Magnetism and Electromagnetism - Short Question and Answer

Magnetism and Electromagnetism - Short Question and Answer Points : magnetism and electromagnetism - short question and answer Q. 1. An electron travelling south to north posses through an electric field directed from west to east. In which direction should a magnetic field be applied so that electron goes undeflected? Ans : The electric force on a charge is parallel to the electric field. In the present case, the electric force on the electron is towards west. In order to balance it, the magnetic force on the electron should be directed towards east. By right-hand rule, the magnetic field should act vertically downward. Q. 2. A circular loop of radius 0.1 m carries a current JA and is placed in a uniform magnetic field of 0.5 T The magnetic field is perpendicular to the plane of the loop. What is the force experienced by the loop? Ans : A current carrying loop is equivalent to a magnetic dipole. A magnetic dipole does not experience any force in a uniform magnetic field. Therefore, this current carrying loop will not experience any net force. Q. 3. Why does the kinetic energy of a charged particle moving in a magnetic field remain constant? Ans : The work done by magnetic field on a charged particle moving in it is zero. Since increase in kinetic energy is equal to the work done on the particle, the kinetic energy of the charged particle moving in a magnetic field is constant. Q. 4. Distinguish between electric and magnetic fields. Ans : The following are the points of differences between electric and magnetic fields:
(i) The source of electric field is the electric charge while that of magnetic field is the current element (IdI).
(ii) The electric field is associated with stationary as well as moving charges. However, magnetic field is associated only with a moving charge.
(iii) Electric lines of force originate from a positive charge and end at a negative charge i.e. they are discontinuous. However, magnetic lines of force form closed loops. Q. 5. A proton is moving vertically upwards when it enters a uniform mage field directed towards north. What is the direction of force on the proton? Ans : The magnetic force F_m acting on the proton moving with velocity in in the magnetic field B is given by:
F_m = e(v x B)
By right-hand rule, the force on the proton is westwards. Q. 6. Imagine that the room in which you are seated is filled with a uniform magnetic field pointing vertically downward. M the centre of the room, an electron is released with a certain speed in the horizontal direction. Describe the motion and path of the electron in the field. Ans : The magnetic field exerts a force on the electron in the horizontal plane. The direction of this force can be found by right-hand rule. Under the given conditions, the electron will keep on revolving clockwise in a circular path with constant speed. Q. 7. How can you produce uniform electric and magnetic fields? Ans : The electric field between the charged plates of a parallel plate capacitor is uniform. A current carrying solenoid produces a uniform magnetic field. Q. 8. An electron moving vertically upward enters a magnetic field directed towards north. What is the direction of magnetic force ? Ans : The magnetic force I acting on the electron moving with velocity V in a magnetic field B is given by: F_m = -e (V x B)

By right-hand rule, the magnetic force on the electron is towards east. Q. 9. An electron beam moving with a uniform velocity is gradually diverging. When it is accelerated to a high velocity, it starts converging. Explain. Ans : Moving electrons experience electrostatic repulsion as well as magnetic attractions. Normally, the repulsive force is much greater than the magnetic attraction and hence the beam diverges. However, as the electrons acquire high velocity, the magnetic attraction predominates and the beam starts converging. Q. 10. Can you prove Biot-Savart law dire city? Ans : No. Biot-Savart law gives the magnetic field at a point due to a very small current element. It is not possible to set up experimental arrangement for this small current element. Q. 11. Both electric and magnetic fields can deflect an electron. What is the difference between these deflections? Ans : The force exerted by the magnetic field on a moving charge is always perpendicular to the motion of the charge. Therefore, no work is done by the magnetic force and kinetic energy of the charge remains unchanged. However, in an electric field, the deflection is in a direction parallel to electric field and hence kinetic energy changes. Q. 12. Why does earth ‘s magnetic field not affect the working of a moving coil galvanometer? Ans : A moving coil galvanometer uses a powerful magnet. The earth’s magnetic field ( = 5 x 10⁴ T) is very weak as compared to that of the galvanometer magnet ( = IT). Q. 13. A voltmeter an ammeter and a cell are connected in series. It is observed that ammeter practically shows no deflection. Why? Ans : The circuit resistance becomes very high due to the large resistance of the voltmeter. As a result, very small current will flow in the circuit. This small current on passing through the coil of the voltmeter will produce some deflection. However, in case of ammeter, most of this small current will flow through the shunt. Consequently, the deflection of the ammeter will be practically nil. Q. 14. A body is suspended from the lower end of a vertical spring. A direct current is passed through the spring. What will happen? Ans : The direction of current in each turn of the spring will be the same. Since parallel currents in the same direction attract each other, the turns come closer. As a result, the body will be lifted upward. The same thing will happen if the direction of current in the spring is reversed. Q. 15. A current carrying solenoid tends to contract. Why? Ans : The turns of the solenoid are parallel and carry current in the same direction. Since parallel conductors carrying current in the same direction attract each other, a current carrying solenoid tends to contract. Q. 16. An electron beam is moving horizontally south to north in a television tube. The vertical component of earth‘s magnetic field is directed downward in which direction will the beam be deflected? Ans : The conventional current is from north to south. By right-hand rule, the magnetic field exerts a force on the electron beam towards east. Hence, the electron beam will be deflected towards east. Q. 17. The net charge in a current carrying conductor is zero, even then it experiences a force in a magnetic field. Why? Ans : In a current carrying conductor, free electrons move with drift velocity and hence magnetic force acts on them. Since the positive ions are stationary, they do not experience any force. Q. 18. Free electrons keep on moving in a conductor But no force acts on them in a magnetic field unless a current is passed through it. Why? Ans : In the absence of electric current through a conductor, the free electrons are in a state of continuous random motion. Therefore, they do not have any net velocity in any direction and hence there is no net magnetic force. However, when current flows through a conductor, the free electrons move with a drift velocity in a definite direction. Hence, magnetic force acts on them. Q. 19. What is the function of soft iron cylinder in a moving coil galvanomerer? Ans : Since soft iron is good conductor of magnetic flux, most of the magnetic lines of force would tend to pass through it. This increases the strength of magnetic field (and hence the torque) in which the coil rotates.