Published by:
CGP EDU Academic Team
Published on: September 13, 2026
In the arrangements shown in Fig. there is a uniform magnetic filed B 0 normal to the plane of paper. The connector is smooth and conducting and is has a mass m and length l.

The connector is pushed against the spring so that the spring has compression x 0 . The connector is released at t = 0. Find the time it will take to come to its original position again. The spring is non-conducting. The resistance of the rails is zero and neglect its self-inductance.
Text Solution
Verified by ExpertsThe correct answer is:
A
Step 1: Analyze the forces acting on the connector. The magnetic force acting on the connector when it moves in the magnetic field is given by the Lorentz force $$ F = B imes I $$, where I is the current flowing through the connector.
Step 2: Since the connector is released from a compressed position, it will experience a spring force directed back towards its equilibrium position, which is given by Hooke's Law as $$ F = -kx $$, where k is the spring constant and x is the displacement from the equilibrium position.
Step 3: Write the equation of motion for the connector. The net force will be the sum of the magnetic force and the spring force:
$$ m \frac{d^2x}{dt^2} = B \times I - kx \text{ (Hooke's Law)} $$
Substitute the expression for current I as $$ I = \frac{dQ}{dt} = \frac{Bl}{dt} $$, leading to a differential equation that resembles simple harmonic motion (SHM).
Step 4: The time period for SHM is given by $$ T = 2\pi \sqrt{\frac{m}{k}} $$. To find the time to return to the original position, note that after half a period, it will be at the equilibrium point. This means the time taken to return to the original position after being released is given by:
$$ t = \frac{T}{2} = \frac{\pi}{2} \sqrt{\frac{m}{k}} $$. Therefore, it takes a time of $$ \frac{\pi}{2} \sqrt{\frac{m}{k}} $$ to return to its original position.
Step 2: Since the connector is released from a compressed position, it will experience a spring force directed back towards its equilibrium position, which is given by Hooke's Law as $$ F = -kx $$, where k is the spring constant and x is the displacement from the equilibrium position.
Step 3: Write the equation of motion for the connector. The net force will be the sum of the magnetic force and the spring force:
$$ m \frac{d^2x}{dt^2} = B \times I - kx \text{ (Hooke's Law)} $$
Substitute the expression for current I as $$ I = \frac{dQ}{dt} = \frac{Bl}{dt} $$, leading to a differential equation that resembles simple harmonic motion (SHM).
Step 4: The time period for SHM is given by $$ T = 2\pi \sqrt{\frac{m}{k}} $$. To find the time to return to the original position, note that after half a period, it will be at the equilibrium point. This means the time taken to return to the original position after being released is given by:
$$ t = \frac{T}{2} = \frac{\pi}{2} \sqrt{\frac{m}{k}} $$. Therefore, it takes a time of $$ \frac{\pi}{2} \sqrt{\frac{m}{k}} $$ to return to its original position.
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