Question

Two long straight parallel conductors are separated by a distance of 5 cm and carrying current 20 A. what work per unit length of a conductor must be done to increases the separation between conductors to 10 cm, if the current flows in the same direction ?
A. `8xx10^(-5) log_(e)2`
B. `log_(e)2`
C. `10^(-7) log_(e) 2`
D. none of these

Answer 1

Correct Answer - a
The force per unit length is
`F_(0)=(mu_(0)l_(1)l_(2))/(2pi) int_(5xx10^(-2))^(10xx10^(-2)) (dr)/r`
`=(mu_(0)l_(1)l_(2))/(2pi) ln 2 =2xx10^(-7) xx20xx20 ln 2`
`=8xx10^(-5) ln 2=8xx10^(-5) log_(e) 2`