Evan's Space

Wonders of Physics

Brightness of bulb as resistance of rheostat increases

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As resistance of the rheostat increases, the total resistance in parallel will increase. Hence potential difference across the parallel bulb Q increases, therefore BRIGHTER.

Total effective resistance of the circuit (include resistance of bulb P) increases, main current thought circuit (bulb P) decreases. Hence bulb P DIMMER.

You can think in terms of p.d of bulb P. Since p.d. across parallel branches increases, p.d across bulb P decreases, hence bulb P is DIMMER.

Without calculation, using resistance and p.d. is easier. To convince yourself, refer to the following diagrams with imaginary numbers.


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