Chapter 17 MAGNATISM & ELECTROSTATICS GALVANOMETER • • • • • Galvanometer is an electromechanical instrument which is used for the detection of electric currents through a circuit. Being a sensitive instrument, Galvanometer can not be used for the measurement of heavy currents. WORKING PRINCIPLE Galvanometer works on the principle of conversion of electrical energy into mechanical energy. ESSENTIAL PARTS OF GALVANOMETER There are five essential parts of a Galvanometer. 1. A U-shaped permanent magnet with concave poles. 2. Flat rectangular coil of wire. 3. A soft iron cylinder. 4. A pointer or needle. 5. A scale. CONSTRUCTION • The flat rectangular coil of thin enamel insulated wire of suitable number of turns wound on an aluminum frame is suspended between the poles of U-shaped magnet by a thin strip. One end of the wire of coil is soldered to connect to an external terminal. The other end is soldered to a loose and soft spiral. A soft iron cylinder is placed within the frame of coil. • WORKING • When the current is passed through the coil it becomes a magnet. There is force of attraction is setup between the poles of magnet and coil. As a result a couple is produced in the coil and it is deflected. The current passes through the coil and the angle of deflection has a direct relation with each other. The deflection is measured by a pointer attached to the coil. AMMETER-VOLTMETER • AMMETER • Ammeter is an electrical measuring device, which is used to measure electric current through the circuit. • CONNECTION OF AMMETER IN CIRCUIT • An ammeter is always connected in series to a circuit. • SYMBOL CONVERSION OF GALVANOMETER INTO AMMETER • Since Galvanometer is a very sensitive instrument therefore it can’t measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low resistance known as "shunt" resistance is connected parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt. Fig . Rs shunt resistance. In this way a Galvanometer is converted into Ammeter and can measure heavy currents without fully deflected. VALUE OF SHUNT RESISTANCE where Rs = Shunt resistance I = Current to be measured Rg = Resistance of galvanometer Ig = Current passing through the galvanometer VOLT METER Voltmeter is an electrical measuring device, which is used to measure potential difference between two points in a circuit. • CONNECTION OF VOLTMETER IN CIRCUIT Voltmeter is always connected in parallel to a circuit. • SYMBOL CONVERSION OF GALVANOMETER INTO VOLTMETER where RX = series resistance V = potential difference to be measured Rg = Resistance of galvanometer Ig = Current passing through the galvanometer PROPERTIES OF MAGNET • PROPERTIES OF MAGNET • 1. Magnets attract objects of iron, cobalt and nickel. 2. The force of attraction of a magnet is greater at its poles than in the middle. 3. Like poles of two magnets repel each other. 4. Opposite poles of two magnets attracts each other. 5. If a bar magnet is suspended by a thread and if it is free to rotate, its South Pole will move towards the North Pole of the earth and vice versa. • CHARACTERISTICS OF MAGNETIC LINES OF FORCE • 1. Magnetic lines of force start from the North Pole and end at the South Pole. 2. They are continuos through the body of magnet 3. Magnetic lines of force can pass through iron more easily than air. 4. Two magnetic lines of force can not intersect each other. 5. They tend to contract longitudinally. 6. They tend to expand laterally. FERROMAGNETIC SUBSTANCES • • • • • Substances that behave like a magnet in the presence of a magnetic field are known as Ferromagnetic Substances. EXAMPLES: Iron, cobalt and nickel are ferromagnetic substances. SOLENOID Solenoid is a coil of wire. Solenoid is a coil wound on a cylindrical frame of iron or any material when an electric current passes through the Solenoid, a magnetic field is produced around it. It has suitable numbers of turns of wire. Magnetic field of solenoids is given by B = (mew)onI Magnetic field inside the solenoid is very strong and uniform but it is very weak outside the solenoid. MAGNETIC FIELD OF INDUCTION DUE TO SOLENOID • SOLENOID • A solenoid is a long tightly wound cylindrical coil of wire. • FIELD DUE TO SOLENOID • When a current is passed through a solenoid the magnetic field is produced, which is strong and uniform inside, while it is negligibly weak outside. EXPRESSION FOR FLUX DENSITY (B) • To calculate B consider a rectangular path a, b, c and d. The path consist of for length elements l1, l2, l3 and l4. Let us calculate the product (B.Dl) for each element. For element l1 EXPRESSION FOR FLUX DENSITY (B) • • • • • • • • • • • • • (B.Delta l)1 = Bl1Cos(theta) Since B is along the axis of l1. i.e. theta = 0 (B.Delta l1) = Bl1Cos0 (B.Delta l1) = Bl1(1) (B.Delta l1) = Bl1 For element l2: (B.Delta l)2 = Bl2Cosq Since l2 is perpendicular to B i.e. theta = 90o (B.Delta l)2 = Bl2Cos90o = Bl2 (0) = 0 For element l3: (B.Delta l)3 = Bl3Cos (theta) Since B is negligible i.e. B = 0 outside the coil. (B.Delta l)3 = 0. l3Cos(theta) EXPRESSION FOR FLUX DENSITY (B) • • • • • • • • • • (B.Delta l)3 = 0 For element l4: (B.Delta l)4 = Bl4Cos(theta) Since l4 is perpendicular to B i.e. q = 90o. (B.Delta l)4 = Bl4Cos90o = Bl4(0) = 0. Now, applying ampere’s circuital law (B.Delta l)n = mewo (current enclosed) B.l1 +0+0+0 = mewo (current enclosed) B.l1 = mewo (current enclosed)law: Now, if number of turns per unit length = n And if current in each turn = I Then current enclosed by the loop abcda = nIl1. EXPRESSION FOR FLUX DENSITY (B) • B.l1 = mewo (nIl1) • B= mewonI • This is the expression for flux density of solenoid. It shows, • B is uniform with in a long solenoid. • The direction of B is along the axis of solenoid. ELECTRIC BELL • MAIN COMPONENT OF ELECTRIC BELL: • Important parts of an electric 1. Electromagnet 2. Armature 3. Spring 4. Armature rod 5. Hammer 6. Gong bell are : CONSTRUCTION • One end of armature winding is connected to terminal T1 and the other to a spring, which is mounted on a soft iron strip. A rod is attached to the armature and the free end of the rod carries a small hammer, which strikes a bell. A very light spring is attached to a screw, which is joined to terminal T2. WORKING OF ELECTRIC BELL • The electric circuit is completed through a battery and push switch button connected to the terminal T1 and T2. When the push button is pressed the electric circuit is completed and the armature is attracted towards the electromagnet as a result, the small spring gets detached from the screw due to which the electric circuit is broken and the electromagnet is demagnetized. Hence, the attraction disappears and the armature is brought back by the spring to its original position. Contact of the spring with the screw is now remade, which completes the electric circuit. The action is repeated over and over again consequently. The armature vibrates and hammer attached to it strikes the gong and the bell rings and sound is produced.