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Question
A cell of emf 'E' and internal resistance 'r' is connected across a variable load resistor R. Draw the plots of the terminal voltage V versus (i) R and (ii) the current I.
It is found that when R = 4 Ω, the current is 1 A and when R is increased to 9 Ω, the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.
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Solution
(i)
Plot for V vs. R
(ii)
When R = 4 Ω and I = 1 A
We know
Terminal voltage, V = E − Ir
So, we have
V = IR = 4 = E − Ir
or
E − r = 4 ..... (i)
When R' = 9 Ω and I' = 0.5 A
V = IR = 0.5 × 9 = E − 0.5r
or
E − 0.5r = 4.5 ..... (ii)
Solving (i) and (ii), we get
r = 1 Ω and E = 5 V
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