# A long straight wire carries a current of 35 A.

Question:

A long straight wire carries a current of 35 A. What is the magnitude of the field at a point 20 cm from the wire?

Solution:

Current in the wire, I = 35 A

Distance of a point from the wire, r = 20 cm = 0.2 m

Magnitude of the magnetic field at this point is given as:

$B=\frac{\mu_{0}}{4 \pi} \frac{2 I}{r}$

Where,

$\mu_{0}=$ Permeability of free space $=4 \pi \times 10^{-7} \mathrm{~T} \mathrm{~m} \mathrm{~A}^{-1}$

$B=\frac{4 \pi \times 10^{-7} \times 2 \times 35}{4 \pi \times 0.2}$

$=3.5 \times 10^{-5} \mathrm{~T}$

Hence, the magnitude of the magnetic field at a point $20 \mathrm{~cm}$ from the wire is $3.5 \times 10^{-5} \mathrm{~T}$.