Question

Four infinite long wires carrying equal current are placed parallel and equidistant as shown in figure. Then magnitude of force per unit length.

• A. on $1$ and $4$ are equal.
• B. on $2$ and $3$ are equal.
• C. on $2$ is maximum.
• D. on $4$ is minimum

Answer 1

Answer: (A), (B)

on $2$ and $3$ are equal.

Magnetic field in different regions are as shown in the figure.



From the figure, it is evident that, Net magnetic field in regions $\text{I}$ and $\text{V}$ are same. Similarly, net magnetic field in regions $\text{II}$ and $\text{IV}$ are same.

Thus, magnetic force acting on wires $1$ and $4$ are equal but in opposite directions.

Also, the magnetic force acting on wires $2$ and $3$ will be equal and opposite.

Since, the wires attract each other if current are in the same direction and repel each other if current are in opposite directions, we can deduce that, options (c) and (d) are wrong alternatives.

Hence, options (a) and (b) are the correct answers.