Magnetism and Electricity ASSIGNMENT 1. Fill in the blank spaces by choosing the correct words from the list given below. List : magnetic field, magnetic keepers, electric bell, stop, magnetic induction, electromagnetism, current, north pole, non-magnetic. (a) The phenomenon due to which magnetism is produced in a magnetic substance by the mere presence of magnet, is called.... (b) Magnetic lines of force can easily pass through... substances. (c) The space around a magnet where its influence can be felt, is called,.... (d) The geographic south pole lies close to magnetic.... (e) A pair of soft iron pieces which preserve magnetism in a pair of bar magnets, are called.... (f) The branch of physics which deals with the relationship between electricity and magnetism, is called.... (g) The magnitude of magnetic intensity in a straight conductor increases with the increase in the magnitude of.... (h) Electromagnet is used in a common electric device, called.... (i) The induced current in a closed coil stops flowing, if the magnetic lines of force... changing within the coil. 2. Statements given below are incorrect. Write the correct statements. (a) The rectangular core of a transformer is made from hard steel. (b) The intensity of induced emf of generator decreases, with the increase in the speed of rotation of the coil. (c) The induced current produced in a closed coil, is direct in nature. (d) Similar poles of a magnet attract each other. 1
(e) An electric bell uses a powerful permanent magnet. (f) The right hand rule tells the direction of lines of electric force around a straight conductor. 3. Write True or False in front of the statements given below : (a) A steel core placed in a solenoid, acts as eletromagnet. (b) Like poles of magnets always repel one another. (c) Magnetic south pole of the Earth is towards its geographic north pole. (d) Transformers can change high voltage DC current to low voltage DC current. (e) An electromagnet is temporary in nature. (f) The strength of magnetic field of solenoid increases with the decrease in number of lines. (g) A permanent magnet is used in an electric bell. (h) When a permanent magnet is rapidly moved in and out of closed coil, an induced direct current is generated. (i) Electromagnets are used to separate iron from scrap of different metals. (j) A step-up transformer increases the AC voltage. 4. Tick ( ) the most appropriate answer. (a) The magnetic south pole of earth is situated near : (i) geographic south pole (ii) geographic north pole (iii) geographic east (iv) geographic west (b) When a straight conductor carrying current is held over a magnetic needle, such that current flows from north to south, then north of needle points towards : (i) east (ii) west (iii) north (iv) south (c) The direction of magnetic lines of force around a straight conductor, pointing vertically downward, such that current flows in vertically upward direction is : (i) clockwise, (ii) anticlockwise, (iii) at one end clockwise and other end anticlockwise, (iv) none of these. (d) The core of transformer is made of : (i) soft iron (ii) steel 2
(iii) copper (iv) aluminium (e) An electric generator converts : (i) mechanical energy into chemical energy. (ii) mechanical energy into magnetic energy. (iii) mechanical energy into electrical energy. (iv) electrical energy into mechanical energy. (f) Electric generators produced electricity, on the principle of : (i) magnets have attractive property (ii) conductors carrying current behave like magnet (iii) electromagnetic induction (iv) none of these (g) The strength of magnetic field in a solenoid, will be maximum, if it has : (i) 100 turns (ii) 75 turns (iii) 5 turns (iv) 20 turns (h) The mechanical energy produced in an electric motor will be maximum, if the current flowing through its coil, is : (i) 2 ampere (ii) 12 ampere (iii) 6 ampere (iv) 8 ampere (i) A freely suspended magnetic needle will always come to rest in : (i) north-east direction (ii) north-west direction (iii) north-south direction (iv) south-west direction (j) An electric device used to stabilise voltage is : (i) dynamo (ii) transformer (iii) voltmeter (iv) ammeter 5. Give four differences between a bar magnet and electromagnet. 6. (a) What do you understand by the term magnetic lines of force? (b) State four properties of magnetic lines of force. 7. Draw a diagram for the magnetic field around a bar magnet. 8. (a) Draw a diagram showing clearly, (i) magnetic north and south poles of the earth, (ii) geographic north and south poles of the earth, (iii) magnetic field around the earth. (b) Where is earth s magnetic field : (i) strongest? (ii) weakest? 3
9. (a) 10. The above figure shows a freely suspended needle over which is held a thick copper wire, parallel to the magnetic axis of magnetic needle. State the direction in which north pole of magnetic needle will turn in the following cases : (i) Conductor is held above magnetic needle and current flows from B to A. (ii) Conductor is held above magnetic needle and current flows from A to B. (iii) Conductor is held below magnetic needle and current flows from B to A. (b) What is your observation when the magnitude of current is increased in conductor in 9 (a)? (c) State the magnitude of maximum angle through which magnetic needle will turn. (d) Why does the magnetic needle turn about its axis in the above experiments? (e) What is your observation, if the current stops flowing in the conductor? (f) Name and state the rule, by which you can find the direction of motion of the north pole of freely suspended magnetic needle in above cases. (a) The above figures, show a cardboard through the centre of which passes a straight conductor. Draw the magnetic field around conductor, at point A in the diagrams, showing clearly the direction of magnetic lines of force. (b) State the rule, which helped you to find the direction of magnetic lines of force. (c) How can you increase the number of magnetic lines of force, around a conductor? 11. State how can you find the magnetic polarity from the face of circular coil carrying current. 4
12. (a) Draw a diagram to show the magnetic field around a circular coil carrying current. (b) State three characteristics of the magnetic lines of force produced by a circular coil carrying current. 13. Study the above figure carefully and answer the following questions : (a) In which direction will north of magnetic needle point, when current is switched on? Show the direction in the diagram. (b) What is the magnetic polarity at : (i) end A? (ii) end B? (c) What is the name given to such a coil? 14. (a) Draw a neat diagram, showing magnetic field around a solenoid. (b) Suggest three ways of increasing magnetic field around a solenoid. 15. (a) How can a solenoid be converted into an electromagnet? (b) How can solenoid be used to make permanent magnets? (c) Suggest four ways of increasing the strength of an electromagnet. (d) Give four uses of electromagnet. 16. Draw a neat and fully labelled diagram of an electric bell. 17. (a) What is the function of the following parts in an electric bell : (i) Electromagnet (ii) Armature (iii) Flat spring (iv) Contact screw 5
(b) Answer the following questions about electric bell : (i) Why are contact points made from silver cadmium alloy? (ii) How can you alter the frequency of ring in an electric bell? 18. (a) Draw a neat and labelled diagram of D.C. motor. (b) Explain briefly the working of D.C. motor. 19. (a) What is a transformer? (b) Draw a neat diagram of step-up transformer. (c) State the characteristics of primary coil of step-up transformer. 6