
∙ When the resistance of the rheostat is increased slightly some current starts flowing through the lamp Q. Thus it starts glowing very dim.
When the resistance of the rheostat is increased considerably the current in the lamp Q and rheostat section moves through the lamp Q owing to increased resistance in the rheostat path. It is the case of a series connection involving lamp P, lamp Q and the battery. This new voltage drop alongside the lamp Q will reduce the voltage drop along lamp P. Thus the lamp P gets less bright and the lamp Q brightens.
Hence, option (a) is correct.
| Lamp P | Lamp Q | |
| (a) | Less brighter | Brighter |
Consider two extreme cases.
∙ When the resistance of the rheostat is zero whole current will pass through the rheostat such that the current through Q is zero or Q will be short circuited. The circuit is then essentially a battery in series with lamp P and lamp P will glow brightly.
∙ When the resistance of the rheostat is increased slightly some current starts flowing through the lamp Q. Thus it starts glowing very dim.
When the resistance of the rheostat is increased considerably the current in the lamp Q and rheostat section moves through the lamp Q owing to increased resistance in the rheostat path. It is the case of a series connection involving lamp P, lamp Q and the battery. This new voltage drop alongside the lamp Q will reduce the voltage drop along lamp P. Thus the lamp P gets less bright and the lamp Q brightens.
Hence, option (a) is correct.
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