# Physics Class 12 NCERT Solutions: Chapter 4 Moving Charges and Magnetism Part 3

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Q: 10. Two moving coil meters, and have the following particulars:

,

,

(The spring constants are identical for the two meters).

Determine the ratio of (a) current sensitivity and (b) voltage sensitivity of and.

Answer:

For moving coil meter:

Resistance,

Number of turns,

Area of cross-section,

Magnetic field strength,

Spring constant

For moving coil meter:

Resistance,

Number of turns,

Area of cross-section,

Magnetic field strength,

Spring constant,

(a) Current sensitivity of is given as:

And, current sensitivity of is given as:

Ratio

Hence, the ratio of current sensitivity of to is.

(B) Voltage sensitivity for is given as:

And, voltage sensitivity for is given as:

Ratio

Hence, the ratio of voltage sensitivity of is.

Q: 11. In a chamber, a uniform magnetic field of is maintained. An electron is shot into the field with a speed of normal to the field. Explain why the path of the electron is a circle. Determine the radius of the circular orbit.

Answer:

Magnetic field strength,

Speed of the electron,

Charge on the electron,

Mass of the electron,

Angle between the shot electron and magnetic field,

Magnetic force exerted on the electron in the magnetic field is given as:

This force provides centripetal force to the moving electron. Hence, the electron starts moving in a circular path of radius r.

Hence, centripetal force exerted on the electron,

In equilibrium, the centripetal force exerted on the electron is equal to the magnetic force i.e. ,

Hence, the radius of the circular orbit of the electron is.