Name of Faculty Prof B S Gujar Subject

Name of Faculty : - Prof. B. S. Gujar Subject : - Basic Electrical Engineering

Electromagnetism

Objectives • Define Magnetism. • Classify the Magnets. • Define Magnetic poles, Magnetic field and Magnetic lines of force. • Define Magnetic circuit. • Define Magneto-motive force, Reluctance and Permeance. • Compare Magnetic circuit and Electric circuit.

Need of Magnetism • Note that, electrical appliances we use in our daily life cannot operate without Magnetism.

Need of Magnetism • Other than the electrical appliances, the devices such as radio, telephone, telegraph, ignition systems of cars, trucks, and airplanes also work on magnetism.

Magnetism • Any substance which possesses the power of attracting pieces of iron is called a Magnet and the property by virtue of which this attraction takes place is known as Magnetism.

Magnetism

Classification of Magnets are classified as: • Natural Magnet: Natural magnets have the inherent property of attracting iron pieces. E. g. iron ore like magnetite, lode stone.

Classification of Magnets • Artificial Magnet: Artificial magnets are prepared either by rubbing iron bars with a magnet or by passing electric current through a wire which is wound around an iron piece.

Classification of Magnets • Thus, the magnets prepared by the second method are known as Electro-magnets and the phenomenon is called Electromagnetism.

Classification of Magnets

Classification of Magnets Artificial magnets are further classified as:

Permanent Magnets • Permanent magnets are prepared from hardened steel and alloys of nickel and cobalt. The magnetism in these magnets is unaltered for long duration even after the removal of magnetizing force.

Permanent Magnets • Permanent magnets are used in electrical instruments such as:

Temporary Magnets • Temporary magnets are prepared from soft iron or nickel which retain magnetism as long as the magnetizing force is applied.

Temporary Magnets • Temporary magnets are used in

Magnetic Poles • When a magnet is suspended and rotated in a horizontal plane, it comes to rest in northsouth direction. • The end of the magnet pointing north is called north pole while the end pointing south is called south pole. A line joining these two poles is termed as magnetic axis.

Magnetic Poles

Magnetic Field • As we know an electric field exists near a charged object, similarly magnetic field exists around a magnet. The space or field in which a magnetic pole experiences a force is known as Magnetic field.

Magnetic Field

Magnetic Field • We can demonstrate the shape and distribution of the magnetic field using a small compass needle and iron fillings.

Magnetic Lines of Force • Observe that the fillings set themselves in the form of curved chains between the poles. • These continuous curves in a magnetic field are known as Magnetic lines of force. • These lines of force travel from north pole to south pole. Each line of force forms a closed loop.

Magnetic Lines of Force

Magnetic circuit • Like electrical circuits, there also magnetic circuits that are being used in today’s modern devices.

Magnetic circuit • As we know, current flows in electrical circuits and magnetic flux flows in magnetic circuits. Thus, the closed path followed by magnetic flux is called a Magnetic Circuit.

Magnetic circuit • A magnetic circuit is made up of materials having high permeability like iron and soft steel.

Magnetic circuit • In a magnetic circuit, the magnetic flux leaves the north pole and passes through the entire circuit and returns to the starting point.

Magnetic circuit Illustration for magnetic circuit: • Consider a coil of N turns wound on an iron core. • When current ‘I’ is passed through the coil, magnetic flux ‘phi’ is set up in the core. • The magnetic flux follows the closed path PQRSP and is known as the magnetic circuit.

Magneto-motive Force • Magneto-motive force m. m. f is a magnetic pressure which sets up flux in a magnetic circuit. • It is defined as the work done in moving a unit magnetic pole once round the magnetic circuit. • m. m. f. is represented as: m. m. f = N. I Ampere-turns (AT)

Magneto-motive Force • Magneto-motive force in a magnetic circuit corresponds to e. m. f. in an electric circuit.

Reluctance • The opposition that the magnetic circuit offers to flux is known as Reluctance. • Reluctance in a magnetic circuit corresponds to resistance in an electric circuit. • Reluctance is given as: • Its unit is Ampere-turns by weber.

Permeance • It is the reciprocal of reluctance and is given as: • Its unit is wb/AT. • Permeance of magnetic circuit corresponds to conductance in an electric circuit.

Comparison between Magnetic and Electric Circuits

Summary • Defined Magnetism. • Classified the Magnets. • Defined Magnetic poles, Magnetic field and Magnetic lines of force. • Defined Magnetic circuit. • Defined Magneto-motive force, Reluctance and Permeance. • Compared Magnetic circuit and Electric circuit.

Time for Students’ Questions! Any Question?