NCERT Physics Class 12 Exemplar Ch 6 Electromagnetic Inductions Part 1

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Very Short Answer

Q 11. Consider a magnet surrounded by a wire with an on/off switch S If the switch is thrown from the off position (open circuit) to the on position (closed circuit), will a current flow in the circuit?

Explain.

Magnet surrounded by wire

Magnet Surrounded by Wire

Magnet surrounded by wire

Ans:

  • No part of the wire is moving and so motional emf. Is zero. The magnet is stationary and hence the magnetic field does not change with time.

  • This means no electromotive force is produced and hence no current will flow in the circuit.

Q 12. A wire in the form of a tightly wound solenoid is connected to a DC source, and carries a current. If the coil is stretched so that there are gaps between successive elements of the spiral coil, will the current increase or decrease? Explain.

Ans:

  • The current will increase. As the wires are pulled apart the flux will leak through the gaps. Lenz’s law demands that induced emf.

  • Resist this decrease, which can be done by an increase in current.

Q 13. A solenoid is connected to a battery so that a steady current flows through it. If an iron core is inserted into the solenoid, will the current increase or decrease? Explain.

Ans:

  • The current will decrease. As the iron core is inserted in the solenoid, the magnetic field increases and the flux increases.

  • Lent’s law implies that induced emf.

  • Should resist this increase, which can be achieved by a decrease in current.

Q 14. Consider a metal ring kept on top of a fixed solenoid (say on a carboard). The centre of the ring coincides with the axis of the solenoid. If the current is suddenly switched on, the metal ring jumps up.

Explain

Metal ring kept on top of a fixed solenoid (ans)

Metal Ring Kept on Top of a Fixed Solenoid (Ans)

Metal ring kept on top of a fixed solenoid (ans)

Ans:

  • No flux was passing through the metal ring initially. When the current is switched on, flux passes through the ring.

  • According to Lenz’s law this increase will be resisted and this can happen if the ring moves away from the solenoid. One can analyze this in more detail.

  • If the current in the solenoid is as shown, the flux (downward) increases and this will cause a counterclockwise current (as seen from the top in the ring).

  • As the flow of current is in the opposite direction to that in the solenoid, they will repel each other and the ring will move upward.

Q 15. Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current I. The centre of the ring coincides with the axis of the solenoid. If the current in the solenoid is switched off, what will happen to the ring?

Metal ring kept on top of a fixed solenoid

Metal Ring Kept on Top of a Fixed Solenoid

Metal ring kept on top of a fixed solenoid

Ans:

  • When the current in the solenoid decreases a current flows in the same direction in the metal ring as in the solenoid.

  • Thus there will be a downward force. This means the ring will remain on the cardboard.

Q 16. Consider a metallic pipe with an inner radius of 1 cm. If a cylindrical bar magnet of radius 0.8cm is dropped through the pipe, it takes more time to come down than it takes for a similar unmagnified cylindrical iron bar dropped through the metallic pipe.

Explain.

Ans:

  • For the magnet, eddy currents are produced in the metallic pipe. These currents will oppose the motion of the magnet.

  • Therefore magnet’s downward acceleration will be less than the acceleration due to gravity g. On the other hand, an unmagnified iron bar will not produce eddy currents and will fall with an acceleration g.

  • Thus the magnet will take more time.