A 10 m long wire of uniform cross section and `20 Omega` resistance is used in a potentiometer. The wire is connected in series with a battery of 5 V along with an external resistance of `480 Omega` . If an unknown emf E is balanced at 6.0 m length of the wire, then the value of unknown emf is:

Options

A. `1.2V`
B. `1.02V`
C. `0.2V`
D. `0.12V`

Answer

D

Explanation

Given `l=10m m,R =20 Omega`
`V=5V. R_("ext")=480 Omega l_(1)=6m`
`I=(E)/(R+R_("ext"))`
`=(5)/(480+20)`
`=(5)/(500)=0.01A`
`V_(PQ)=IR=0.2V`
Potential gradient
`phi_(V)=(V_(PQ))/(l)=0.02V//m`
`E=phi_(V)l_(V)=0.02xx6`
`=0.12V`