# NCERT Physics Class 12 Exemplar Ch 6 Electromagnetic Inductions Part 6

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Q 30 Find the current in the sliding rod AB (resistance = R) for the arrangement shown in. B is constant and is out of the paper. Parallel wires have no resistance. v is constant. Switch S is closed at time

Ans:

At

Q 31. A metallic ring of mass m and radius l (ring being horizontal) is falling under gravity in a region having a magnetic field. If z is the vertical direction, the z-component of magnetic field is If R is the resistance of the ring and if the ring falls with a velocity v, find the energy lost in the resistance. If the ring has reached a constant velocity, use the conservation of energy to determine v in terms of and acceleration due to gravity g.

Ans:

= rate of change in flux

Energy lost/second

This must come from rate of change in PE

(as kinetic energy is constant for constant)

Thus,

Or,

Q 32. A long solenoid ‘S’ has ‘n’ turns per meter, with diameter ‘a’. At the centre of this coil we place a smaller coil of ‘N’ turns and diameter ‘b’ (where a). If the current in the solenoid increases linearly, with time, what is the induced emf appearing in the smaller coil. Plot graph showing nature of variation in emf, if current varies as a function of

Ans:

Magnetic field due to a solenoid S,

Magnetic flux in smaller coil where

So

Negative sign signifies opposite nature of induced emf. The magnitude of emf varies with time as shown in the figure.