Q14 of 32 Page 35

Consider a metal ring kept on top of a fixed solenoid (say on a cardboard) (Fig 6.5). The center of the ring coincides with the axis of the solenoid. If the current is suddenly switched on, the metal ring jumps up. Explain.


When current is suddenly switched on, the upper end becomes south-pole. By Lenz’s law the induced current produce south-pole lower side and north upper side so repelled and jumps up.


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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.

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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.

 15

Consider a metal ring kept (supported by a cardboard) on top of a fixed solenoid carrying a current I (see Fig 6.5). The center 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?

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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 un-magnetized cylindrical iron bar dropped through the metallic pipe.