In an electric circuit shown in the figure, the external resistance R is variable. It is found that same power is developed in external resistance R if its value is either 5 Ω or 20 Ω. With this information find the internal resistance (r) of the voltage source.

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20 Ω

8 Ω

103 Ω

10 Ω

answer is D.

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Detailed Solution

I= ER+r, P=I2R=E2RR+r2Now P1=P2 ⇒ E255+r2 = E22020+r2 ⇒ r=10 Ω

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