MAGNETIC EFFECTS OF CURRENT

Transcription

1 Magnet A magnet is an object, which attracts pieces of iron, steel, nickel and cobalt. Naturally Occurring Magnet Lodestone is a naturally occurring magnet. It is actually a black coloured, oxide ore of iron called magnetite (Fe 3 O 4 ), which behaves like a magnet. MAGNETIC EFFECTS OF CURRENT 1. Used in radio and stereo speakers, refrigerator doors. 2. In audio and video cassettes, tapes, hard disks. 3. In children s toys. 4. In hospitals to scan inner human body parts. Magnetic Field Space surrounding a magnet in which magnetic force (attraction or repulsion) is exerted is called magnetic field. Direction of magnetic field at a point is the direction of the resultant force acting on a hypothetical north pole placed at that point. Bar Magnet A bar magnet is a long rectangular bar of uniform cross section that attracts pieces of iron, steel, nickel and cobalt. Magnetic Needle (Plotting Needle, Plotting Compass, Magnetic Compass) It is a tiny magnet which is free to move in the horizontal plane. It is in the form of an arrow. The tip of compass represents its north pole and the tail of compass represents its south pole. Magnetic Lines Of Force Magnetic lines of force are the lines drawn in a magnetic field along which a north magnetic pole would move. The direction of magnetic lines of force at any point gives the direction of the magnetic force on a north pole placed at that point. Method To Plot Magnetic Lines Of Force Horse Shoe Type Magnet 1. By Using Iron Filings If iron filings are sprinkled around a bar magnet placed on a horizontal plane, then the iron filings arrange themselves in a regular pattern giving a rough picture of magnetic lines of force. For A Bar Magnet A magnet has two poles: - 1. North Pole Of Magnet The end of a freely suspended magnet which points towards the geographic north direction of earth is called the north pole of magnet. 2. South Pole Of Magnet The end of freely suspended magnet which points towards the geographic south direction of earth is called the south pole of magnet. For A U Shaped Magnet Properties Of The Poles Of Magnet 1. Like magnetic poles repel each other. 2. Unlike magnetic poles attract each other. Uses Of Magnet

2 2. By Using A Plotting Compass Steps 1. Place a bar magnet on a sheet of paper. 2. Bring north pole of the plotting compass near the north pole of the bar magnet. Due to the repulsion between the two north poles the tip of the compass moves away from the north pole of the magnet and tail of the compass comes near to the north pole of magnet. 3. Mark the positions of tip and tail of compass. 4. Now move the plotting compass to the marked point and get another point. 5. In this manner we get various dots denoting the path in which the north pole of the plotting compass moves. 6. Joining the various dots we get a smooth curve representing a magnetic line of force. In this way we can draw a large number of lines of forces by starting from different points near the magnet. 2. Place the compass needle directly under the wire. The needle points north - south when there is no current. 3. Pass the current towards the north, the needle deflects towards the west. When the current is passed towards the south the needle deflects towards the east. 4. Place the compass directly above the wire, now the needle deflections are reversed. Magnetic Field Due To A Straight Current Carrying Conductor Procedure 1. Take a copper wire AB. 2. Pass it through a cardboard. Connect the wire to a battery through a key. 3. Sprinkle some iron fillings on the cardboard. 4. Switch on the key and hold the cardboard gently. The iron fillings will arrange themselves in the form of concentric circles. 5. Reverse the direction of current by changing polarity of the battery. This time too the iron fillings arrange themselves in concentric circles but in opposite direction. This shows that the magnetic lines of force of a straight current carrying conductor are circular in nature. Properties Of Magnetic Lines Of Force 1. They travel from north pole to south pole outside the magnet and south to north inside the magnet. 2. They are continuous closed curves. 3. They emerge out normally from the magnetized surfaces. 4. The tangent drawn at any point of magnetic lines of force represents the direction of the magnetic field at that point. 5. Two magnetic lines of force do not intersect each other. 6. They contract in length and repel each other laterally. 7. The field lines of a uniform magnetic field are parallel to each other. 8. The more crowded the magnetic field lines, the stronger is the magnetic field. Magnetic Effects Of Current (Electromagnetism) Magnetic effects of current means that, the current flowing in a wire produces a magnetic field around it. In 1820 Oersted was able to show that an electric current flowing through a wire produces a magnetic field around it. It was the first observation that indicates a connection between electricity and magnetism. Electrically produced magnetism is also called electromagnetism. Experiment Demonstrating Magnetic Effects Of Current 1. Take a thin insulated copper wire connected to a battery. Right Hand Thumb Rule To Find Direction Of Magnetic Field Produced By Straight Current Carrying Conductor According To Maxwell s Right Hand Thumb Rule: Hold the conductor in your right hand in such a way that the thumb points in the direction of current flowing in it and then the direction in which fingers encircle (curve) gives the direction of magnetic field. Magnetic Field Pattern Due To A Circular Coil Carrying Current Circular Coil

3 A circular coil consists of twenty or more turns of insulated copper wire closely wound together. It has been found that the magnetic effect of current increases if, instead of using a straight wire, the wire is converted into a circular coil. To make things simple circular coil consisting of only one turn is taken. Procedure 1. Take a short circular coil and fixed it to a thin cardboard sheet. 2. Pass the current through the circular coil, a magnetic field is produced around it. The pattern of magnetic lines of force is circular near the wire, but they become straight and parallel at the middle of coil. Factors On Which Strength Of Magnetic Field Produced By A Current Carrying Solenoid Depends: - 1. The number of turns in the solenoid. 2. Strength of current in solenoid. 3. The nature of core material used in making solenoid. Electromagnet The combination of a solenoid and a soft iron core is called electromagnet. The magnetic field produced by a current carrying circular wire behaves as a thin disc of magnet, whose one face is a north pole and other face is a south pole. Magnetic Field Due To A Current Carrying Solenoid The solenoid is a long coil containing a large number of close turns of insulated copper wire. Procedure 1. Connect the two ends of a solenoid to a battery through a switch. 2. When electric current is passed through it, it produces a magnetic field around it. The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet. If a current carrying solenoid is suspended freely, it comes to rest pointing north and south. Thus, an electromagnet consists of a long coil of insulated copper wire wound on a soft iron core. How Electromagnets Are Made? 1. To make an electromagnet take a rod of soft iron. 2. Wind a coil of insulated copper wire round it. 3. Connect the two ends of the copper coil to a battery. An electromagnet is formed. A solenoid containing soft iron core in it acts as a magnet only as long as the current is flowing in the solenoid. If we switch off the current in the solenoid, it no more behaves as a magnet. All the magnetism of the soft iron core disappears as soon as the current in the coil is switched off. It is the iron piece inside the coil which becomes a strong electromagnet on passing the current. Factors Affecting Strength Of An Electromagnet Strength of electromagnet is directly proportional to: - 1. The number of turns in the coil. 2. Strength of current flowing in the coil. And, The strength of electromagnet is inversely proportional to the length of air gap between its poles. A U shaped electromagnet has small air gaps hence it acts as a strong electromagnet. Bar Magnet (Permanent Magnet) Permanent magnets are usually made up of alloys such as : carbon steel, chromium steel, tungsten steel, cobalt steel,

4 alnico (alloy of aluminium, nickle, cobalt, iron) nipermag (alloy of iron, nickle, aluminium, titanium). Permanent magnets of these alloys are much stronger than those made up of ordinary steel. Such strong permanent magnets are used in microphones, loudspeakers, electric clocks, ammeter, voltmeter and speedometer etc. Force On Current Carrying Conductor Placed In A Magnetic Field Oersted s experiment shows that a current carrying wire exerts a force on a freely suspended magnetic needle and deflects it. We can also say that a current carrying wire exerts a mechanical force on a magnet and this force produce motion in the magnet. In reverse we can say that a magnet exerts a mechanical force on a current carrying wire, and if the wire is free to move this force can produce motion in the wire. How The Direction Of Force On A Current Carrying Conductor Placed In Magnetic Field Can Be Reversed? 1. Reversing the direction of flow of current in the conductor. 2. Reversing the direction of magnetic field. Fleming s Left Hand Rule To Find The Direction Of Force Applied On The Conductor According to Fleming s Left Hand Rule: - Hold the forefinger, the centre finger and thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current, then the direction in which the thumb points, gives the direction of force acting on a conductor. When a current carrying conductor is placed in a magnetic field, it experiences a force and begins to rotate continuously in a direction given by Flemings Left Hand rule. Construction It consists of following components: - 1. Armature Coil It consists of large number of turns of copper wire wound over a rectangular core of soft iron. 2. Strong Field Magnets The coil is mounted between the curved poles of a U-shaped permanent magnet in such a way that it can rotate between the poles N and S. 3. Split Rings The two ends of the coil are welded to two semicircular metallic rings. These rings are called split rings or half rings or commutator. Function Of Split Rings Split rings are meant to change the direction of current flowing through the coil after each half rotation. 4. Carbon Brushes Two carbon brushes B1 and B2 make a contact with the slip rings of the commutator and through them the current is supplied to the coil. The system of two half rings and the associated brushes are referred to as a split ring commutator. Electric Motor An electric motor is a device that converts electrical energy into mechanical energy. Every motor has a shaft or spindle which rotates continuously when current is passed into it. The rotation of its shaft is used to derive the various types of machines in homes and industry e.g. electric fans, washing machines, refrigerator, mixer and grinder etc. Types Of Motor 1. AC Motor (Alternating Current Motor) Uses AC (Alternating Current) supply e.g. motor of a fan. 2. DC Motor (Direct Current Motor) Uses DC (Direct Current) supply e.g. motors of battery operated toys. Direct Current Motor (DC Motor) Principle Of A Motor Working 1. When the current is passed through the coil of copper wire from the battery, it enters the coil through the left brush and half ring, goes around the coil and then leaves through the right half ring and brush. 2. As the direction of current is perpendicular to magnetic field, a torque (one force in upward direction and one in downward direction) acts on it. According to Fleming s Left Hand Rule, applied on the left side AB of the coil we find that it will experience a force in upward direction and the right side DC of the coil will experience a force in downward direction. 3. This turns the coil due to which the coil starts rotating in the anticlockwise direction. 4. While rotating when the coil reaches the vertical position then the brushes will touch the gap between the two commutator rings and current to the coil is cut off, but the coil does not stop rotating due to gain of momentum. 5. When the coil goes beyond the vertical position and the half rings are again connected to brushes, but now on opposite sides, this reverses the direction of current in the coil which in turn reverses the direction of forces acting on the two sides of the coil. 6. In this position also couple of forces act on the coil (torque) which rotate it in the same anticlockwise direction.

5 7. This process is repeated again and again and the coil continues to rotate as long as the current is passing. Note The coil experiences maximum force at 0 and 180 and no force at 90 and 270. Electromagnetic Induction (Electricity From Magnetism) The phenomenon of producing electric current by moving a magnet near a coil or a coil near a magnet is called electromagnetic induction. If magnetic field through a circuit changes an emf and a current is induced in the circuit, the emf is called induced emf (voltage) and the current is called induced (brought about) current. Condition For Electromagnetic Induction To Take Place The condition is that there must be a relative motion between magnetic field and coil. Faraday s Experiment There are a number of ways a magnetic field can be used to generate an electric current. First Method 1. Place the coil in the field of a bar magnet to which galvanometer is connected. 2. When there is no relative motion between the bar magnet and the coil, the galvanometer reads zero, indicating there is no current. 3. However, when the magnet moves towards the coil, a current appears in the coil. As the magnet approaches the magnetic field that it creates at the location of the coil becomes stronger and stronger and it is this changing magnetic field which produces current. 4. When the magnet moves away from the coil a current also exists but the direction of the current is reversed. Second Method A current would be created if the magnet were held stationary and the coil were moved because the magnetic field at the coil would be changing as the coil approaches or recede from the magnet. Only relative motion between the magnet and the coil is needed to generate a current it does not matter which one moves. Factors On Which Induced Current In The Coil Depends: - 1. Strength of the magnetic field 2. Number of turns in the coil 3. Relative speed between the coil and the magnet Direction Of Induced EMF Induced Current changes its direction with the motion of the magnet in opposite direction. The direction of induced current can be obtained by Fleming s Right Hand Rule. Fleming s Right Hand Rule Hold the forefinger, the middle finger and the thumb of your right hand mutually perpendicular to each other. Now position your hand in such a way that the thumb represents the direction of motion of conductor, and the fore finger gives the direction of magnetic field, then the direction in which the middle finger points gives the direction of induced current. Electric Generators (Dynamo) An electric generator converts mechanical energy into electrical energy. An electric generator is a machine, which generates electricity from mechanical energy by using the principle of electromagnetic induction. Types Of Generators 1. AC Generator (Alternating Current Generator) Induces AC (Alternating Current) supply. 2. DC Generator (Direct Current Generator) Induces DC (Direct Current) supply. DC Generator Also called DC dynamo, converts mechanical energy into electrical energy. The DC generator produces direct current and not alternating current. Principle When a straight conductor is moved in a magnetic field, then current is induced in the conductor. In electric generator a rectangular coil having straight sides is made to rotate rapidly in the magnetic field between the poles of a horse shoe type magnet. When the coil rotates, it cuts the lines of magnetic force, due to which a current is induced in the generator coil. This current can be used to run various electrical appliances. Construction Of D.C. Generator A simple D.C. Generator consists of the following: - An armature coil of copper wire wound on a soft iron core. The armature coil rotates between the pole pieces of a strong magnet. The free ends of the coil are connected to the commutator or slip rings R1 and R2. Two metallic brushes B1 and B2 are in contact with the slip rings. These brushes are called contact brushes. When the coil is rotated, the two half rings R1 and R2 touch the two carbon brushes B1 and B2 one by one. In this way the current produced in the rotating coil can be tapped out through the commutator half rings into the carbon brushes. From the carbon brushes B1 and B2 we can take the current into the various electrical appliances.

6 1. As the coil rotates in the anticlockwise direction, the side AB moves down and CD moves up cutting the magnetic lines near N pole and S pole respectively. Due to this the current is induced in side AB and DC of the coil. On applying Fleming s Right Hand rule to the sides AB and CD, the direction of induced current is found out to be BADC. Working Of A D.C. Generator Suppose that generator coil ABCD is initially in the horizontal position and is being rotated in the anticlockwise direction. 1. As the coil rotates in the anticlockwise direction, i.e., the side AB moves downwards and side CD upwards cutting the magnetic lines of force near the N Pole and S Pole of the magnet respectively. Due to this motion of the coil in the magnetic field, current is induced in the coil from D to C and B to A, according to the Fleming s right hand rule respectively. Thus, the induced current in the two sides of coil are in the same direction i.e. BADC. Due to this the brush b1 becomes positive and brush B2 becomes negative. 2. After half rotation, the arms of the coil interchange their positions. Arm AB comes to the right and arm CD to the left. But when sides of the coil interchange their positions then the two commutator half rings R1 and R2 automatically change. This keeps current flowing in the same direction i.e., B1 always remain +ve and B2 always remain ve. Thus a DC generator supplies current always in same direction. Comparison Of D.C. Generator with D.C. Motor DC Generator Coil is rotated to produce direct current. Electric Motor Direct current is supplied to rotate the coil. AC Generator AC Generator produces alternating current which changes its polarity continuously. Construction A simple AC generator consists of an armature coil of copper wire of many turns wound over a soft iron core. The armature is placed between the poles a strong magnet. The ends of the coil are connected to two cylindrical full rings called slip rings R1 and R2 mounted on the shaft. Each slip ring is permanently in contact with a carbon brush. The brushes are connected to the fixed terminals P and Q. As the slip rings R1 and R2 rotate with the coil, the two carbon brushes B1 and B2 keep contact with them and the current can be tapped out through the slip rings into the carbon brushes and further into the circuit. Working Suppose that the generator coil ABCD is initially in the horizontal position and then is being rotated in the anticlockwise direction between the poles N and S of a horse shoe type magnet. 2. After half rotation the sides AB and CD of the coil interchange their positions, arm AB moves up and arm CD moves down. Now the direction of induced current in each side of the coil is reversed after half revolution. So the polarity of the two ends of the coil also changes after half revolution. Thus, in one complete rotation of the coil, the current changes its direction two times. Household Electric Circuit (Domestic Wiring) Electricity is generated in a power station. It is usually alternating. Its voltage is volts to volt. The power stations are located far away from the cities and industrial area and hence transmission of electricity is done with the help of high tension underground or overhead cables. From electric poles situated in our street, two insulated wires, live wire (red in color) and neutral wire (black in color) are used. Live wire is at high potential of 220 volts whereas neutral wire is at ground potential of 0 volt. Thus the potential difference between the live wire and neutral wire in India is 220 volts. There is no harm when we touch the neutral wire but we will get an electric shock if we touch live wire directly. The two insulated wires L and N coming from the electric pole enter a box fitted inside our house. In this box, a main fuse is put in the live wire. This fuse has a high rating of about 50 A. The two line wires then enter the electric meter which records the electrical energy consumed in Kilowatt Hours. The two wires coming out of the meter are connected to the main switch from which the electrical supply can be switched off

7 when required. After the main switch there is another fuse in the live wire. This is called consumer s fuse. Circuit In House There are usually two separate circuits in a house. 1. The lighting circuit with a 5 A fuse for lighting. 2. The power circuits with a 15 A fuse for radios, fans, lighting electric iron, room heater, electric stove, refrigerator etc. Each distribution circuit is provided with a separate fuse so that if a fault like short circuiting or overloading occurs in one circuit, its corresponding fuse blows off but the other circuits remain unaffected. Also various distribution circuits are connected in parallel so that if a fault occurs in one circuit, its fuse will melt but the other circuits remain operational. Wiring In Rooms Along with the live wire and neutral wire, a third wire called Earth wire (green in colour) also goes into our rooms. Earth wire is usually an uncovered copper wire having no plastic insulation on it. One end of the earth wire is connected to a copper plate which is burried deep under the earth or at the nearest electric sub station. All Switches Are Put In The Live Wire. Why? Usually all the electrical appliances are provided with separate switches. So, if we switch off the electrical appliance, then connection with the live wire is cut off and there will be no danger of the electric shock if we touch metal case of electrical appliance. What Is Earthing Of Electrical Appliance? Earthing means to connect the metal case of an electrical appliance to the earth (at zero potential) by means of a metal wire called earth wire. One end of the earth wire is burried in the earth. Earth wire is connected to the metal case of the appliance by using a three-pin plug. The metal casing of the appliance will now always remain at the zero potential of the earth. This is called grounding or earthing of appliance. Why An Electric Fuse Is Needed? Electric fuse is very useful in case of short circuiting or overloading to avoid the circuit or device from getting damaged by any undue hike in the electric current. Short Circuiting Touching of live wire and neutral wire directly is known as short circuiting. It occurs if the plastic insulation of the live wire and neutral wire gets torn, then the two wires touch each other. When the two wires touch each other, the resistance of the circuit becomes very small. Since the resistance is very small the current flowing through the wire becomes very large and heats the wire to a dangerously high temperature and may cause the fire. Overloading If too many electrical appliances of high power rating like electric iron, water heater and air conditioner etc. are switched on at the same time, they draw extremely large current from the circuit. This is known as overloading of the current flowing through the circuit. Since the current flowing through the wire becomes very large it heats the wire to a dangerously high temperature and may cause the fire. Why Earthing Is Done? To avoid the risk of electric shocks, the metal body of an electrical appliance is earthed. The appliances which draw heavy current and which are liable to touch like electric iron, electric heater, room cooler, geyser, refrigerator should all be provided with earth connections so that the user will not get any kind of electric shock. Electric Fuse A fuse is a safety device having a short length of a thin wire made of tin or tin lead alloy having low melting point, which melts and breaks the circuit if a current exceeds a safe value. An electric fuse works on the principle of the heating effect of current. A fuse wire is always connected in series in the electrical circuit or with the device. It is obvious that we should have some devices which may disconnect the electricity supply when a short circuit or overloading occurs. To avoid this danger electric fuse is used. Miniature Circuit Breaker (MCB) (Fuse) These days MCBs are used to protect household wiring from the excessive flow of electric current through it. If the current becomes too large, the MCB puts off a switch cutting off the electric supply. The MCB can be reset when the fault has been corrected.

8 Fuse Used In Electrical Appliances The fuses which are put on the main switch-board in our houses are used to protect the whole wiring of the house. Fuses are also used to protect domestic electrical appliances from damage which may be caused due to excessive current flowing through them. Costly electrical appliances like T.V., refrigerators have their own fuses which protect them against damage by too much current. Precautions While Harnessing Electricity 1. If a person accidentally touches a live wire or if an electric fire starts in a house, the main switch should be turned off at once as to cut off the electric supply. This will prevent fire from spreading. 2. Use wires of high quality, proper amperage and good insulating material. 3. All connections at plugs, switches, sockets must be tight. 4. Replace any defective plugs, switches and sockets. 5. Never touch any part of the circuit being wet or without putting on rubber shoes or rubber gloves. 6. Use fuse of proper rating and material. 7. All electrical appliances must be earthed. 8. Connect switches and fuse to live wire.