Question

Comprehension:

A conducting rod of mass $m$ and length $l$ is released from rest on smooth metallic rails placed in vertical plane in a uniform horizontal magnetic field $\left(B\right)$ as shown in figure.

When rod falls, it would cut magnetic field lines and motional emf will be induced. Velocity and acceleration of rod will change with time and after a long time rod will achieve a maximum velocity called as terminal velocity.


Answer the following question based on above discussion.

$\left(i\right)$ The initial acceleration $(t=0)$ of the rod will be

• A. $g$ downwards
• B. Less than $g$ downwards
• C. More than $g$ downwards
• D. Less than $g$ upwards

Answer 1

The correct option is A $g$ downwards

From FBD of the rod we get,


At $t=0$, the only force acting on the rod is $mg$ downwards,

$\because$ At $t=0$, no current flows through the rod, we can say that magnetic force is zero.

$\therefore a=\frac{F}{m}=\frac{mg}{m}=g$ downwards

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(A\right)$ is the correct answer.