# Physics Class 12 NCERT Solutions: Chapter 4 Moving Charges and Magnetism Part 10 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 26. A solenoid long and of radius has layers of windings of turns each. long wire of mass lies inside the solenoid (near its centre) normal to its axis; both the wire and the axis of the solenoid are in the horizontal plane. The wire is connected through two leads parallel to the axis of the solenoid to an external battery which supplies a current of in the wire. What value of current (with appropriate sense of circulation) in the windings of the solenoid can support the weight of the wire?

Length of the solenoid,

It is given that there are layers of windings of turns each.

Total number of turns,

Length of the wire,

Mass of the wire,

Current flowing through the wire,

Acceleration due to gravity,

Magnetic field produced inside the solenoid,

Where,

Permeability of free space

Current flowing through the windings of the solenoid

Magnetic force is given by the relation,

Also, the force on the wire is equal to the weight of the wire.

Hence, the current flowing through the solenoid is 108 A.

Q: 27. A galvanometer coil has a resistance of and the metre shows full scale deflection for a current of . How will you convert the metre into a voltmeter of range to ?

Resistance of the galvanometer coil,

Current for which there is full scale deflection,

Range of the voltmeter is 0, which needs to be converted to .

Let a resistor of resistance R be connected in series with the galvanometer to convert it into a voltmeter. This resistance is given as:

A resistor of resistance is to be connected in series with the galvanometer.

Q: 28. A galvanometer coil has a resistance of and the metre shows full scale deflection for a current of . How will you convert the metre into an ammeter of range to ?