# NCERT Physics Class 12 Exemplar Ch 4 Moving Charges and Magnetism Part 4

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Q.17 A current carrying loop consists of identical quarter circles of radius , lying in the positive quadrants of the and planes with their centers at the origin, joined together. Find the direction and magnitude of at the origin.

Solution:

Q.18 A charged particle of charge and mass is moving in an electric field and magnetic field . Construct dimensionless quantities and quantities of dimension .

Solution:

No dimensionless quantity

Q.19 An electron enters with a velocity into a cubical region (faces parallel to coordinate planes) in which there are uniform electric and magnetic fields. The orbit of the electron is found to spiral down inside the cube in plane parallel to the plane. Suggest a configuration of fields and that can lead to it.

Solution:

Q.20 Do magnetic forces obey Newton’s third law. Verify for two current elements located at the origin and located at . Both carry current.

Solution:

Force due to on is zero.

Force due to on is non-zero.

Q.21 A multirange voltmeter can be constructed by using a galvanometer circuit as shown in Fig. We want to construct a voltmeter that can measure using a galvanometer of resistance and that produces maximum deflection for current of . Find that have to be used.

Solution:

For range

For range

And For range

Gives

And

Q.22 A long straight wire carrying current of rests on a table as shown in Fig. Another wire of length , mass carries the same current but in the opposite direction. The wire

is free to slide up and down. To what height will rise?