# NCERT Physics Class 12 Exemplar Ch 6 Electromagnetic Inductions Part 2

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Q 17. A magnetic field in a certain region is given by and a coil of radius a with resistance R is placed in the x-y plane with its centre at the origin in the magnetic field. Find the magnitude and the direction of the current at at and.

Ans:

Flux through the ring

Current at

Q 18. Consider a closed loop C in a magnetic field. The flux passing through the loop is defined by choosing a surface whose edge coincides with the loop and using the formula . Now if we chose two different surfaces and having C as their edge, would we get the same answer for flux?

Ans:

• One gets the same answer for flux. Flux can be through of as the number of magnetic field lines passing through the surface (we draw lines in an area), as lines of of B cannot end or start in space (they form closed loops) number of lines passing through surfaceust be the same as the number of lines passing through the surface

Q 19. Find the current in the wire for the configuration shown in. Wire has negligible resistance. B, the magnetic field is coming out of the paper. is a fixed angle made by travelling smoothly over two conducting parallel wires Separated by a distance d.

Ans:

Motional electric field E along the dotted line CD E.M.F. along

Therefore,

and is independent of q.

Q 20. A (current vs time) graph of the current passing through a solenoid is shown in. For which time is the back electromotive force a maximum. If the back emf at is e, find the back emf at s, 15s and 40s. OA, AB and BC are straight line segments.

Ans:

Maximum rate of change of current is in AB. So maximum back emf will be obtained between.

If (L is a constant)

For

Thus at

For

e

For

Q 21 There are two coils A and B separated by some distance. If a current of 2 A flows through A, a magnetic flux of Wb passes through B (no current through B). If no current passes through A and a current of 1 A passes through B, what is the flux through A?

Ans:

Mutual inductance

Flux

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