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1. Electromagnetism 2. Magnetic Effects of an Electric Current 3. Electromagnetic Induction 1. When the current flows through a straight conductor: (a) It behaves like a magnet. (b) The magnitude of magnetic intensity is directly proportional to the magnitude of current flowing through the conductor. (c) The magnetic field set up by the conductor is at right angles to the direction of the flow of current. (d) The direction of magnetic field depends upon the direction of the flow of current. 2. Ampere s Swimming Rule : Imagine a swimmer (always looking towards magnetic needle), swimming in the direction of current, such that current enters from his feet and leaves from his head. Then, the direction in which left hand of swimmer points gives the direction of the north pole of magnetic needle. 3. Properties of magnetic lines of force around straight conductor carrying current : (1) The magnetic lines of force are in the form of concentric circles. (2) The plane of magnetic lines of force is at right angles to the direction of the current. (3) The direction of magnetic lines of force depends upon the direction of current. (4) On increasing the magnitude of current, the number of magnetic lines of force increases. Class-X 1 Key points 4. Right Hand Thumb Rule or Right Hand Palm Rule : Imagine you are holding a conductor with the palm of your right hand, such that 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 magnetic lines of force around the conductor. 5. Properties of magnetic lines of force around a circular loop : (1) The magnetic lines of force are circular at the points, where the current enters or leaves the coil. (2) Within the space enclosed by the coil, the lines of force are in the same direction. (3) Near the centre of coil the magnetic lines of force are almost parallel to each other. (4) At the centre of coil, the plane of magnetic lines of force is at right angles to the plane of circular coil. (5) With the increase in the strength of the current, the number of magnetic lines of force increases and hence magnetic intensity increases. 6. If the direction of current flowing through coil is anticlockwise, then the north polarity is created. If the direction of the current is clockwise, then the polarity is south. 7. 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 current is made to flow through it. Class-X 2 Key points 8. The intensity of magnetic field of a solenoid can be increased by: (1) Increasing the number of turns in the solenoid. (2) By increasing the strength of the current flowing through the solenoid. (3) By placing soft iron core along the axis of the solenoid. (4) By laminating the soft iron core. 9. Rough handling, heating and hammering, demagnetising with other magnet and by using alternating current are some of the methods of demagnetisation of a permanent magnet. 10. Electromagnets are used : (i) in electrical appliances, such as electric bell; electric relay; electric motor; electric generator, etc. (ii) for magnetic separation and for lifting heavy ferromagnetic loads. 11. Electric bell is based on the principle that when an electric current flows through a closed coil, it behaves like a magnet. 12. A freely-suspended conductor carrying current experiences a force, when placed in a magnetic field. The force experienced by it is : (i) directly proportional to the strength of the current. (ii) directly proportional to the magnetic field. (iii) directly proportional to the length of the conductor within the magnetic field. Class-X 3 Key points 13. The direction in which a freely suspended conductor carrying current moves, depends upon : (i) the direction of the flow of current. (ii) the direction of magnetic lines of force, relative to the direction of flow of current. 14. 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 current. Then, the thumb points in the direction of the motion of the conductor. 15. An electric motor is a device which changes electrical energy into mechanical energy. 16. The jerky motion of electric motor is converted into smooth circular motion by winding number of coils on the same soft iron core at angles of a few degrees to each other. 17. The speed of rotation of electric motor can be increased by: (1) increasing the number of turns in the coil (2) increasing area of cross-section of the coil. (3) increasing the strength of radial magnetic field. (4) increasing the current flowing into the coil. (5) increasing number of coils wound on the same soft iron core. (6) laminating soft iron core. Class-X 4 Key points 18. 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. 19. 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. 20. 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. 21. Direction of induced current is found by : (i) Fleming s right hand rule, (ii) Lenz s law. (i) Fleming s Right Hand Rule : 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. (ii) Lenz s Law : In all cases of electromagnetic induction, the direction of induced current is such that it always opposes the cause, which produces it. Class-X 5 Key points 22. Electric generator is a device which converts mechanical energy into electrical energy. It is based on the principle of electromagnetic induction. 23. The induced e.m.f. in an electric motor is alternating in nature. It is maximum at 90 and 270 and minimum at 180 and 360 . 24. 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. 25. The direction of induced e.m.f. in an electric generator is found by Fleming s right hand rule. 26. Transformer : An electric device, which can increase or decrease e.m.f. of an alternating current is called transformer. 27. Principle of transformer : It is based on the principle of mutual induction, i.e., when an alternating current is passed in a primary coil wound over a soft iron core, an induced e.m.f. is produced in the secondary coil, wound on the same soft iron core. 28. Step-up transformer : A transformer which increases the applied e.m.f. of an alternating current is called step-up transformer. 29. Step-down transformer : A transformer which decreases the applied e.m.f. of an alternating current is called step-down transformer. Class-X 6 Key points 30. In a step-up transformer, the primary coil has less number of turns, is thicker and less heavily insulated as compared to the secondary coil. 31. In a step-down transformer, the primary coil has more number of turns, is thinner and more heavily insulated as compared to the secondary coil. Class-X 7 Key points
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