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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