# Conducting loop placed in magnetic field

**1. The problem statement, all variables and given/known data**

A uniformly conducting wire is bent to form a ring of mass m and radius r, and the ring is placed on a rough horizontal surface with its plane horizontal. There exists a uniform and constant horizontal magnetic field of induction B. Now a charge q is passed through the ring in a very small time interval Δt. As a result the ring ultimately just becomes vertical. Find the value of g. Assume that friction is sufficient to prevent slipping and ignore any loss in energy.

**2. Relevant equations**

**3. The attempt at a solution**

Let the initial angular velocity of the loop be ω. Since energy is conserved,

[itex]\dfrac{I \omega ^2}{2} = mgr[/itex]

I know the moment of inertia but I’m still perplexed as why does the loop becomes vertical when a charge is passed through it? I have spent almost a day thinking about this.:confused: I really need some help.

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