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ICSE NOTES FOR ELECTROMAGNETISM

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Mira Sree
Lakshmi School, Veerapanchan, Madurai

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Electromagnetism 1. When an electric current flows through a wire, it behaves as a magnet. This is called 2. 3. 4. 5. the magnetic effect of electric current. Electric bell works on the principle of magnetic effect of electric current. A compass needle shows deflection when brought near a current carrying wire. An iron nail behaves as a electromagnet when a current is allowed to flow through a wire, which is wrapped around the nail. Magnet is used to separate iron objects from a heap of garbage. An electric bell works on the principle of electromagnetism. Deflection of compass Right-hand thumb rule Solenoid A cylindrical coil having many turns of insulated wires wrapped closely Right-hand thumb rule/ Maxwell s corkscrew rule If one holds a current-currying wire in the right hand such that the thumb is pointing in the direction of the current, then the direction in which the other finger encircles the wire will give the direction of the produced magnetic field lines around the wire. Corkscrew rule If one drives a corkscrew in the direction of the current, then the direction in which the handle is turned is the direction of the magnetic field on the magnetic field lines. The magnitude of the field lines produced by a circular loop at its centre is directly proportional to the amount of current inversely proportional to the radius of the loop The strength of the magnetic field produced by a current carrying a solenoid 1. is directly proportional to the number of turns in the solenoid 2. is directly proportional to the strength of the current in the solenoid 3. depends upon the nature of the core material A current carrying the rod experiences a force when placed between two poles of strong magnets. The direction of force exerted on the rod is related with the direction of current. Fleming s left-hand rule If the thumb, forefinger, and middle finger of the left hand are stretched in such a way that they are mutually perpendicular to each other and the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb will point in the direction of the force acting on the conductor. Fore finger = Magnetic field Middle finger = Current Thumb = Force on conductor Application Electric motor An electric motor is a rotating device that converts electrical energy into mechanical energy. Motor principle: The basic principle on which the electric motor works is the magnetic effect of current. A current carrying rectangular coil starts rotating when placed in a magnetic field. Simple electric motor An electric device that reverses the direction of current in a circuit is called a commutator. The split ring acts as a commutator of the electric motor. The phenomenon of the generation of induced current in a conductor by changing the magnetic field or by moving a conductor in the magnetic field is known as electromagnetic induction. The working of a transformer is based on the phenomenon of mutual induction. The direction of the current induced with respect to the directions of the magnetic field and motion of the coil is given by Fleming s right hand rule. If the thumb, forefinger, and middle finger of the right hand are stretched in such a way that they are mutually perpendicular to each other and the forefinger points in the direction of the magnetic field and the thumb points towards the motion of the conductor, then the middle finger will point in the direction of the current through the conductor. Fore finger = magnetic field Thumb = movement of conductor Middle finger = direction of current Application Generator Eddy currents Currents induced in a thick conductor when the conductor is placed in a changing magnetic field. Applications of Eddy Currents Electromagnetic damping Some galvanometers have a fixed core, which is made of non-magnetic metallic materials. When the coil oscillates, the eddy currents generated in the core oppose the motion and bring the coil to rest quickly. Induction furnace In an induction furnace, very high temperature can be produced by producing large eddy currents. Magnetic braking in trains Strong electromagnets are situated above the rails in some electrically powered trains. When the electromagnets are activated, the eddy currents induced in the rails oppose the motion of the train. Self-induction Property of a coil by virtue of which the coil opposes any change in the strength of current flowing through it by inducing an emf in itself Self-Inductance of a Long Solenoid of length l N is the number of turns in the coil Mutual induction: It is the phenomenon of production of induced emf in one coil, due to varying current in the neighbouring coil. Induced emf, Where, M is mutual inductance of the coil Mutual inductance of two long solenoids: Where, Np Number of turns in the primary coil Ns Number of turns in the secondary coil A Cross-sectional area of the solenoid l Length of the solenoid In an AC generator, mechanical energy is converted into electrical energy by virtue of electromagnetic induction. If a coil of N turns and area A is rotated at n revolutions per second in a uniform magnetic field B, then the motional emf produced is Where, we have assumed that at t = 0 s, the coil is perpendicular to the field. A transformer consists of an iron core, on which are bound a primary coil of N P turns and a secondary coil of N S turns. If the primary coil is connected to an AC source, the primary and secondary voltages are related by And the currents are related by If N S > N P If N S < N P The voltage is stepped up The voltage is stepped-down

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