Q18 of 31 Page 1

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 W, the current is 1 A and when R is increased to 9 W, the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.

Capture.PNG


(i)


Capture.PNG


(ii) Graph between terminal voltage (V) and resistance(R)


Graph between terminal voltage (V) and current (I).


(iii) When, R=4Ω and I=1A


We know that,


Terminal voltage V=E-Ir


V=IR=4=E-Ir


R-r=4 ------ (i)


When, R=9Ω and I=0.5A


V=IR=0.5× 9= E-0.5r


E-0.5r=4.5 --------- (ii)


Subtracting (i) from (ii), we get:


E-0.5r-E+r=4.5-4


0.5r=0.5


So, r= 1Ω


Substituting value of r in (i)


E-1 = 4


E=5V


Thus, r= 1Ω and E = 5V


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