Question

the magnetic field at a distance r from a long wire carrying current I is 0.4 tesla. The magnetic field at a distance 2r from the wire is

Answer 1

We know , magnetic field due to current carrying wire = u(I)/2πr
Here , u and I are constant
So , B(magnetic field ) will be inversely proportional to r
Let initial magnetic field be B and final magnetic field be B'
So , B'/B = r/2r = 1/2
B' = B/2 = 0.4/2 = 0.2T

Answer 2

Magnetic field due to Current carrying wire (i) = (ui)/2×22/7×r)

Let B1 be the magnetic field at distance r:

B1 = (ui)/(2×22/7×r)

Let B2 be the magnetic field at distance 2r:

B2 = (ui)/2×22/7×2r)

Divide the two equations we get:

B1/B2 = 2

B2= B1/2

Since B1 = 0.4 tesla

B2 = 0.4/2

B2 = 0.2 tesla.