A straight wire is placed in a magnetic field that varies with distance from origin as . The ends of the wire are at and and it carries a current . If the force on the wire is , then the value of is
1
1
2
5
3
-1
4
1/2
Official Solution
Correct Option: (3)
Given: - Magnetic field: - Wire lies along x-axis from to - Current in wire = - Force on a current element in magnetic field:
Wire description: - -
Total force:
Evaluating the integral: Given: Equating: Now, since ,
Final Answer:–1
02
PYQ 2022
medium
physicsID: wbjee-20
A horizontal semi-circular wire of radius is connected to a battery through two similar springs and to an electric cell, which sends current through it. A vertically downward uniform magnetic field is applied on the wire, as shown in the figure. What is the force acting on each spring?
1
2πrBI
2
1/2πrBI
3
BIr
4
2BIr
Official Solution
Correct Option: (3)
To determine the force acting on each spring, we need to find the total magnetic force acting on the semi-circular wire and then consider how that force is distributed between the two springs.
Total Magnetic Force on the Semi-circular Wire:
The magnetic force on a current-carrying wire in a magnetic field is given by . For a straight wire of length perpendicular to the magnetic field, the magnitude of the force is simply . However, we have a curved wire.
Instead of integrating, we can calculate the magnetic force on the curved semi-circular wire by considering the effective length. The effective length is the straight-line distance between the endpoints of the curved wire. In this case, the endpoints are separated by a distance equal to the diameter of the semi-circle, which is .
Therefore, the total magnetic force acting on the semi-circular wire is: The direction of this force is outward from the center of the semicircle, due to the right-hand rule.
Force on Each Spring:
Since there are two identical springs and supporting the wire, and the magnetic force acts along the diameter of the semicircle, we can assume that the force is equally distributed between the two springs.
Therefore, the force acting on each spring is half of the total magnetic force: