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JEE Advanced 2016 Paper 1 · Physics · Current Electricity · medium

Two cells of emf 6 V (internal resistance 1 Ω) and 4 V (internal resistance 2 Ω) are connected in parallel, positive terminal to positive terminal…

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Two cells of emf 6 V6\ \mathrm{V} (internal resistance 1 Ω1\ \Omega) and 4 V4\ \mathrm{V} (internal resistance 2 Ω2\ \Omega) are connected in parallel, positive terminal to positive terminal, and the combination is connected across an external resistor of 3 Ω3\ \Omega. Which of the following statements is/are correct?
  1. A. The 4 V4\ \mathrm{V} cell is being charged by the 6 V6\ \mathrm{V} cell.
  2. B. The current through the 3 Ω3\ \Omega resistor is 16/11 A16/11\ \mathrm{A}.
  3. C. The equivalent emf of the parallel combination is 5 V5\ \mathrm{V}.
  4. D. The potential difference across the 3 Ω3\ \Omega resistor is 4 V4\ \mathrm{V}.
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Answer: A, B

Let VV be the common terminal voltage. Kirchhoff's current law at the positive node gives 6−V1+4−V2=V3\dfrac{6-V}{1}+\dfrac{4-V}{2}=\dfrac V3, i.e. 8=116V8=\dfrac{11}{6}V, so V=4811≈4.36 VV=\dfrac{48}{11}\approx4.36\ \mathrm{V}. The current in the resistor is V/3=16/11 A≈1.45 AV/3=16/11\ \mathrm{A}\approx1.45\ \mathrm{A} (B true) and the potential difference is 4.36 V4.36\ \mathrm{V}, not 4 V4\ \mathrm{V} (D false). The current in the 6 V6\ \mathrm{V} cell is (6−V)/1≈1.64 A(6-V)/1\approx1.64\ \mathrm{A}, while for the 4 V4\ \mathrm{V} cell (4−V)/2≈−0.18 A(4-V)/2\approx-0.18\ \mathrm{A} is negative: since V>4V>4 the current enters its positive terminal, so it is being charged (A true). The equivalent emf of two parallel cells is E1/r1+E2/r21/r1+1/r2=81.5=5.33 V\dfrac{E_1/r_1+E_2/r_2}{1/r_1+1/r_2}=\dfrac{8}{1.5}=5.33\ \mathrm{V}, not 5 V5\ \mathrm{V} (C false).
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