Quick Review - CH - 13 Magnetic Effect of Electric Current

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CH-13   Magnetic Effects of Electric Current

Class X

Syllabus of Magnetic effects of current: Magnetic field, field lines, field due to a current-carrying conductor, field due to current carrying coil or solenoid; Force on the current-carrying conductor, Fleming's left-hand rule. Electromagnetic induction. The induced potential difference, Induced current. Fleming's Right Hand Rule, Direct current. Alternating current: frequency of AC. Advantage of AC over DC. Domestic electric circuits.

1. A compass needle is a small magnet. It's one end, which points towards the north, is called a north pole, and the other end, which points towards the south, is called a south pole.
2. A magnetic field exists in the region surrounding a magnet, in which the force of the magnet can be detected.
3. Magnetic the field is a quantity that has both direction and magnitude.
4.     The field lines emerge from the North Pole and merge at the South Pole. The direction of field lines inside the magnet is from its south pole to its north pole. So the magnetic field lines are closed curves. The relative strength of the magnetic field is stronger at the pole, where the field lines are crowded.
5. No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
6. The magnetic field produced at a given point increases as the current through the wire increases. Therefore the magnetic field is directly proportional to current
7. The magnetic field produced by a given current in a conductor decreases as the distance between the magnet and current-carrying conductor increases. Therefore the magnetic field is indirectly proportional to the distances between a magnet and current-carrying conductor.
8. The concentric circles representing the magnetic field around a current-carrying straight wire becomes larger and larger as we move away from it.
9. If we are holding a current-carrying straight conductor in our right hand such that the thumb points towards the direction of the current. Then our fingers will wrap around the conductor in the direction of the field lines of the magnetic field. This is known as the Right-hand thumb rule.
10. The magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it. Therefore, if there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.
11. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
12. The pattern of the magnetic field lines around a current-carrying solenoid and the pattern of the field with the magnetic field around a bar magnet is similar. In fact, one end of the solenoid behaves as a magnetic north pole, while the other behaves as the South Pole.
13. The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid.
14. A strong magnetic field produced inside a solenoid can be used to magnetize a piece of magnetic material, like soft iron, when placed inside the coil The magnet so formed is called an electromagnet.
15. French scientist Andre Marie Ampere (1775–1836) suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor.
16. Force is exerted on the current-carrying conductor when it is placed in a magnetic field. The direction of force is reversed when the direction of current through the conductor is reversed. If the direction of the field vertically downwards by interchanging the two poles of the magnet, the direction of force acting on the current-carrying rod gets reversed. It shows that the direction of the force on the conductor depends upon the direction of the current and the direction of the magnetic field.
17. 1.  The displacement of the rod is largest or the magnitude of the force is the highest when the direction of current is at right angles to the direction of the magnetic field. By using Fleming’s left-hand rule, we can find the direction of the force on the conductor.
18. Fleming’s left-hand rule - According to this rule, stretch the thumb, forefinger, and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of the magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of motion or the force acting on the conductor.
19. 1.  Devices that use current-carrying conductors and magnetic fields include electric motor, electric generator, loudspeakers, microphones, and measuring instruments. 
20. ELECTRIC MOTOR -- An electric motor is a rotating device that converts electrical energy to mechanical energy. The electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc.
21. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.
22. The coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.
23. The commercial motors use (i) an electromagnet in place of the permanent magnet; (ii) a large number of turns of the conducting wire in the current-carrying coil; and (iii) a soft iron core on which the coil is wound.
24. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.
25. First studied by English physicist Michael Faraday. In 1831, Faraday made an important breakthrough by discovering how a moving magnet can be used to generate electric currents.
26. A galvanometer is an instrument that can detect the presence of a current in a circuit. The pointer remains at zero for zero current flowing through it. It can deflect either to the left or to the right of the zero marks depending on the direction of the current.
27. Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.
28. If we move a strong bar magnet with its north pole towards the one end of the coil( in left direction), there is a momentary deflection in the needle of the galvanometer, say to the right. This indicates the presence of a current in the coil in the right direction. The deflection becomes zero the moment the motion of the magnet stops. If we withdraw the north pole of the magnet away from the coil ( in the right direction), now a galvanometer is deflected toward the left, showing that the current is now set up in the direction opposite to the first(in left). 
29.  When the electric current through the primary coil is changing, the magnetic field associated with it also changes, and due to this the magnetic field lines around the secondary coil also change, and thus a potential difference is induced in the secondary coil. Hence the change in magnetic field lines associated with the secondary coil is the cause of induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.
30. Electromagnet – An electromagnet is a type of magnet consists of a core of soft iron wrapped around with a coil of insulated copper wire. When a current is passed through it the magnetic field is produced which is concentrated at the center of the coil. 
31. The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field (According to Fleming’s right-hand rule.)
32. ELECTRIC GENERATOR- A generator converts mechanical energy into electrical energy. It works on the basis of electromagnetic induction and generates electricity for use in homes and industry. In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
33.  A current, which changes direction after equal intervals of time, is called an alternating current is known as AC current and the device which produces AC current is called an AC generator.
34.  A unidirectional current is called DC current that flows in the same direction and devices which produce DC current is called a DC generator.
35. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
36. Most power stations constructed these days produce AC. In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz. (Or In 1 sec – 100 times)
37. An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.
38. Often, two separate circuits are used, one of 15 A current rating for appliances with higher power ratings such as geysers, air coolers, etc. The other circuit is of 5 A current rating for bulbs, fans, etc.
39. The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc. The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current. Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.
40. 1. A fuse is another safety measure in the circuit that prevents damage to the appliances and the circuit due to overloading. The Joule heating that takes place in the fuse melts it to break the electric circuit.
41. 1. Overloading can occur when the live wire and the neutral wire come into direct contact. This occurs when the insulation of wires is damaged or there is a fault in the appliance. In such a situation, the current in the circuit abruptly increases. This is called short-circuiting.
42. 1. Overloading is caused by connecting too many appliances to a single socket and can also occur due to an accidental hike in the supply voltage. 
43. 1. Field lines are used to represent a magnetic field and it helps to detect the direction of the magnetic force on the north pole and magnetic strength.
44. A pattern of the magnetic field around a conductor due to an electric current flowing through it depends on the shape of the conductor. The magnetic field of a solenoid carrying a current is similar to that of a bar magnet.
45. A current-carrying conductor, when placed in a magnetic field, experiences a force. If the direction of the field and that of the current are mutually perpendicular to each other, then the force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. This is the basis of an electric motor.
46. The electric motor is a device that converts electric energy into mechanical energy.
47. The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time.
48. The magnetic field may change due to relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current-carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor.
49. In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside the earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.