Conceptual Questions Based on Moving Charges and Magnetism

Conceptual Questions Based on Moving Charges and Magnetism

Here we are providing conceptual questions for Class 12 Physics Chapter 4 Moving Charges and Magnetism.

Q.1. How is a magnetic field produced ?
Sol. A magnetic field can be produced by the following ways:
(i) by a magnet
(ii) by a current carrying conductor
(iii) by a moving charge and
(iv) by changing electric field.

Q.2. What is the nature of magnetic field produced by a current flowing in a straight conductor ? Name the rule to find the direction of magnetic field.
Sol. When current flows through a straight conductor, the magnetic field is circular i.e. the magnetic lines of force are in the form of concentric circles with the conductor as centre. The plane of the magnetic lines of force is perpendicular to the length of the conductor. The direction of magnetic lines of force can be given by Right hand thumb rule.

Q.3. The net charge in a current-carrying conductor is zero, even then it experiences a force in a magnetic field. Why?
Sol. We know that a magnetic force acts on a charge moving in a magnetic field. In a conductor carrying current, the free electrons move towards the positive end of the conductor with some drift velocity hence magnetic force acts on them in a magnetic field. The positive ions of the conductor being stationary, do not experience any magnetic force.

Q.4. What is the nature of magnetic field produced by a current flowing in a circular wire loop. Name, the rule to find the direction of magnetic field.
Sol. When current flows through a circular wire loop, the magnetic field is nearly uniform near the centre of the circular wire. The magnetic lines of force are perpendicular to the wire loop and are circular near the wire and practically straight near the centre of the wire loop. If the radius of the current loop is very large, the magnetic field near the centre of the current loop is almost uniform. The direction of magnetic field at the centre of circular current loop is given by Right Hand Palm Rule.

Q.5. How will you identify whether the magnetic field at a point is due to the earth or due to some current carrying conductor ?
Sol. We know that a freely suspended magnetic needle always points in north-south direction due to earth’s magnetic field. If a freely suspended magnetic needle always rests in north-south direction, the magnetic field is due to earth, and if this magnetic needle turns to some other direction and returns back to north-south when the current is switched off, then the magnetic field is due to a current carrying conductor.

Q.6. Why should an ammeter have a low resistance?
Solution. An ammeter is connected in series in a circuit so that whole of the current, which it is required to measure, passes through it. In order that its insertion in the circuit does not affect the current in the circuit, the ammeter must have least possible resistance.

Q.7. Why should the resistance of an ideal voltmeter be infinite?
Solution. A voltmeter is connected in parallel with the component across which the p.d. is required to be measured. The voltmeter must have very high resistance so that a very small current passes through it and the p.d. across that component is not affected. To measure the exact value of p.d., an ideal voltmeter has to have infinite resistance.

Q.8. Why is a voltmeter always connected in parallel with a circuit element across which voltage is to be measured?
Solution. A voltmeter is a high resistance galvanometer. When it is connected in parallel across any element of a circuit, it draws a very small current from the main circuit. Most of the current passes through that element. Hence potential difference across that component is not affected materially.

Q.9. Why is an ammeter connected in series in a circuit?
Solution. An ammeter is connected in series in a circuit so that whole of the current, which it is required to measure, passes through it. Moreover, an ammeter has a low resistance, so its insertion in the series circuit does not practically change the main current.

Q.10. What happens when an ammeter is placed in parallel with a circuit?
Solution. An ammeter is a low resistance device. When it is placed in parallel with a circuit, the resistance decreases and current in the circuit increases to a large extent. Moreover, it measures the current flowing through it only and not the current in the circuit.

Q.11. A galvanometer is first converted into a voltmeter of range 0 – 3 V and then into a voltmeter of range 0 – 6 V. In which case the resistance would be higher one?

Solution. The voltmeter with range 0 – 6 V will have a higher resistance. This is because a higher range voltmeter requires a higher resistance to be put in series with the galvanometer.

Q.12. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the circuit? Give reason.

Solution. An ammeter reads slightly less than the actual current. It has a small resistance. When it is connected in a circuit, it decreases the current slightly.

Q.13. When a voltmeter is put across a part of the circuit, does it read slightly less or more than the actual voltage drop across that part? Give reason.

Solution. Voltmeter reads slightly less than the actual voltage drop. When it is connected across a part of the circuit, it draws some current from that part. As a result, potential drop across that part slightly decreases.

Q.14. Why does a solenoid contract when a current is passed through it?
Solution. The current in adjacent turns of the solenoid flows in the same direction. So different turns attract one another and the solenoid contracts.

Q.15. Does the torque on a planar current loop in a magnetic field change, when its shape is changed without changing its area?
Solution. No. The torque on a planar current loop remains same as long as its area remains unchanged.

Q.16. Why do we prefer phosphor-bronze alloy for the suspension wire of a moving coil galvanometer?
Solution. The phosphor bronze alloy is used for the suspension wire of a moving coil galvanometer because of its following properties:
(i) Small restoring torque per unit twist. This makes galvanometer highly sensitive.
(ii) High tensile strength so that even thin fibre does not break under the weight of the suspended coil.
(iii) Rust resisting.

Q.17. What is the main function of soft iron cylinder between the poles of a galvanometer?
Solution. Due to the high permeability of soft iron, the magnetic lines of force crowd through the soft iron core. This increases the magnetic field and hence sensitivity of the galvanometer.

Q.18. Why are pole pieces of galvanometer made concave? [Haryana 93, 94]
Solution. Concave poles produce strong, uniform and radial magnetic field.

Q.19. What is the importance of radial magnetic field in a moving coil galvanometer?
Solution. Radial magnetic field makes the arm of the couple fixed and hence the torque on the coil is always same in all positions of the coil in the magnetic field. This provides a linear current scale.

Q.20. Two parallel wires carrying currents in the same direction attract each other, while the two beams of electrons travelling in the same direction repel each other. Give reason.

Solution. In case of parallel wires, only attractive magnetic interaction acts. In case of electron beams, the electrostatic repulsion is stronger than the attractive magnetic interaction.

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