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JEE Advanced 2024 Paper 2 · Physics · Current Electricity · medium

A semi-infinite ladder network is made of identical sections. Each section has a 1 Ω resistor in series along the top rail followed by a 2 Ω resistor…

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A semi-infinite ladder network is made of identical sections. Each section has a 1 Ω1\ \Omega resistor in series along the top rail followed by a 2 Ω2\ \Omega resistor connected across the two rails. A battery of emf 12 V12\ \mathrm{V} and internal resistance 1 Ω1\ \Omega is connected across the input terminals of the first section. The power dissipated, in watts, in the first 2 Ω2\ \Omega shunt resistor is
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Answer: 8

Let the input resistance of the infinite ladder be RxR_x. Removing one section leaves an identical ladder, so Rx=1+2Rx2+RxR_x=1+\dfrac{2R_x}{2+R_x}, giving Rx2−Rx−2=0R_x^2-R_x-2=0 and Rx=2 ΩR_x=2\ \Omega. The battery current is I=121+2=4 AI=\dfrac{12}{1+2}=4\ \mathrm{A} and the terminal voltage is 12−4×1=8 V12-4\times1=8\ \mathrm{V}. The first series resistor drops 4×1=4 V4\times1=4\ \mathrm{V}, so the voltage at the first node (across the first 2 Ω2\ \Omega shunt resistor) is 8−4=4 V8-4=4\ \mathrm{V}. Check: the shunt 2 Ω2\ \Omega is in parallel with the remaining ladder of 2 Ω2\ \Omega, giving 1 Ω1\ \Omega, and 4 A×1 Ω=4 V4\ \mathrm{A}\times1\ \Omega=4\ \mathrm{V}. The power in the first shunt resistor is P=V2R=422=8 WP=\dfrac{V^2}{R}=\dfrac{4^2}{2}=8\ \mathrm{W}.
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