# Physics Class 12 NCERT Solutions: Chapter 4 Moving Charges and Magnetism Part 7

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Q: 20. A magnetic field set up using Helmholtz coils (described in Exercise16 (Reference: Chapter 4 Moving Charges And Magnetism Part 5)) is uniform in a small region and has a magnitude of . In the same region, a uniform electrostatic field is maintained in a direction normal to the common axis of the coils. A narrow beam of (single species) charged particles all accelerated through enters this region in a direction perpendicular to both the axis of the coils and the electrostatic field. If the beam remains undeflected when the electrostatic field is, make a simple guess as to what the beam contains. Why is the answer not unique?

Magnetic field,

Accelerating voltage,

Electrostatic field,

Mass of the electron

Charge of the electron

Velocity of the electron

Kinetic energy of the electron

Since the particle remains undeflected by electric and magnetic fields, we can infer that the electric field is balancing the magnetic field.

Putting equation (2) in equation (1), we get

This value of specific charge e/m is equal to the value of deuteron or deuterium ions. This is not a unique answer. Other possible answers are, etc.

Q: 21. A straight horizontal conducting rod of length and mass is suspended by two vertical wires at its ends. A current of is set up in the rod through the wires.

(A) What magnetic field should be set up normal to the conductor in order that the tension in the wires is zero?

(B) What will be the total tension in the wires if the direction of current is reversed keeping the magnetic field same as before? (Ignore the mass of the wires.) .

Length of the rod,

Mass suspended by the wires,

Acceleration due to gravity,

Current in the rod flowing through the wire,

(A) Magnetic field (B) is equal and opposite to the weight of the wire i.e.,

A horizontal magnetic field of normal to the length of the conductor should be set up in order to get zero tension in the wire. The magnetic field should be such that Fleming’s left hand rule gives an upward magnetic force.

(b) If the direction of the current is revered, then the force due to magnetic field and the weight of the wire acts in a vertically downward direction.

∴Total tension in the wire

Q: 22. The wires which connect the battery of an automobile to its starting motor carry a current of (for a short time). What is the force per unit length between the wires if they are long and apart? Is the force attractive or repulsive?

Current in both wires,

Distance between the wires,

Length of the two wires,

Force between the two wires is given by the relation,

Where,

Permeability of free space

Since the direction of the current in the wires is opposite, a repulsive force exists between them.

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