In a moving coil galvanometer, there is a coil of copper having number of insulated turns N, each of area A. The coil is suspended in a radial magnetic field B. The moment of inertia of the coil about its rotational axis is I. The scale divisions in the galvanometer are n and resistance of the coil is R.
If the charge Q is passed almost instantaneously through the coil, the angular speed of the coil immediately after this is

Options

A. `(NBAQ)/(I)`
B. `(2NBAQ)/(I)`
C. `(3NBAQ)/(I)`
D. `(NBAQ^2)/(I)`

Answer

A

Explanation

Corresponding to change Q, if the current i flows for time `dt`, then
Torque, `tau=nBAi` or `taudt=nBAidt=nBAQ`
It `omega` is the angular speed of the coil produced when charge Q passes through the coil, then, change in angular momentum
`=Iomega=tau dt=n BAQ`
or `omega=(nBAQ)/(I)`