Q. 18

# A cell of emf ‘E’ (i) (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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