For `c=2a` if, the magnetic field at point P will be zero when

Options

A. `a=b`
B. `a=3/5b`
C. `a=5/3b`
D. `a=1/3b`

Answer

C

Explanation

(c) Magnetic field at the centre
`B=(mu_0I)/(2c)ox+(mu_0I)/(2b)(3/4)odot +(mu_0I)/(2a)(3/4)ox`
As per problem:
`0=(mu_0I)/2[1/c-3/(4b)+3/(4a)] implies 3/4[1/b-1/a]=1/c`
If `c=2a`,
`3/4[1/b-1/a]=1/(2a) implies a=5/3b`