MAGNETIC EFFECTS OF ELECTRIC CURRENT

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1 MAGNETIC EFFECTS OF ELECTRIC CURRENT It is observed that when a compass is brought near a current carrying conductor the needle of compass gets deflected because of flow of electricity. This shows that electric current produces a magnetic effect. MAGNETIC FIELD The region around a magnet where the force of attraction or repulsion can be detected is called magnetic field. Magnetic field around a magnet can be detected by using a magnetic compass. MAGNETIC FIELD LINES The imaginary lines of magnetic field around a magnet are called magnetic field line. The magnetic field lines around a magnet can be observed by sprinkling iron filings around a magnet. It can also be observed by moving a magnetic compass around a magnet. Properties Magnetic lines of force start from the North Pole and end at the South Pole. They are continuous, that is, they form closed loops. The magnetic field lines are closer at the poles. The magnetic field lines do not intersect each other. If they do so, there will be two directions for the magnetic field at a single point. That is not possible. Stronger the field, closer are the field lines. MAGNETIC FIELD DUE TO CURRENT THROUGH A STRAIGHT CONDUCTOR: A current carrying straight conductor has magnetic field in the form of concentric circles; around it. The direction of magnetic field through a current carrying conductor depends upon the direction of flow of electric current. The direction of magnetic field gets reversed in case of a change in the direction of electric current. The direction can be given by right hand thumb rule. RIGHT HAND THUMB RULE: It is also known as Maxwell s Corkscrew Rule. If a current carrying conductor is held by right hand; keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.

2 MAGNETIC FIELD DUE TO CURRENT THROUGH A CIRCULAR LOOP: In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the conductor. At the centre, the field lines would appear as straight lines. The direction of magnetic field can be identified using Right Hand Thumb s Rule. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction; at the top of the loop. Moreover, it would be in anticlockwise direction at the bottom of the loop. MAGNETIC FIELD AND NUMBER OF TURNS OF COIL: Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are n turns of coil, magnitude of magnetic field will be n times of magnetic field in case of a single turn of coil. MAGNETIC FIELD DUE TO A CURRENT IN A SOLENOID: Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of cylinder. A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the North Pole and another end behaves as the South Pole. Magnetic field lines are parallel inside the solenoid; similar to a bar magnet; which shows that magnetic field is same at all points inside the solenoid. By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.

3 FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD: A current carrying conductor exerts a force when a magnet is placed in its vicinity. Similarly, a magnet also exerts equal and opposite force on the current carrying conductor. The direction of force over the conductor gets reversed with the change in direction of flow of electric current. It is observed that the magnitude of force is highest when the direction of current is at right angles to the magnetic field. FLEMING S LEFT HAND RULE: If direction of electric current is perpendicular to the magnetic field, the direction of force is also perpendicular to both of them. The Fleming s Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions; then the index finger and middle finger of a stretched left hand show the direction of magnetic field and direction of electric current respectively and the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes. Many devices, such as electric motor, electric generator, loudspeaker, etc. works on the Fleming s left Hand Rule. ELECTRIC MOTOR: Electrical energy is converted into mechanical energy by using an electric motor. Electric motor works on the basis of Fleming s Left Hand Rule. In an electric motor, a rectangular coil is suspended between the two poles of a magnetic field. The electric supply to the coil is connected with a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit. When electric current is supplied to the coil of electric motor, it gets deflected because of magnetic field. As it reaches the half way, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of current reverses the direction of forces acting on the coil. The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.

4 In commercial motor, electromagnet; instead of permanent magnet; and armature is used. Armature is a soft iron core with large number of conducting wire turns over it. Large number of turns of conducting wire enhances the magnetic field produced by armature. ELECTROMAGNETIC INDUCTION The phenomenon of producing electric current in a conductor by moving it perpendicular to a magnetic field or vice- versa, is called electromagnetic induction. The current so produced is called induced current. The voltage so produced called induced emf. Electromagnetic induction is opposite to the electric motor effect. The direction of induced current is given by Fleming s Right Hand Rule. FLEMING S RIGHT HAND RULE:- It states that If the thumb, fore finger and middle finger of the right hand is held perpendicular to each other such that the thumb points in the direction of motion of the conductor, the fore finger points in the direction of the magnetic field, then the middle finger shows the direction of induced current. ELECTRIC GENERATOR: An electric generator converts mechanical energy into electrical energy. Principle: The principle behind the electric motor is based on Fleming s right hand rule. When a coil of insulated copper wire is forced to rotate inside a magnetic field then electric current is induced. Construction: 1. Field magnets: It is a powerful horseshoe magnet 2. Armature: It is consists of a soft iron core on which a large number of turns of insulated copper wire are wound. The armature is rotated in a perpendicular magnetic field about a horizontal axis. 3. Slip rings and brushes: The current generated is taken out to the external circuit by means of an arrangement called slip rings and brushes. The end of the coil is connected to the slip rings which rotates the same axis of coil. One end of each brush touches the slip ring and the other end is connected to the external circuit.

5 Working: The structure of electric generator is similar to that of an electric motor. The rectangular armature is placed within the magnetic field of a permanent magnet. The armature is attached to wire and is positioned in way that it can move around an axle. When the armature moves within the magnetic field an electric current is induced. After half the rotation the current the arms of the coil interchange their position. Thus the direction of current changes once in every rotation. Due to this, the electric generator usually produces alternate current, i.e. AC. To convert an AC generator into a DC generator, a split ring commutator is used. This helps in producing direct current. DOMESTIC ELECTRIC CIRCUIT: In our homes, the electric power supplied through the mains with potential difference of V = 220V and frequency 50Hz. It consists of three wires: 1. Wire with Red insulation: LIVE WIRE (positive wire). It is at a high potential of 220V. 2. Wire with Black insulation: NEUTRAL WIRE (negative wire). It is at zero potential. The potential difference between the two wires is 220V. 3. Wire with green insulation: EARTH WIRE is connected to a metal plate kept in the ground. The appliances having metallic body like electric iron, refrigerators etc., and their metallic body is connected to the earth wire so that if there is leakage of current, it passes to the earth and prevents electric shock. ELECTRIC FUSE: Electric fuse is a safety device used in electric circuits to protect the circuit and appliances from damage due to overloading and short circuit. It is a wire having high resistance and low melting point. If excess current flows through the circuit, the fuse wire melts and breaks the circuit. Fuse wire is made of a metal or an alloy of metals like lead, tin, aluminum and copper. Fuse wire is connected in series with the live wire. OVERLOADING: Overloading is caused due to increase in voltage, or if the live wire and neutral wire comes in contact or if too many appliances are connected to a single socket. It results in overheating of the wires and can cause damage to the circuit and appliance. SHORT CIRCUIT: Short circuit is caused when the live wire and neutral wire comes in contact and the current suddenly increases in the circuit. It causes spark, fire and damage to the circuit and appliances.

6 DIFFERENTIATE BETWEEN AC AND DC CURRENT: AC CURRENT Current in which direction is changed periodically is called Alternate Current DC CURRENT Current that flows in one direction only is called Direct current. AC is transmitted up to a long distance without much loss of energy. Voltage of DC cannot travel very far until it begins to lose energy. The frequency of alternating current is 50Hz or 60Hz depending upon the country. It reverses its direction while flowing in a circuit. Produced by A.C Generator and mains. The frequency of direct current is zero. It flows in one direction in the circuit. Produced by Cell or Battery.