1. (d) The field consists of concentric circles centred on the wire.
2. (c) producing induced current in a coil due to relative motion between a magnet and the coil.
3. (a) generator
4. (d) A.C. generator has slip rings while the D.C. generator has commutator.
5. (c) increase heavily
6. (a) False
7. Three methods of producing magnetic fields are as follows:
(a) Magnetic field can be produced by placing a permanent magnet or a horse-shoe magnet at the place, where magnetic field is required.
(b) Magnetic field is produced around a current carrying straight conductor or a current carrying
(c) A very good method to produce magnetic field is due to flow of current in a solenoid.
8. When current is passed through a solenoid coil, magnetic field is produced due to presence of turns in same direction. As a result, the resultant magnetic field is very strong and uniform. Solenoid behaves like a strong bar magnet.
We can determine the poles of magnet formed by solenoid. The end of solenoid connectd with positive terminal behaves like South Pole and the end connected with negative terminal behaves as North Pole.
9. The force experienced by a current carrying conductor placed in a magnetic field is largest when the conductor is placed with its length in a direction perpendicular to that of magnetic field.
10. An electron beam moving horizontally from back wall towards the front wall is equivalent to a current flowing in the opposite direction. The deflection of electron beam as seen by the observer is to his right side. On applying Fleming’s left-hand rule we find that the magnetic field is acting in vertically downward direction.
11. Electric motor labelled diagram of an electric motor is as follows:
Principle: A current-carrying conductor, when placed in a magnetic field, experiences a force. if the direction of magnetic field and that of current are mutually perpendicular then force acting on the conductor will be perpendicular to both and will be the given by Fleming’s left-hand rule. Due to this force the conductor begins to move, if it is free to rotate.
Working: Let the current in the coil ABCD of motor enters from the source battery through the conducting brush X, flow along ABCD and finally flows back to the battery through brush Y. On applying Fleming’s left-hand rule we find that force acting on arm AB due to magnetic field pushes it downwards. But the force acting on arm CD pushes it upwards. Thus, the coil and the axle rotate anticlockwise. Due to action of split rings P and Q change their contacts with brushes. Now, P makes contact with Y and Q with X. As a result, Current begins to flow in coil along DCBA. The arms are pushed in opposite direction and coil continues to rotate in same direction.
12. Electric motors are used in all devices where we want to convert electrical energy into mechanical energy. In our houses, electric motors, coolers, mixer grinders, washing machines, computers etc motor is used.
13. (i) When a bar magnet is pushed into the coil of insulated copper wire connected to a galvanometer, galvanometer gives a deflection towards left.
(ii) When the bar magnet is withdrawn from inside the coil, again an induced current is set in coil that deflect the galvanometer towards right.
(iii) If the bar magnet is held stationary inside the coil, then no induced current is set and galvanometer does not show any deflection.
14. Yes, a current is induced in the coil B.
When the current in the coil A is changed, the magnetic field associated with it also changes. As coil B is placed close to A, hence magnetic field lines around this coil also change. Due to change in magnetic field lines associated with coil B, an induced current is also induced in it.
15. (i) To know the direction of magnetic field produced around a straight conductor we make use “Right hand thumb Rule”.
(ii) To find the direction of force experienced by a current carrying straight conductor placed in a magnetic field we make use “Fleming’s left hand rule”.
(iii) For finding the direction of current induced in a coil we use “Fleming’s right hand rule”.
16. Electric generator labelled diagram is as follows:
Principle: An electric generator works on the principle of electromagnetic induction.
Working: Let the in the beginnings brushes B1 and B2 are kept pressed separately on rings R1 and R2 respectively. Let the axle attached to the rings is rotated such that arm AB of the coil moves up and arm CD moves down in the magnetic field. Due to rotation of arms AB and CD induced current are set up in them. As per Fleming’s right hand rule, induced current in these arms along the directions AB and CD respectively and current flows into B1 and B2.
After half rotation, arm AB moves downward and arms CD upward to change the direction opposite to first case. Thus, after every half rotation current changes its direction and an alternate current is obtained in the generator.
Brushes are kept pressed on the two slip rings separately. Outer ends of the brushes are connected to the galvanometer. Thus, brushes help in transferring current from the coil ABCD to external circuit.
17. If either the insulation of wires used in an electric circuit is damaged or there is a
fault in the appliances, live wire and neutral wire may come in direct contact. As a result, the current in the circuit abruptly rises and short circuiting occurs.
18. The earth wires functions as a safety measure, especially for those appliances that have a metallic body, like heater, electric, press, room cooler etc.
The metallic body of the appliance is connected to the earth wire, which provides a low resistance conducting path for electric current. It ensures that any leakage of current to the metallic body of an appliance keeps it potential same as of earth. As a result, the user would not get severe electric shock, even if he touches the body of appliance.
19. The compass needle is small bar magnet. When a compass needle is brought near a bar magnet then due to repulsive force between unlike poles and attraction between unlike poles, the compass needle is deflected and settle in the direction of net magnetic field.
20. Magnetic field lines are as follows:
21. Properties of magnetic field lines of force as follows:
a. Outside a magnet, the field lines are directed from N-pole of magnet towards S-pole and inside the magnet lines are directed form S-pole to N-pole.
b. Magnetic filed lines are closed curves.
c. No two magnetic field lines intersect each others.
d. Relative strength of magnetic field lines is given by degree of closeness of the filed lines.
22. No, two magnetic field lines can ever intersect each other. If they do, then it would mean that at the point of intersection there are two directions of magnetic field, which is not possible.
23. As per right-hand rule, we find that inside the loop, the magnetic field lines are directed perpendicular to the plane of paper in the inward direction. Outside the loop magnetic field lines are directed out of the plane paper.
24. The uniform magnetic field is represented by parallel equispaced lines of equal length as follows:
25. (d) is the same at all points.
26. (c), (d) Velocity as well as momentum will change.
27. (i) 0 increasing the current in rod AB its displacement will increase.
(ii) If stronger horse-shoe magnet is used then the displacement of rod AB will increase.
(iii) If length of the rod is increased, force acting on it will increase and hence, displacement of the rod increases.
28. (d) the direction of magnetic field is vertically upward.
29. Fleming’s left hand rule states that stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger
shows the direction of the magnetic field and central finger that of the current, then
the thumb will point towards the direction of motion of the conductor.
30. An electric motor is based on the principle that the current carrying conductor
experiences a force when placed in a magnetic field. If the direction of the magnetic
field and that of the current are mutually perpendicular then the direction of the
force is given by Fleming’s left-hand rule.
31. In electric motor, the split ring acts as commutator. Due to its action, the direction of current flowing in motor coil reverses after half turn, giving rise to a continuous
rotation of the coil and the axle.
32. Different ways to induce current in a coil are as follows:
(a) If a magnetic field is changed around a coil then an induced current is set up in the coil.
(b) If a coil is moved in magnetic field, then again an induced current is set up in the coil.
(c) If a coil is rotated in a uniform magnetic field
33. An electric generator is based on the principle of electromagnetic induction. When a rectangular coil is rotated in a uniform magnetic field, an induced emf is generated between the ends of the coil.
34. Some sources of direct current are a cell, a battery and a D.C. generator.
35. A.C. generator and common inverter used in houses for emergency power supply produce alternating current.
36. (c) half-rotation.
37. Two safety measures are:
(a) use of earth wire and proper earthing.
(b) use of fuse.
38. Power rating of electric oven (P) = 2 kW = 2000 W
Current drawn (I) = P/V = 2000/220 = 9.09 A.
As the current rating of domestic electric circuit is only 5 A the oven draws a current of 9.09 A. Which is more than the current rating; hence the circuit will be damaged due to overheating/overloading.
39. We should take following precaution to avoid the overloading of domestic electric circuit:
(a) Two separate circuits should be used, one of 5A current rating of bulbs, fans,
tubes etc. and the other 15 A current rating for appliances with higher current rating
such as geysers, air coolers, electric iron, electric stoves etc.
(b) Too many appliances should never be connected to a single socket.
(c) A fuse of appropriate current rating should be used with the electric circuit.