Question Bank. 1. Electromagnetism 2. Magnetic Effects of an Electric Current 3. Electromagnetic Induction

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1 1. Electromagnetism 2. Magnetic Effects of an Electric Current 3. Electromagnetic Induction 1. Diagram below shows a freely suspended magnetic needle. A copper wire is held parallel to the axis of magnetic needle. (a) Describe the directions in which the north pole of the needle will move in the following situations. (i) When conductor is above needle and the current flows from A to B. (ii) When conductor is below needle and the current flows from B to A. (b) Why does the needle move in the above situations? (c) Name and state the law which will determine the direction of motion of magnetic needle. 1

2 Ans.(a) (i) The north pole of needle will deflect towards west. (ii) The north pole of needle will deflect towards west. (b) The current flowing through conductor, sets up a magnetic field, which is at right angles to the direction of flow of current. Thus, magnetic field due to the conductor is at right angles to the direction of magnetic field due to the magnetic needle. Thus, a couple acts, which deflects the freely suspended magnetic needle. (c) Ampere s Swimming Rule : Imagine a swimmer (always looking towards magnetic needle), swimming in the direction of the current, such that current enters from his feet and leaves from his head. Then, the direction in which left hand of the swimmer points, gives the direction of the north pole of the magnetic needle. 2. Describe a set up for plotting magnetic field lines in a straight conductor carrying current. Ans. 2

3 3. Draw a diagram showing magnetic field lines due to a straight wire carrying current. Ans. 4. Two straight conductors A and B, carrying strong equal currents in opposite directions, pass through a cardboard, as shown in the diagram below. Copy the diagram and sketch separately the lines of force produced by each current. Show the direction of magnetic field at X. What will be the effect of magnetic field at X, if the current in B is reversed? Explain why, the lines of force at distance may differ in shape from those in the immediate vicinity of conductors. 3

4 Ans. The direction of magnetic field at X will be in the downward direction as is illustrated by diagram. If the direction of current in B is reversed, then no magnetic lines of force will pass through X. The lines of force at distance interacts with the earth s magnetic field and hence are elliptical in nature. 5. State three characteristics of the magnetic field produced by a straight current carrying conductor. Ans. (1) The magnetic lines of force are in the form of concentric circles near the conductor. (2) The plane of magnetic lines of force is at right angles to the straight conductor carrying current. (3) The magnetic intensity increases with the increase in the magnitude of current. 4

5 6. Describe a set up for plotting magnetic field lines in a circular loop carrying current. Ans. 7. A wire, bent into a circle, carries current in an anticlockwise direction. What polarity does this face of coil exhibit? Ans.The face of the coil will exhibit North polarity. 8. What is the direction of magnetic field at the centre of coil carrying current in : (i) clockwise, (ii) anticlockwise direction? Ans. (i) Magnetic field is inward and along the axis of the coil. (ii) Magnetic field is outward and along the axis of the coil. 9. (a) Copy the diagram given below and draw the lines of the magnetic field produced due to the flow of current. (b) State one way of increasing the magnetic strength of the coil. (c) Give three characteristics of magnetic lines of force in (a). 5

6 Ans. (a) The lines of magnetic field are shown in the figure. (b) By increasing the magnitude of current, the magnetic strength of the of coil increases. (c) (i) The points where the current enters, the magnetic lines of force are in the form of concentric circles. (ii) Within the space enclosed by the coil, the magnetic lines of force are in the same direction. (iii) The direction of the magnetic lines of force is at right angles to the direction of flow of current. 10. State the rule for finding the direction of magnetic field lines around a conductor carrying current. Ans. Right Hand Thumb Rule : Imagine you are holding a conductor with the palm of your right hand, such that the fingers encircle the conductor and the thumb points in the direction of the current. Then, the direction of fingers encircling the conductor gives the direction of the magnetic field lines of force around the conductor. 6

7 11. (a) What is a solenoid? (b) Draw magnetic field around solenoid, showing clearly the magnetic polarities and the direction of magnetic lines of force. (c) If the solenoid is suspended freely, in which direction is it likely to point? (d) State three ways of increasing the magnetic strength of a solenoid. Ans. (a) Solenoid is an insulated copper coil, wound on cylindrical cardboard, such that its length is greater than its diameter and it behaves like a magnet when electric current is made to flow through it. (b) Diagram drawn below. (c) It will point in the North-South direction. (d) The intensity of magnetic field of a solenoid can be increased by: (1) Increasing number of turns in the solenoid. (2) By increasing strength of current flowing through solenoid. (3) By placing soft iron core along the axis of the solenoid. (4) By laminating soft iron core. 7

8 12. The diagram below shows a small magnet placed near a solenoid. When current is switched on in the solenoid, will the magnet be attracted or repelled? Give a reason for your answer. Ans. The current of the end A flows in clockwise direction. Thus, towards end A south polarity is created. Thus magnet is attracted by the coil. 13. The diagram below shows a spiral coil wound on a hollow cardboard tube. A magnetic compass is placed close to it. When the current flows by closing the key, how will the compass needle be affected? State two ways in which the magnetic field due to the coil can be made stronger. Ans. The current on the end A flows in the anti-clockwise direction. Thus, the north polarity is created towards the end A. Thus, the south pole of the compass needle will point towards the end A. The magnetic field can be made stronger by : (1) increasing the magnitude of the current in the coil. (2) placing a laminated soft iron core within the coil. 8

9 14. Why does a current carrying freely suspended solenoid rest along a particular direction? Ans.When the current flows through a freely suspended solenoid, it behaves like a bar magnet. Thus, it aligns itself in the direction of magnetic field of the earth i.e., in north-south direction. 15. You are required to make an electromagnet from a U-shaped soft iron bar. Draw a circuit diagram to represent the process. In your diagram show the electric cell, insulted copper coil, U-shaped iron bar and switch. Label the poles of electromagnet. Ans. 16. (a) What do you understand by the term electromagnet? (b) State three ways of increasing strength of an electromagnet. (c) State three practical applications of electromagnets. Ans. (a) A magnet made by winding insulated copper coil over a piece of soft iron such that it behaves like a magnet only when current flows through the coil is called an electromagnet. 9

10 (b) (i) By increasing the number of turns in the coil. (ii) By increasing the magnitude of current flowing through the coil. (iii) By laminating the soft iron core. (c) (i) Electromagnets are used in electric bell, electric relay; microphone, etc. (ii) Electromagnets are used for separating ferromagnetic substances from ores. (iii) Electromagnets are used in electric motors and electric generators. 17. Diagram below shows a circuit, containing a coil, wound on a long and thin hollow cardboard tube. (i) Copy the diagram and show the polarity acquired by each face of the solenoid. (ii) Draw the magnetic lines of force inside the coil and show their direction. (iii) Mention two methods to increase magnetic strength of magnetic lines of force inside the coil. 10

11 Ans. (i) Shown in the diagram the polarity acquired by each face of the solenoid. (ii) Shown in the diagram. (iii)(a) By increasing the magnitude of current. (b) By placing a soft iron core within the coil. 18. State two differences between an electromagnet and permanent magnet. Ans. 1. Electromagnets exhibit very strong magnetic field as compared to permanent magnets. 2. Polarity of an electromagnet can be easily reversed, but not in the case of a permanent magnet. 19. (a) Incomplete diagram of an electric bell is shown in the figure below. Copy and complete the diagram. 11

12 (b) What is the function of the following parts of an electric bell? (i) Electromagnet (ii) Soft iron armature (iii) Flat steel spring. (c) Why is special attention paid to the contact points of an electric bell? (d) How can you alter the frequency of ring for the electric bell? (e) Can this bell operate on a.c. mains? If so, explain your answer. If no, then suggest alterations in the circuit, so that bell may adapt to a.c. mains. Ans. (a) Shown in the diagram. (b) (i) Electromagnet pulls the soft iron armature towards itself when current flows through it. The movement in armature makes the hammer strike against gong, and hence, sound is produced. (ii) Soft iron armature gets pulled strongly towards electromagnet, when the latter is activated. (iii) Flat spring gets under tension, when armature moves towards electromagnet. When the electrical circuit breaks at contact point, the tension in spring brings the armature back to its original position. 12

13 (c) At contact points, sparking takes place, when the electrical circuit breaks. Due to sparking, the contact points are oxidised, and hence bell stops working. To avoid oxidation, the contact points are made from silvercadmium alloy. (d) By moving the contact screw towards armature, the frequency of ring increases and vice versa. (e) Yes, it can operate on a.c. mains. It is because the electromagnet works equally well on the a.c. or d.c. supply. Only difference is that in case of a.c. supply, the polarity of electromagnet changes rapidly. 20. Figure below shows the current flowing in the coil of wire wound around the soft iron horse-shoe core. State the polarities developed in the ends A and B. Ans. (i) At the end A north polarity. (ii) At the end B south polarity. 13

14 21. (a) A freely suspended copper conductor is held between the pole pieces of a magnet, as shown in the diagram below. State whether the conductor will move into the plane of paper or out of the plane of paper. (b) Why does the conductor move? (c) State the rule which determines the motion of the conductor. (d) State three factors which determine the force acting on the conductor. Ans. (a) It will move into the plane of paper. (b) The magnetic field set up by the conductor, due to the passage of electric current is at right angles to the magnetic field of the permanent magnet. Thus, a magnetic couple acts, which makes the freely suspended conductor to move out of the magnetic field of permanent magnet. (c) Fleming s Left Hand Rule : It tells the direction of motion of conductor carrying current in a magnetic field. It states : Stretch the thumb, the forefinger and the middle finger of left hand, such that forefinger points in the direction of magnetic field; middle finger points in the direction of the current. Then, the thumb points in the direction of the motion of the conductor. 14

15 (d) The force experienced by it is : (i) directly proportional to the strength of current intensity. (ii) directly proportional to the magnetic field. (iii) directly proportional to the length of the conductor within magnetic field. 22. The diagram below shows a rectangular coil ABCD, suspended freely between the concave pole pieces of a permanent horse-shoe magnet, such that the plane of coil is parallel to magnetic field. (a) State your observation when the current is switched on. (b) Give an explanation for your observation in (a). (c) State the rule which will help you to find the motion of rotation of the coil. (d) In which position will the coil ultimately come to rest? (e) State four ways of increasing the magnitude of the force acting on the coil. Ans. (a) The coil ABCD will turn about its axis. The arm AB of the coil will move out of the plane of the paper and arm CD into the plane of the paper. Thus, coil will turn in the anti-clockwise direction. 15

16 (b) A magnetic field is set up by this coil due to the passage of electric current. The magnetic field of the coil is at right angles to the magnetic field of permanent magnet. Thus, a magnetic couple acts, which turns the coil. (c) Fleming s Left Hand Rule : It tells the direction of motion of conductor carrying current in a magnetic field. It states : Stretch the thumb, the forefinger and the middle finger of left hand, such that forefinger points in the direction of the magnetic field; middle finger points in the direction of the current. Then, the thumb points in the direction of the motion of the conductor. (d) The coil will come to rest at right angles to the direction of the magnetic field. (e) (1) By increasing the number of turns in the coil. (2) By increasing the area of cross-section of the coil. (3) By placing a laminated soft iron core within the coil (4) By increasing the magnitude of current in the coil. 23. What is an electric motor? State its principle. Ans. An electric device, in which electric energy is converted into mechanical energy is called electric motor. It is based on the principle that when a coil carrying current is placed in a permanent magnetic field, experiences a force due to interaction of the magnetic field of the magnet and the field of the coil. 16

17 24. What energy conversions take place during the working of a d.c. motor. Ans. The electric energy changes into mechanical energy during the working of a d.c. motor. 25. Complete the following sentence : energy is converted into energy by an electric motor. Ans.1. Electric 2. mechanical 26. (a) The diagram below shows a simple form of D.C. motor. State the observation in ammeter A, after switch is put on. (b) What is the cause of the above observation? (c) State the direction of rotation of the coil. (d) How do you determine the direction of rotation of the coil? Ans. (a) The ammeter will record the maximum value of current very slowly. It means that the needle of ammeter will slowly move over the scale till it records maximum value. 17

18 (b) When the current flows into the coil of motor, an induced current due to self induction flows in the opposite direction. It is on account of this reverse current that the needle of this ammeter moves slowly. (c) The coil will rotate in the anticlockwise direction. (d) Fleming s Left Hand Rule : It tells the direction of motion of conductor carrying current in a magnetic field. It states : Stretch the thumb, the forefinger and the middle finger of left hand, such that forefinger points in the direction of the magnetic field; middle finger points in the direction of the current. Then, the thumb points in the direction of the motion of the conductor. 27. (a) State the function of the following parts in D.C. motor : (1) Insulated copper coil. (2) Soft iron core. (3) Split ring commutator. (4) Carbon brushes. (b) In which positions the deflecting couple acting on coil is : (1) Maximum. (2) Minimum. (c) How is the jerky motion of the coil turned into smooth circular motion? (d) State three ways of making motor more powerful. 18

19 Ans.(a) (1) It behaves like a magnet when electric current flows through it. Thus, it sets up a magnetic field which further helps in the setting up of magnetic couple. (2) It intensifies the magnetic field set up by the coil and hence helps in setting up strong magnetic couple. (3) Split ring commutator helps in altering the direction of the current in the coil after every 180. This in turn helps in the rotation of the coil in the same direction. (4) They rub against split rings and hence provide continuous current to the coil. (b) (1) At 0 and 180, the deflecting couple is maximum. (2) At 90 and 270, the deflecting couple is zero. (c) The jerky motion of the coil is converted into smooth circular motion by winding number of coils on the same soft iron core, such that when deflecting couple decreases in one coil, it becomes maximum in the following coil. (d) (1) By increasing number of turns in the coil. (2) By increasing area of cross-section of the coil. (3) By increasing the strength of radial magnetic field. 28. Name two appliances in which electric motor is used. Ans. (i) Mixer grinder, (ii) Fans. 19

20 29. (a) What do you understand by the following terms : (i) Electromagnetic induction? (ii) Induced e.m.f? (b) State two laws of electromagnetic induction. Ans.(a) (i) Electromagnetic induction : It is the phenomenon due to which an induced alternating e.m.f. is set up in a conductor, when magnetic flux changes in it. (ii) Induced e.m.f. : It is the p.d. set up in a conductor, when magnetic lines of force are allowed to change within it. (b) Faraday s laws of electromagnetic induction : 1.Whenever magnetic flux changes within a closed coil, an induced e.m.f. is set up within the coil, which gives rise to induced electric current. 2. The magnitude of induced e.m.f. is: (1) directly proportional to the rate of change of magnetic flux. (2) directly proportional to the number of turns in the coil. (3) directly proportional to the area of the cross-section of the coil. 20

21 30. The diagram below, shows a coil AB, connected to centre zero galvanometer G. The galvanometer shows a deflection to right when the north pole of a powerful magnet is moved to right as shown. (i) Explain why deflection occurs in the galvanometer. (ii) Does the direction of current appear clock-wise or anticlockwise, when viewed from end A? (iii) State the observation in G when the coil is moved away from N. (iv) State the observation in G when both coil and the magnet are moved to right, with the same speed. Ans. (i) When the magnetic lines of force increase within the coil, it drives the free electrons (dipoles) in one particular direction. This gives rise to induced current, and hence, galvanometer shows deflection. (ii) Anticlockwise is the direction of current. (iii) The needle of galvanometer shows a momentary deflection towards left. (iv) The galvanometer shows no deflection. It is because there is no relative change in the position of magnet and the coil. Thus, no magnetic lines of force change within the coil. Hence, no induced e.m.f. is produced. 21

22 31. What kind of energy change takes place when a magnet is moved inside a coil having a galvanometer at its ends? Name the phenomenon. Ans. The mechanical energy of the moving magnet changes into electric energy through the agency of magnetic lines of force. The generation of electric energy is shown by the momentary deflection of the galvanometer needle. This phenomenon is called electromagnetic induction. 32. Describe briefly one way of producing induced current. State one factor that determines the magnitude of induced e.m.f. Ans. Take an insulated copper coil of large number of turns, wound on a hollow cardboard tube and connect its bare ends to a sensitive galvanometer. Take a powerful magnet and rapidly move it in and out in the cardboard tube. You will observe the galvanometer registers deflection. The magnitude of induced e.m.f. can be increased by rapidly moving the bar magnet in and out. 33. State Fleming s right hand rule. Ans. Stretch the thumb, the forefinger and the middle finger of your right hand, mutually at right angles to each other, such that forefinger points in the direction of magnetic field, the thumb in the direction of motion of conductor, then the direction in which middle finger points gives the direction of induced current. 22

23 34. What is lenz s law : Ans. In all cases of electromagnetic induction, the direction of the induced current is such that it always opposes the causes, which produces it. 35. Why is it more difficult to move a magnet into the coil which has large number of turns? Ans. More are the number of turns in a coil, more is the magnitude of induced e.m.f. Now, by Lenz s law the direction of induced e.m.f. is such that it always opposes the cause. Thus, the moving magnet experiences a lot of force in the direction, opposite to its direction of motion and hence it is difficult to move within the coil. 36. Explain, why an induced current must flow in such a direction, so as to oppose the change producing it. Ans. In nature, forces always occur in pairs which are mutually equal and opposite to one another (Newton s third law of motion). Thus, the force (magnetic) produced by the induced current must be equal and opposite to the direction of force applied on the moving magnet. 37. The diagram below shows a coil of several turns of copper wire connected to a sensitive centre zero galvanometer G near a bar magnet NS. The coil is free to move. 23

24 (i) Describe the observation, if the coil is rapidly moved in the direction of arrow. (ii) How would the observation be altered, if (a) coil has twice as many turns, (b) coil is made to move three times faster? Ans. (i) The needle of galvanometer shows a momentary deflection and then comes to centre zero, when the coil stops. (ii) (a) The magnitude of deflection of the needle of galvanometer increases twice as compared to (i) (b) The magnitude of deflection of the needle of galvanometer increases thrice as compared to (i) 38. The diagram below shows a fixed coil of several turns connected to a centre zero galvanometer G and a magnet NS which can move. (a) Describe the observation in the galvanometer if (i) the magnet is moved rapidly in the direction of arrow, (ii) the magnet is kept still after it has moved into the coil, (iii) the magnet is then rapidly pulled out of the coil. (b) How would the observation in (a) (i) alter, if more powerful magnet is used? 24

25 Ans.(a) (i) The needle of galvanometer shows momentary deflection, say towards right. (ii) The needle of galvanometer will return back to centre zero position. (iii) The needle of galvanometer shows momentary deflection, opposite to the direction to (a) (i), i.e., say towards left. (b) The extent of deflection of galvanometer needle increases with more powerful magnet as the magnitude of induced e.m.f. increases. 39. The diagram below shows a coil X connected to centre zero galvanometer G and a coil P connected to d.c. supply through switch S. Describe your observations when the switch S is (i) closed suddenly, (ii) then kept closed, (iii) finally opened. Name and state the law which explains the above observation. Ans. (i) The galvanometer needle shows a momentary deflection and then comes back to centre position. Say the deflection of needle is towards right. (ii) The galvanometer needle shows no deflection. (iii) The galvanometer needle shows momentary deflection opposite to (i) and then comes back to centre zero position. 25

26 Faraday s law of electromagnetic induction explains the above observation. It states : whenever, there is a change in magnetic flux linked with a coil and e.m.f. is induced in the coil, as long as there is a change in the magnetic flux within the coil. 40. For what purpose are transformers used? Can they be used with direct current source? Ans. Transformers are used to step up or step down the a.c. voltage required for operating various electrical devices operating on some specified voltage. For example, a door bell may require an e.m.f. of 6 V, whereas a TV set may require an e.m.f. of several thousand volts. Transformers cannot be used with direct current, because magnetic flux does not change with it. 41. How is e.m.f. in primary and secondary coils of a transformer related with the number of turns in these coils? Ans. e.m.f. in primary coil e.m.f. in secondary coil = number of turns in primary coil number of turns in secondary coil 42. Draw a labelled diagram to show various components of a step-up transformer. Ans. 26

27 43. (a) Draw a labelled diagram of a device you would use to transform 200 V a.c. to 15 V. a.c. Name the device and explain how it works. (b) (i) Give two uses of this device. (ii) State three characteristics of its primary coil with respect to secondary coil. Ans. (a) The device is known as step-down transformer. Working : When a.c. at 200 V flows into primary coil P the magnetic flux rapidly changes within the soft iron, with the change in the direction of current. This changing magnetic flux links with secondary coil S and hence induces a.c. e.m.f. in it, the magnitude of which is given by : e.m.f. in secondary coil e.m.f. in primary coil = number of turns in secondary coil number of turns in primary coil (b) (i) A step-down transformer is used in operating door bells, battery eliminators, battery chargers and emergency lights. (ii) (1) The primary coil has more number of turns as compared to secondary coil. (2) The primary coil has smaller diameter as compared to the secondary coil. (3) The primary coil is more heavily insulated as compared to the secondary coil. 27

28 44. The figure below shows a transformer and name of its parts A and B. Complete the diagram and name the parts A and B. Name the part you have drawn to complete the diagram. What is the material of this part? Is this transformer stepup or step-down and why? Ans. A is primary coil. B is secondary coil. The part drawn for completing diagram is laminated core. The material of laminated core is soft iron. The above transformer is step-down transformer as the number of turns in its secondary coil are less than the number of turns in its primary coil. 28

29 45. The secondary windings of a transformer in which voltage is stepped down are usually made of thicker wire than the primary. Explain, why? Ans. The magnitude of current in the secondary windings of a step-down transformer is far higher than the primary windings. As the power wasted in secondary windings is given by the expression I 2 R, where R is the resistance of secondary windings, therefore, value of R is reduced as far as possible. This is achieved by making the winding of secondary coil thicker, because, thicker the wire, lesser is its resistance. 46. Why is the iron core of a transformer made laminated (thin sheets), instead of being one solid piece? Ans. (1) In a laminated iron core the magnitude of magnetic intensity is higher than the solid iron core. (2) In a laminated iron core the least amount of eddy currents are generated. Thus, wastage of energy, in the form of heat is minimised. 47.Fill in the blank spaces with appropriate words. (i) In a step-up transformer the number of turns in the primary are than number of turns in the secondary. (ii) The transformer is used in current circuits. Ans. (i) less (ii) alternating 29

30 48. What is the function of a transformer in an a.c. circuit? How does the input and output powers of a transformer compare? Name two causes of energy loss in a transformer. Ans. A transformer in an a.c. circuit, step up or steps down, the e.m.f. depending upon the requirement. In a transformer the output power is always less than the input power. Causes of energy loss in a transformer. 1. A part of energy is lost on account of the resistance of primary and secondary coils. 2. A part of energy is lost due to the formation of eddy currents in the soft iron core. 49. What energy losses take place in the core of a transformer and how are they minimised? Ans. The energy losses in the core of transformer take place due to formation of eddy currents which in turn heat up the core. The energy losses can be minimised by using laminated soft iron core. 50. Give one point of difference between a step-up transformer and a step-down transformer. Ans. (1) The primary of step up transformer has lesser number of turns as compared to step down transformer. 30

31 51. The diagram below shows a core of a transformer and its input and output connection. (a) State the material used for the core and describe its structure. (b) Use the given data in the diagram to calculate the turn ratio for the transformer. (c) Complete the diagram of the transformer and connections by labelling all parts added by you. (d) If a current of 2A is taken from the output, calculate the current in the input circuit. (Assume transformer is ideal). Ans. (a) The material used in the core is soft iron. The soft iron is cut into thin rectangular pieces which are then bound together. Such an arrangement is called laminated soft iron core. (b) number of turns in secondary coil (N ) number of turns in primary coil (N ) p s = 31

32 s N N N :N =10 :1 s p p e.m.f. in secondary coil = e.m.f. in primary coil 220V = 22V (c) (d) For ideal transformer : Power input = Power output IP VP = IS VS IP 220 V = 2A 22V IP = 2A 22V 220V = 0.2 A 32

33 52. A primary coil of 800 turns is connected to 220 V a.c. mains supply and the Ans. secondary coil has 8 turns. What will be the output voltage. N N s p = V V s p V s 8 = V V s = V = 2.2 Volts 53. A transformer is designed to work from a 240 V a.c. mains and to give a supply Ans. of 8 V to ring a house bell. The primary coil has 4800 turns. How many turns would you expect in the secondary? N N N s p s = V V s p Np Vs V = = V 240 V p = 160 turns 54. (a) What is an electric generator? (b) Draw a neat and labelled diagram of a.c. generator? (c) What is the effect of increasing the speed of rotation of coil in a generator? (d) What energy conversion takes place in a generator? (e) What is the magnitude of e.m.f. induced in the coil, when its plane becomes parallel to magnetic field? 33

34 Ans. (a)a device which converts mechanical energy into electric energy is called electric generator. a.c. generator (b) Shown in the figure above. (c) It increases the magnitude of induced e.m.f. (d) The mechanical energy is converted into electric energy. (e) The magnitude of induced e.m.f. is maximum as maximum number of magnetic lines of force cut through the coil. 55. (a) An a.c. generator, running at a constant speed is connected to an external circuit, produces alternating current of 50 Hz. Draw graph to show how the current in external circuit varies with time. (b) Why is the e.m.f. produced by generator zero at certain instant and maximum at some other instant when coil is rotating at same speed? 34

35 (c) When resistance in external circuit is decreased, more energy is required to drive the generator at the same speed. Why? (d) State four ways of increasing the magnitude of induced current produced by generator. Ans. (a) Shown in the figure below. (b) When the number of magnetic lines of force cutting the coil is maximum, the magnitude of induced e.m.f. is maximum. Conversely, when least number of magnetic lines of force cut the coil, the magnitude of induced e.m.f. is zero. (c) With the decrease in external resistance, the magnitude of current increases. Thus, output power increases. This is possible only, if there is matching input. Hence, more energy is required to drive the generator. (d) The magnitude of induced e.m.f. in an electric generator depends upon : (i) Number of turns in the coil (ii) Area of cross-section of the coil (iii) Intensity of radial magnetic field (iv) Rate of rotation of the coil in magnetic field. 35

36 56. In an a.c. generator the speed at which the coil rotates is doubled. How would this affect : (a) the frequency of out put voltage, (b) the maximum out put voltage? Ans. (a) The frequency of out put voltage shall be double. (b) The maximum out put voltage shall be double. 57. Why energy conversion takes place in a generator? Ans. The mechanical energy of some primary source of energy changes to electric energy. 58. State two differences between d.c. motor and a.c. generator. Ans. (1) In d.c. motor the electric energy changes to mechanical energy, whereas in a.c. generator the mechanical energy changes to electric energy. (2) In d.c. motor split ring commutator is employed to alter the direction of current in its coil. In a.c. generator slip rings are used to take out electric energy from the coil. 59. Fill in the blank by writing (i) only soft iron, (ii) only steel, (iii) both soft iron and steel for the material of core and / or magnet. (a) Electric bell : (b) Electromagnet : (c) d.c. motor : (d) a.c. generator (e) transformer : 36

37 Ans. (a) Electric bell : electromagnet core uses soft iron. (b) Electromagnet : core is of soft iron. (c) d.c. motor : Radial magnets steel and core soft iron. (d) a.c. generator : Radial magnets steel and core soft iron (e) transformer : Core of soft iron. 37

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