All About Electromagnetism

Transcription

1 5. All About Electromagnetism Maglev or magnetic levitation is a system of transportation that suspends, guides and propels vehicles. This technology has the potential to exceed 4000 mph (6437 km/h) if deployed in an evacuated tunnel. 106 SCHOOL SECTION

2 MT EDUCARE LTD. SCIENCE & TECHNOLOGY Q.I (A) Choose the correct alternatives and rewrite the complete sentences : *1. The device used for producing current is called a.... (a) voltmeter (b) ammeter (c) galvanometer (d) generator *2. At the time of short circuit, the current in the circuit.... (a) increases (b) decreases (c) remains (d) increases in steps *3. The direction of the magnetic field around a straight conductor carrying current is given by.... (a) right hand rule (b) Fleming s left hand rule (c) Fleming s right hand rule (d) non of these 4. The relation between electricity and magnetism was first established by.... (a) Ohm (b) Ampere (c) Oersted (d) Moseley 5. The region around magnet is called as.... (a) magnetic Area (b) magnetic field (c) magnetic loop (d) magnetism 6. Oersted proved that when current passes through a conducting wire it produces.... (a) charge (b) current (c) magnetic field (d) potential difference 7. The relative strength of magnetic field is shown by.... (a) degree of closeness of field lines (b) optical lines (c) current flowing (d) potential difference 8. The magnetic field produced at centre of circular wire is inversely proportional to.... (a) magnitude of current (b) radius of circular wire (c) resistance of wire (d) time for which current is passed 9. Solenoid has properties similar to.... (a) magnet (b) resistance (c) electric motor (d) optical instruments 10. The magnetic field in solenoid depends on.... (a) current and number of turns per unit length (b) current only (c) Number of turn per unit length (d) material 11. In India, the frequency of A.C is... cycles per second. (a) 50 (b) 100 (c) 60 (d) none 12. The magnetic field produced at centre of circular wire is directly proportional to.... (a) magnitude of current passing through it (b) radius of the loop (c) resistance of wire (d) time for which the current passes through it. 13. The non-oscillating current is called as.... (a) A.C. (b) D.C. (c) electric current (d) non-oscillating current 14. The... at any point on the magnetic lines of force gives the direction of the magnetic field at that point. (a) tangent (b) circle (c) radius (d) chord SCHOOL SECTION 107

3 SCIENCE & TECHNOLOGY MT EDUCARE LTD is an alloy of iron, nickel, aluminium and titanium. (a) Nipermag (b) Nichrome (c) Alnico (d) Tin is aluminium-nickel-cobalt alloy. (a) Alnico (b) Tin (c) Nichrome (d) Nipermag 17. If a conductor is moving inside a magnetic field or magnetic field is changing around a fixed conductor... is generated. (a) electric current (b) heat (c) sparks (d) light 18. The process, by which a changing magnetic field in a conductor induces a current in another conductor, is called.... (a) electromagnetic induction (b) potential difference (c) electrostatics (d) sparks 19. If a live wire and a neutral wire comes in direct contact or touch each other,... takes place. (a) short circuiting (b) magnetic effect (c) heating (d) electro-magnetism means the flow of large amount of current in the circuit beyond the permissible value of current. (a) Overloading (b) Drip (c) Sparkling (d) Resistance 21. French scientist... suggested that the magnet must also exert an equal and opposite force on the current carrying conductor. (a) joule (b) newton (c) ampere (d) fleming reverses the direction of current in the armature coil of electric motor. (a) Brushes (b) Armature coil (c) Magnet (d) Commutator 23. The direction of induced current in an AC generator changes once in each.... (a) two revolutions (b) one revolution (c) half revolution (d) one fourth revolution 24. A... is used to detect the presence of current in a circuit. (a) voltmeter (b) variable resistance (c) galvanometer (d) Tap key 25. Electro-magnetic induction was first studied by.... (a) Ampere (b) Faraday (c) Oersted (d) Joule Answers : 1. generator 2. increases 3. right hand rule 4. Oersted 5. magnetic field 6. magnetic field 7. degree of closeness of field lines 8. radius of circular wire 9. magnet 10. current and number of turns per unit length magnitude of current passing through it 13. D.C. 14. tangent 15. Nipermag 16. Alnico 17. electric current 18. electromagnetic induction 19. short circuiting 20. Overloading 108 SCHOOL SECTION

4 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 21. ampere 22. Commutator 23. half revolution 24. galvanometer 25. Faraday. Q.I (B) Match the following : 1. Column I Column II (i) Electric motor (a) Transmission over long distance (ii) Electric generator (b) Domestic appliances like mixers, refrigerators (iii) A.C. current (c) One directional current (iv) D.C. current (d) Mechanical energy to electric energy ( e) Behaves like a magnet (i - b), (ii - d), (iii - a), (iv - c). 2. Column I Column II (i) Solenoid (a) Magnetic effect of current (ii) Magnetic lines of force (b) Direction of current and magnetic field (iii) Hans Oersted (c) Behaves like a magnet (iv) Right hand rule (d) Never intersect ( e) Electric motor (i - c), (ii - d), (iii - a), (iv - b). 3. Column I Column II (i) Earthing (a) Electric motor (ii) Overloading (b) Electric generator (iii) Magnetic force (c) Prevents shocks (iv) Magnetic induction (d) Electric geyser (e) Flow of high current (i - c), (ii - e), (iii - a), (iv - b). Q.I (C) State whether the following statements are true or false. If false write the corrected statement : *1. Magnetic poles exists in pairs. True. *2. Magnetic field increase as we go away from a magnet. False. Magnetic field decreases as we go away from a magnet. *3. Magnetic lines of force always cross each other. False. Magnetic lines of force never cross each other. *4. Electric generator is used to generate current. True. 5. The magnetic field of solenoid has few properties similar to bar magnet. False. The magnetic field of solenoid has all properties similar to bar magnet. 6. The direct current always flows in one direction. True. 7. Direct current is used for long distance transportation of electricity. False. A.C. is used for long distance transportation of electricity. 8. Water should be used in electrical fire. False. Water should not be used in electrical fire. 9. If there is fire caused by sparking, the main switch should be switched off immediately. True. 10. Magnetic lines of force start from south pole and ends on south pole. False. Magnetic lines of force start from north pole and ends on south pole. SCHOOL SECTION 109

5 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 11. Alnico is an alloy used to make temporary magnet. False. Alnico is an alloy used to make permanent magnet. 12. Commutator increases the direction of current in the armature coil. False. Commutator reverses the direction of current in the armature coil. 13. Galvanometer is an instrument used to detect presence of current in circuit. True. 14. Overloading means flow of large current within permissible value of current. False. Overloading means flow of large current beyond permissible value of current. Q.I (D) Find the odd man out : *1. Loud speaker, Microphone, Electric motor, Bar magnet. Bar magnet. It is a magnet while the rest are devices working on electromagnetism. *2. Armature coil, Brushes, Commutator, Direct current. Direct current. The remaining are parts of a generator or motor. *3. Fuse, Insulating material, Rubber shoes, Generator. Generator. The remaining are safety devices. 4. Refrigerator, Electric heater, Electric fan, Mixer grinder. Electric heater. It works on the principle of heating effect of current, while the others work on electromagnetism. 5. Fuse wire, Phase wire, Neutral wire, Earth wire. Fuse wire. It is a safety device whereas the others are the types of cables used in the electric supply. *6. Voltmeter, Ammeter, Galvanometer, Thermometer. Thermometer. The rest work on the principle of electromagnetism. Q.II (A) Define the following : *1. Magnetic field. The area adjoining the magnet comprising of magnetic lines of force is called magnetic field. *2. Solenoid. A coil of many turns of insulated copper wire wrapped in the shape of cylinder is called a solenoid. *3. Electric motor. A device which converts electrical energy into mechanical energy is called an electric motor. 4. Electric generator. An electric device which converts mechanical energy into electrical energy is called an electric generator. 5. Electro-magnetic induction. The process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electro-magnetic induction. Q.II (B) State the following : *1. Right hand thumb rule. Imagine that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current, then the curled fingers around the conductor will give the direction of the magnetic field. Current Right hand Magnetic field 110 SCHOOL SECTION

6 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. Fleming s left hand rule. Motion of the conductor Thumb Magnetic field Magnetic field Middle finger Forefinger Current Current Motion of the conduction Stretch the forefinger, the central finger and the thumb of you left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger shows the direction of the current, then the thumb will point towards the direction of the motion of the conductor. *3. Fleming s right hand rule. Motion of the conductor Thumb Magnetic field Magnetic field Motion of the conduction Induced current Forefinger Induced current Middle finger Stretch the thumb, forefinger and middle finger of the right hand so that they are perpendicular to each other. It the forefinger indicates the direction of the magnetic field and the thumb shows the direction of the motion of conductor, the middle finger will show the direction of induced current. Q.III (A) Give scientific reasons : *1. Wires carrying electricity should not be touched bare footed. 1. If we touch the wire bare footed, a large current may pass through our body. 2. So we may receive severe shock. This shock may sometimes cause death. 3. Therefore while working with electricity we must wear gloves made of insulated material and rubber soled shoes. 4. Hence wires carrying electricity should not be touched bare footed. *2. We should not use many electrical appliances simultaneously. 1. If many electrical appliances of high power rating such as geyser, heater, motor, oven are switched on simultaneously, overloading occurs. 2. This may result in switching off of the fuse resulting in lose of power supply. SCHOOL SECTION 111

7 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 3. It may also cause fire. Hence we should not use many electrical appliances simultaneously. *3. Alloys like alnico or nipermag are used in industry. 1. Alloys like alnico or nipermag are very hard and are used in the production of permanent magnet. 2. Nipermag is an alloy of iron, nickel aluminium and titanium. Alnico is aluminium, nickel and cobalt alloy of iron. 3. Permanent magnet of such alloys are used in microphones, loudspeakers, electric clocks and many other devices like ammeter, voltmeter and speedometer. *4. A magnetic crane is used to load and transport scrap iron. 1. A magnetic crane consist of a magnetic disc to which all scrap and loose iron material get attracted. 2. Magnetism is induced in the disc, as it is not feasible to create a permanent magnet of such a big size and store such a big magnet. 3. Hence magnetic crane is used to load and transport scrap iron. 5. Most of the electrical appliances need earthing. 1. Earthing connects the metallic body of electrical appliances deep inside the earth. 2. In case the appliance is faulty, the live wire comes in contact with the metal body of the appliances. 3. Due to earthing the current flowing through the metal body passes to earth and the user handling the device does not get shock. Therefore most of the electrical appliances having a metallic body need earthing. 6. In rainy season, we should not touch the wall of building. 1. In rainy season, the wall of building becomes wet due to rain. 2. If bare wire comes in contact with such wet wall, electric current begins to flow through the wall. 3. A person touching the wall gets severe shock. Therefore in rainy season, we should not touch the wall of building. 7. Magnetic resonance imaging (MRI) is used to study the body of patients. 1. Electric current produces magnetic field even if the current is very weak. 2. If we touch something, our nerves carry an electrical impulse to the muscles we need to use. This produces weak magnetic field. 3. This forms the basis of obtaining images of heart and brain or images of different parts of the body. 4. This principle is used in magnetic resonance imaging (MRI). 8. During a short-circuit, the circuit catches fire. 1. During a short-circuit, the resistance of the circuit becomes very small and hence, huge amount of current flows through it. 2. This produces a large amount of heat and raises the temperature. 3. As a result, the circuit catches fire. Q.III (B) Answer the following questions in short : *1. State the characteristics of magnetic lines of force. 1. Magnetic lines of force are closed continuous curves. They start from north pole and ends on south pole. 2. The tangent at any point on the magnetic lines of force gives the direction of the magnetic field at that point. 3. No two magnetic lines of force can intersect each other. 4. Magnetic lines of force are crowded where the magnetic field is strong and far from each other where the field is weak. 112 SCHOOL SECTION

8 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. State the applications of electric motor. 1. These motors are used in domestic appliances like mixers, blenders, refrigerators and washing machines. 2. These motors are used in electric fans, hair dryers, record players, tape recorders and blowers. 3. These motors are used in electric cars, rolling mills, electric cranes, electric lifts and electric trains. 3. State the factors on which magnetic field due to current carrying conductor depends. The factors on which magnetic field due to current carrying conductor depends are : 1. Magnitude of the field produced at a given point is directly proportional to the magnitude of the current passing in the wire. 2. Magnetic field in the wire decreases as the distance from the wire increases. 4. State any four applications of magnetic effect of electric current. 1. Electric bell 2. Telephone earpiece 3. Galvanometer 4. Ammeter. 5. How is the magnetic force used to study structure of the matter? 1. Current is a flow of charge. 2. The moving charges in a magnetic field would experience a force. 3. This property is used to accelerate charged particles to very high energies. 4. These high energies of the particles are used to study structure of the matter. Current Restoring spring Galvanometer can be considered as an electric motor with a needle. Only for reference 6. What is a galvanometer? How does it work? 1. A galvanometer is an instrument which is used to detect the presence of current in a circuit. 2. If the current in a circuit is zero, the galvanometer will show zero deflection or no deflection. 3. The deflection is either side of the zero mark depending on the direction of the current. 7. What are the main components of AC generator? 1. Armature 2. Strong magnets 3. Slip rings 4. Brushes. 8. What is short circuiting? 1. If a live wire and a neutral wire come in direct contact or touch each other, short-circuiting takes place. 2. During a short circuit, the resistance of the circuit becomes very small and hence huge amount of current flows through it. 3. This produces a large amount of heat and raises the temperature. SCHOOL SECTION 113

9 SCIENCE & TECHNOLOGY MT EDUCARE LTD. *9. Observe the figure. If the current in the coil A is changed, will some current be induced in the coil B? Explain. A B + G 1. If the current in coil A is changed, then some current will be induced in coil B. 2. As the current in coil A changes, the magnetic field related to it also change. 3. Due to the changing magnetic field, current is induced in coil B. Q.III (C) Write short notes for the following : 1. Earthing. 1. Many electrical appliances have a metallic body such as electric heater, toaster, oven, refrigerators. 2. If by chance insulation of the wire melts the live wire may touch the metallic body of the appliance. 3. If we touch this body with bare hands, we may receive a severe shock. To avoid such accidents, metal casings of all electrical appliance must be earthed. 4. One end of a copper wire is connected to the metal case of the appliance and the other end is connected to the copper plate which is buried deep inside the earth. 5. This system of connecting the metal casing of an electrical appliance with a copper plate buried deep inside the earth is called as earthing. 2. Domestic electric circuit. 1. State board of electricity provides us the electric power supply for the houses and factories. 2. This is supplied through underground cables or overhead wires on poles. These wires or cables are of three types : phase wire (or live wire), neutral wire and earth wire. 3. These wires coming from power sub stations are connected to an electric meter installed in a house or a factory. The potential difference between the live wire and neutral wire is 220V-250V. 4. The output power from electric meter is taken to the main switch. A fuse known as main fuse is placed in the path of live or phase wire. 5. The output electric power from the main switch is distributed to the domestic electric circuit. 6. In each separate circuit, different appliances can be connected across the live and neutral wires. Each appliance has equal potential difference, they are connected parallel to each other. 114 SCHOOL SECTION

10 MT EDUCARE LTD. SCIENCE & TECHNOLOGY Earth wire Live wire Neutral wire 220 V Electricity board s fuse Electricity meter Distribution box containing main switch and fuses for each circuit 3. Use of magnetism in medical sciences. 1. Electric current produces magnetic field, even if the current is very weak. 2. Ion current (very weak) which travels along the nerve cells in our body produces magnetic fields. 3. If we touch something, our nerves carry an electrical impulse to the muscles we need to use. This produces weak magnetic field. 4. Heart and brain are the two main organs where significant magnetic fields can be produced. 5. This forms the basis of obtaining images of heart and brain or images of different parts of the body. 6. This is done using a technique called Magnetic Resonance Imaging (MRI). 4. Force on current carrying conductor is a magnetic field. 1. Electric current flowing through a conductor produces a magnetic field. 2. The field so produced exerts a force on a magnet placed in the vicinity of the conductor. 3. The magnet must also exert an equal and opposite force on the current carrying conductor. 4. The direction of the force depends on the direction of the current and the direction of magnetic field. 5. The displacement of the rod is maximum, when the direction of the current is at right angles to the direction of magnetic field. Q.IV (A) Distinguish between : *1. Electric motor and Electric generator. Electric motor 1. A device which converts electrical energy into mechanical energy is called an electric motor. 2. Electric motor is based on a principle, when current carrying conductor is placed in a magnetic field a force acts on it. 3. It uses electricity. Electric generator 1. An electric device which converts mechanical energy into electrical energy is called an electric generator. 2. It works on the principle of electromagnetic induction. 3. It generates electricity. SCHOOL SECTION 115

11 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 2. Direct current and Alternating current. Direct current 1. The magnitude and direction of the current is constant 2. This type of current cannot be used for large scale, for household purpose. 3. The frequency of direct current is zero. 3. Permanent bar magnet and Electromagnet. Permanent bar magnet 1. It is a permanent magnet. 2. It produces a weak magnetic force. 3. Its strength can not be changed. 4. The north-south polarity of a permanent magnet is fixed. 4. AC generator and DC generator. AC generator 1. AC generator generates alternating current. 2. It converts mechanical energy into electrical energy. Alternating current 1. The magnitude and direction of current reverses periodically. 2. This type of current is used in electrical household appliances such as electric heater, iron, refrigerator etc. 3. Frequency of alternating current is 50Hz in India. Electromagnet 1. It is a temporary magnet. Its magnetism is only as long as current flows through it. 2. It produces a strong magnetic force. 3. The strength of an electromagnet can be changed by changing the number of turns in the coil or by changing the quantity of current passing through it. 4. The north-south polarity of an electromagnet can be changed by changing the direction of current in its coil. DC generator 1. DC generator generates direct current. 2. It converts mechanical energy into electrical energy. Q.IV (B) Draw a neat and labelled diagram of the following : 1. Magnetic line of force around a bar magnet. N S Magnetic lines of force around a bar magnet 116 SCHOOL SECTION

12 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. Magnetic field of a straight conductor carrying current. A P + 3. Magnetic field lines due to current through circular loop. N S Magnetic field lines due to current through circular loop 4. Magnetic lines of force through a solenoid. S N + K Magnetic lines of force through a solenoid SCHOOL SECTION 117

13 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 5. Electric motor. Iron core B C N S A D B 1 R 1 R 2 B 2 + Battery + 6. Domestic electric circuit. Earth wire Live wire Neutral wire 220 V Electricity board s fuse Electricity meter Distribution box containing main switch and fuses for each circuit Q.V Answer the following in detail : 1. Give the construction and working of electric motor. 1. A device which converts electrical energy into mechanical energy is called an electric motor. 2. Electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force. Construction : Iron core B C N S A D B 1 R 1 R 2 B 2 + Battery SCHOOL SECTION

14 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 1. Armature coil : A large number of turns of insulated copper wire wound on iron core in rectangular shape forms an armature coil ABCD as shown in the figure. 2. Strong magnet : The armature coil is placed in between two pole pieces (N and S) of a strong magnet. This provides a strong magnetic field. 3. Split ring commutator : It consist of two halves (R 1 and R 2 ) of a metallic ring. The ends of the armature coil are connected to these rings. Commutator reverses the direction of current in the armature coil. 4. Brushes : Two carbon brushes B 1 and B 2 are used to press the commutator. 5. Battery : The battery supplies the current to the armature coil. Working of the electric motor : (a) When current is passed through the coil ABCD, arms AB and CD experience force. (b) According to Fleming s left hand rule the force experienced by arm AB is in the upward direction and arm CD in the downward direction. Both these forces are equal and opposite. (c) This force rotates the coil in clockwise direction until the coil is vertical. At this position, the contact between commutator and brushes break. So the supply to the coil is cut off. Thus no force acts on the coil. (d) But the coil does not stop due to inertia. It goes on rotating until the commutator again comes in contact with the brushes B 1 and B 2. Again the current starts passing through the coil and the arm AB rotates through 90º, 180º, 270º and 360 degrees. ( e) Now the force acting on arm AB is upward and CD is downward. Again this force moves the coil in clockwise direction. (f) Thus, the coil rotates with the help of electrical energy. The coil of DC motor continues to rotate in the same direction. 2. Give the construction and working of AC generator. B C N S A D B 1 R 1 G B 2 R 2 Construction : The main components of AC generator are as shown in figure. 1. Armature 2. Strong magnets 3. Slip rings and 4. Brushes. SCHOOL SECTION 119

15 SCIENCE & TECHNOLOGY MT EDUCARE LTD. Armature, strong magnets and brushes are the same as used for electric motors. 3. Slip rings : The two ends of the armature coil are connected to two brass slip rings R 1 and R 2. These rings rotate along with the armature coil. Working : (a) When the armature coil ABCD rotates in the magnetic field provided by the strong magnets, it cuts the magnetic lines of forces. (b) Thus the changing magnetic field produces induced current in the coil. The direction of induced current is determined by the Flemings right hand rule. (c) The current flows out through the brush B 1 in one direction in the first half of the revolution and through the brush B 2 in the next (d) half revolution in the reverse direction, this process is repeated. Therefore, the induced current produced is of alternating nature. Such a current is called as alternating current. 3. Give the construction and working of D.C. generator or Dynamo. Iron core B C N S A B 1 R 1 R 2 D B Construction : The main components of D.C. generator are : 1. Armature coil 2. Strong magnets 3. Split rings or commutator 4. Brushes 5. Bulb. The components such as armature coil, strong magnet, brushes are the same which are used for AC generators. Slip rings or commutator is the same as used in electric motor. Working of DC generator : (a) When the coil of DC generator rotates in the magnetic field, potential difference is produced in the coil. This gives rise to the flow of current. This is shown by glowing of the bulb. (b) In D.C. generator, the flow of current in the circuit is in the same (c) direction as long as the coil rotates in the magnetic field. This is because one brush is always in contact with the arm of the armature moving up and other brush is in contact with the arm of the armature moving downward in the magnetic field. 4. State general precautions for safety with respect to electricity. 1. Electric fuse must be used in electric circuits. It protects the electrical appliances by stopping the flow of any excess electric current. 2. Wires carrying electricity should not be touched bare footed. SCHOOL SECTION

16 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 3. While working with electricity we must wear gloves made of insulated material and rubber soled shoe so that current may not flow through our body. 4. The insulations of the wire must be checked periodically. 5. To avoid electric shocks, metal casing of all electrical appliances must be earthed. 6. Electric circuits should be switched off during lightning. This will save the electrical appliances from burning. Q.VI Answer the following questions in one sentence each : 1. What will happen when a magnetic needle is brought near a current carrying conductor? The magnetic needle will be deflected because a magnetic field is produced around a current carrying conductor. 2. What is a solenoid? A coil of many turns of insulated copper wire wrapped in the shape of cylinder is called a solenoid. 3. What are permanent magnets made? Permanent magnets are usually made of carbon steel chromium steel, cobalt and tungsten steel and some alloys, like nipermag and Alnico. 4. What is Nipermag and Alnico? Nipermag is an alloy of iron, nickel, aluminium and titanium. Alnico is an alloy of aluminium, nickel and cobalt. 5. What is electric motor? A device which converts electrical energy into mechanical energy is called an electric motor. 6. Name a device which acts on the magnetic effects of current. Electric motor is the device which acts on the magnetic effect of current. 7. What is the frequency of AC of India? The frequency of AC in India is 50 Hz. 8. What is an electric generator? An electric device which converts mechanical energy into electrical energy is called electric generator. 9. Which are the three wires used in electrical connections? The three wires used in electrical appliances are phase wire or live wire, neutral wire and earth wire. 10. What is meant by short circuit. If a live wire and a neutral wire come in direct contact or touch each other, sparks are caused between them. This is called as short circuit. 11. What is the conclusion of Oersted s experiment? Oersted s experiment proves that an electric current produces a magnetic field around it. 12. What is meant by overloading? Overloading means the flow of large amount of current in the circuit beyond the permissible amount of current. 13. What is the voltage of domestic supply? The voltage of domestic supply is 220V - 250V. 14. Name the safety device used to save the electrical appliances. Electric fuse is the safety device used to save electrical appliances. SCHOOL SECTION 121

17 SCIENCE & TECHNOLOGY MT EDUCARE LTD. ACTIVITY BASED QUESTIONS ACTIVITY : 5.1 (For question refer to Text Book page No. 49) The iron filing will arrange themselves along the magnetic lines of force as shown in the figure. ACTIVITY : 5.2 (For question refer to Text Book page No. 49) It is seen that at each point the magnetic needle is tangential to the magnetic lines of force. ACTIVITY : 5.3 (For question refer to Text Book page No. 50) When the key is pressed the magnetic needle gets deflected, the deflection of the needle is reversed if polarity of the cell is reversed. ACTIVITY : 5.4 (For question refer to Text Book page No. 51) 1. If the current is increased, then the deflection of the needle also increases. 2. As the needle is moved away from the current carrying wire,, deflection of the needle decreases. ACTIVITY : 5.5 (For question refer to Text Book page No. 51) The iron filing arrange themselves in the form of concentric circle at every point along the circular loop of wire. ACTIVITY : 5.6 (For question refer to Text Book page No. 52) 1. When the wire is connected to the cell the iron pins get attracted to it. 2. When the wire is disconnected from the cell the iron pin does not get attracted. ACTIVITY : 5.7 (For question refer to Text Book page No. 52) This activity shows that when a current carrying conductor is placed in a magnetic field it experiences a force. ACTIVITY : 5.8 (For question refer to Text Book page No. 53) The coil rotates because when current is passed through the coil, a magnetic force is created on it. This force rotates the coil. 122 SCHOOL SECTION

18 MT EDUCARE LTD. SCIENCE & TECHNOLOGY ACTIVITY : 5.9 (For question refer to Text Book page No. 55) 1. When north pole of the magnet is moved towards B end of the coil. The galvanometer will deflect to the right. 2. When south pole of the magnet is moved towards B end of the coil. The galvanometel will deflect to the left. 3. If both coil and magnet are kept stationary, then no deflection is observed. ACTIVITY : 5.10 (For question refer to Text Book page No. 55) 1. The deflection in the needle is observed only when the current in the coil (I) varies. 2. There is no deflection when the current is zero or constant. ACTIVITY : 5.11 (For question refer to Text Book page No. 57) 1. The components in the fig 5.14 are Armature coil, strong magnets, split ring commutator, Brushes and Batlary. 2. The components in fig 5.18 are armature coil, strong magnets, split rings or commutator, Brushes Bulb. 3. The components in fig 5.19 are Armature coil, strong magnets, split rings, Brushes. Similarity : All there figures have an armature coil, strong magnet, split ring and brushes. Difference : Fig 5:14 includes a battery, where as Fig 5:18 and Fig 5:19 have a bulb in the circuit. SCHOOL SECTION 123

19 SCIENCE & TECHNOLOGY MT EDUCARE LTD. HOTS QUESTIONS & ANSWERS 1. State the rule you will use. (a) To find the direction of the motion of the conductor, if direction of current and magnetic field are known to you. (b) To find the direction of induced current, if direction of motion of conductor and magnetic field are known to you. (a) If I know the current and directions of magnetic field, then I will use Fleming s Left Hand Rule to know the direction of motion of the conductor. (b) If I know the direction of motion and magnetic field, I will use Fleming s Right Hand Rule to know the direction of induced current. 2. I have an electromagnet. Give me two methods to increase the magnetism. The two methods to increase the magnetism are : (a) Increase the current in the coil. (b) Increase the number of turns of the coil. 3. Observe the figure. A B + G (a) If the value of resistance is changed in the first circuit what effect would be observed in (i) coil B (ii) in galvanometer? (b) If the terminals of battery are reversed what change would you observe in galvanometer? (a) When the value of resistance is changed in the 1st circuit there will be a change in (i) the induced current in the coil B. (b) If the terminals of battery are reversed the galvanometer will show deflection in the opposite direction. 124 SCHOOL SECTION

20 S.S.C. Marks : 30 CHAPTER 5 : ALL ABOUT ELECTROMAGNETISM SCIENCE Duration : 1 hr. Q.I [A] Choose the correct alternatives and rewrite the complete 4 sentences : 1. Electro-magnetic induction was first studied by.... (a) Ampere (b) Faraday (c) Oersted (d) Joule is aluminium-nickel-cobalt alloy. (a) Alnico (b) Tin (c) Nichrome (d) Nipermag 3. The direction of the magnetic field around a straight conductor carrying current is given by.... (a) Right hand rule (b) Fleming s left hand rule (c) Fleming s right hand rule (d) None of these 4. The... at any point on the magnetic lines of force gives the direction of the magnetic field at that point. (a) tangent (b) circle (c) radius (d) chord Q.I [B] Match the columns : 4 Column A Column B 1. Solenoid (a) Electric motor 2. Magnetic lines of force (b) Magnetic effect of current 3. Hans Oersted (c) Direction of current and magnetic field 4. Right hand rule (d) Behaves like a magnet (e) Never intersect Q.I [C] State whether True or False : 2 1. Commutator increases the direction of current in the armature coil. 2. Magnetic lines of force always cross each other. Q.I [D] Find the odd man out : 2 1. Fuse wire, Phase wire, Neutral wire, Earth wire. 2. Fuse, Insulating material, Rubber shoes, Generator.

21 Q.II [A] Define the following : 2 1. Solenoid. 2. Electro-magnetic induction. Q.II [B] State the following : 2 1. Fleming s left hand rule. 2. Right hand rule. Q.III Answer the following : (Any Three) 6 1. Distinguish between Permanent bar magnet and Electromagnet. 2. A magnetic crane is used to load and transport scrap iron. Why? 3. Most of the electrical appliances need earthing. Why? 4. Wire carrying electricity should not be touched bare footed. Why? Q.IV Draw a neat and labelled diagram of the following : 4 1. Solenoid. 2. Domestic electric circuit. Q.V Answer the following questions in brief : (Any One) 4 1. Give the construction and working of AC generator. 2. Give the construction and working of electric motor. Best Of Luck