Two long and parallel straight wires A and B carrying currents of 8.0 A and 5.0 \mathrm{~A} in the same direction are separated by a distance of 4.0 \mathrm{~cm}. Estimate the force on a 10 \mathrm{~cm} section of wire A.
Two long and parallel straight wires A and B carrying currents of 8.0 A and 5.0 \mathrm{~A} in the same direction are separated by a distance of 4.0 \mathrm{~cm}. Estimate the force on a 10 \mathrm{~cm} section of wire A.

Ans: We are given the following:
Current flowing in wire A_{1} I_{A}=8.0 \mathrm{~A}
Current flowing in wire B, I_{B}=5.0 \mathrm{~A}
Distance between the two wires, r=4.0 \mathrm{~cm}=0.04 \mathrm{~m}
Length of a section of wire \mathrm{A}_{4} \mathrm{I}=10 \mathrm{~cm}=0.1 \mathrm{~m}
Force exerted on length I due to the magnetic field could be given as:

    \[\begin{array}{l} \mathrm{B}=\frac{\mu_{0} 2 I_{\mathrm{A}} \mathrm{I}_{\mathrm{A}} \mathrm{I}}{4 \pi r} \\ \text { Where, } \mu_{0}=\text { Permeability of free space }=4 \pi \times 10^{-7} \mathrm{~T} \mathrm{~m} \mathrm{~A}^{-1} \\ \Rightarrow \mathrm{B}=\frac{4 \pi \times 10^{-7} \times 2 \times 8 \times 5 \times 0.1}{4 \pi \times 0.04} \\ \Rightarrow \mathrm{B}=2 \times 10^{-5} \mathrm{~N} \end{array}\]

The magnitude of force is found to be 2 \times 10^{-5} \mathrm{~N}. This is an attractive force that is normal to A towards B because the direction of the currents in the wires are the same.