Chapter 4 Moving Charges and assertation & reasoning Questions assertation & reasoning Questions class 12th physics

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1. If both assertion and reason are true and reason is the correct explanation of assertion.
2. If both assertion and reason are true, but reason is not the correct explanation of assertion. 
3. If the assertion is true, but the reason is false.
4. If the assertion is false, but the reason is true.

1 . Assertion When a test charge moves through the magnetic field, its momentum changes but kinetic energy remains same.

Reason The magnetic force acts as a centripetal force, which is perpendicular to the instantaneous velocity and so does no work.

1 . (1)

Kinetic energy of the charged particle remains same in the circular path while velocity and momentum of the particle changes because of continous change in the direction of motion.

2. Assertion Magnetic field interacts with a moving charge.

Reason A moving charge produces a magnetic field.

2. (1)

A moving charge experience a force in magnetic field. It is because of interaction of two magnetic fields, one which is produced due to the motion of charge and other in which charge is moving.

3. Assertion Free electron always keeps on moving in a conductor even then no magnetic force act on them in magnetic field unless a current is passed through it.

Reason The average velocity of free electron is zero.

3. (1)

In the absence of electric field, the free electron in a conductor are in a state of random motion, like molecules in a gas. Their average velocity is zero, i.e, they do not have any net magnetic force on the free electrons in the magnetic field. On passing the current, the free electrons acquire drift velocity in a definite direction, hence magnetic force acts on them, unless the field has no perpendicular component.

4. Assertion Two beam of electrons traveling in the same direction repel each other.

Reason The electrostatic interaction is less than the magnetic interaction.

4. (3)

Two beams of electron traveling in the same direction repel each other because the electrostatic interaction is more that the magnetic interaction.

5. Assertion If the current in a solenoid is reversed in direction while keeping the same magnitude, the magnetic field energy stored in the solenoid decreases.

Reason Magnetic field energy density is proportional to square of current.

5. (4)

Reversing the direction of the current reverses the direction of magnetic field. However, it has no effect on the magnetic field energy density, which is proportional to the square of the magnitude interaction.

6. Assertion The magnetic field produced by a current carrying solenoid is independent of its length and cross-sectional area.

Reason The magnetic field inside the solenoid is uniform.

6. (2)

The earth’s magnetic field is towards north and the velocity of electron is vertically downward. Applying Fleming’s left hand rule, the direction of force is towards west. Therefore, an electron coming from outer space will be deflected toward west.

7. Assertion If two long wire, hanging freely an connected to a battery in series, they come closer to each other.

Reason Force of attraction acts between the two wires carrying current.

7. (4)

When two long parallel wires, are connected to a battery in series. They carry current in opposite directions, hence they repel each other.

8. Assertion In a shunted galvanometer only 10% current passes through the galvanometer. The resistance of the galvanometer is G. Then resistance of the shunt in G/9.

Reason If S is the resistance of the shunt, then voltage across S and G is same.

8. (2)

IgG = (I-Ig)S

S = (Ig / I – Ig) G

Ig = I / 10

S = G / 9

9. Assertion To convert a galvanometer into an ammeter a small resistance is connected in parallel with it.

Reason The small resistance increases the combined resistance of the combination.

9. (3)

An ammeter should have a low resistance which we get when we connect low resistance in parallel with galvanometer.

10. Assertion An ammeter is always connected in series whereas a voltmeter is connected in parallel.

Reason An ammeter is a low resistance galvanometer while a voltmeter is high resistance galvanometer.

10. (1)

An ammeter is a low resistance device and is connected in series so as the whole circuit current flows through it for an accurate measurement. A voltmeter is a device having a high resistance. So, if we connect it in series, it would hinder the current flow in the circuit hence open circuit results.