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CBSE Class 12 Physics 2016 Outside Delhi Set 1 Paper

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Question : 5 of 26
Marks: +1, -0
The plot of the variation of potential difference across a combination of three identical cells in series, versus current is shown below. What is the emf and internal resistance of each cell?
Solution:  
Voltage across cell combination
V=E−IrV = E - I r
When current I=0⇒V=EI = 0 \Rightarrow V = E
Form graph, when I=0,V=6 VI = 0, V = 6\,\mathrm{V}
⇒emfE=6 V\Rightarrow \mathrm{emf} \\ E = 6\,\mathrm{V}
As     I=1 A\; \; I = 1\,\mathrm{A}, at V=0V = 0 (from graph)
∴V=E−Ir⇒0=6−1r\therefore V = E - I r \Rightarrow 0 = 6 - 1 r ⇒−6=−1r\Rightarrow -6 = -1 r
r=6 Ωr = 6\,\Omega
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