# Important Questions and Answers: Magnetic field

1. What do you mean by current element?

Ans: Current element is the product of current (I) and small length dl.

i.e., Current element = I

It is used to generate magnetic field.

2. Under what conditions does the charge affect a magnet ?

Ans: Fast moving current can induce magnetic field around the conductor. So, it can affect the magnet.

3. Define 1 Ampere in terms of force.

Ans: We know that the force between two parallel straight conductor carrying conductor is given by:

$$F = \frac{\mu_oI_1I_2l}{2\pi a}$$

When I1 = I2 = 1A, l = 1m, a = 1 m2 then:

$$F = \frac{4 \pi \times 1-^{-7} \times 1\times 1\times 1}{2\pi \times 1}$$

$$\therefore F = 2\times 1-^{-7}N$$

Thus, the equal amount of current passing in two straight parallel conductor placed at 1 metre distance and having length 1 metre  produces force of 2 × 10-17 N on either conductor is called on ampere current.

4. Does a charged particle moving through a magnetic field experience a force? Mention the condition for maximum and minimum force.

Ans: Yes, a charged particle moving through a magnetic field experience force. This force is called Lorentz force..  It is written as F=B.q.v.sinθ

Here, B is the magnetic flux density, q is the charge, v is the velocity of the charge and θ is the angle between direction of v and B.

When θ = 90°, then force = Bqvsin90° = Bqv (maximum)

When θ = 0°, then force = Bqvsin0° = 0(minimum)

5. A current carrying conductor tends to contract. Why?

Ans: When current is passed through a solenoid, there  will be small straight parallel placed conductors carrying current in same direction. So, magnetic force will be developed in the conductors towards each other. Thus, the current carrying solenoid tends to contract.

6. What will happen to the reading of a spring balance when current is passed through it?

Ans: When current is passed through a spring balance, the magnetic force will be developed in the current carrying conductor. So, the spring bends after passing the current.

7. Two straight parallel placed conductors carrying current in the same direction tends to contract. Why?

Ans: If current is passed in two parallel straight conductors, they will be in magnetic field of one another. We know that a current carrying conductor experiences force. As  they experience force towards each other, they tend to contract.

8. Two straight parallel conductors carrying current in opposite direction tends to repel each other. Why ?

Ans: If current is passed in two parallel placed conductors, they will be in magnetic field of one another. They experience force and the direction of force is away from each other. So, they tend to repel each other.

9. Why is cylindrical core of soft iron is used in moving coil galvanometer?

Ans: The cylindrical soft iron core is used in the moving coil galvanometer because:

• It makes the field radial such that  θ = 1.
• It makes the galvanometer dead beat.
• It increases the sensitivity of the galvanometer.

10. What is current sensitivity and voltage sensitivity of a galvanometer?

Ans: The deflection of a galvanometer per unit current is called current sensitivity of galvanometer.$$\text{i.e., } S_I = \frac{ \theta }{I}$$

The deflection of galvanometer per unit voltage is called voltage sensitivity of a galvanometer. $$\text{i.e., } S_v = \frac{ \theta }{V}$$

11. The net charge in a current carrying conductor is zero. But, it experience force in a magnetic field. Why?

Ans: In a current carrying conductor, electron moves with drift velocity. Magnetic force acts on these drifting electrons. The positive ions are almost at rest and hence do not experience any magnetic force. So, the current carrying conductor experience force even when it is neutral.

12. Birds sitting on a high voltage wire flies away as soon as the current is switched on. Why?

Ans: We know that current can pass through the body of the bird as its body is conducting. So, as soon as the current is switched on, the induced current flows through the body of the bird. Now, according to Lenz’s law, the flow of current through the wings is in opposite directions. So, the wings experience mutual repulsion. As a result, the  birds flies away.

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