XII PHYSICS: XII PHYSICS MOVING COIL GALVANOMETER MAHENDRA SINGH PGT PHYSICS KV,NO1,AFS,JODHPUR(RAJ.) Definition: Definition Moving coil galvanometer is a device which is used to detect small electric current . Being a sensitive instrument, Galvanometer can not be used for the measurement of heavy currents Use of galvanometer: Use of galvanometer It is used to construction of both voltmeter (a electric potential measuring device )and ammeter (a current measuring device) Principle of Galvanometer: Principle of Galvanometer Galvanometer works on the principle of conversion of electrical energy into mechanical energy. When a current carrying coil is placed in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional to the current flowing through it. ESSENTIAL PARTS OF GALVANOMETER: ESSENTIAL PARTS OF GALVANOMETER There are five essential parts of a Galvanometer. 1. A permanent magnet with concave poles.(for radial magnetic field) 2. Flat rectangular coil of thin enameled insulated wire ‘C’. 3. A soft iron cylinder 'B'. 4. A pointer or needle. 5. A scale. CONSTRUCTION: CONSTRUCTION CONSTRUCTION: CONSTRUCTION 1. House shoe magnet: 1. House shoe magnet House shoe magnet produces radial magnetic field. So we get a m.f . with its lines of force pointing along the radii of a circle. Such a field is called a radial field. The plane of a coil in such a field is always parallel to the field as shown in fig. 2. rectangular coil : 2. rectangular coil In a galvanometer consists of rectangular coil of fine insulated copper wire wound on a light non-metallic (Al) frame. It is suspended from a movable torsion head T by means of a fine phosphor –bronze strip. The lower end of the coil is connected to a hair spring. 3. A soft iron cylinder: 3. A soft iron cylinder A soft iron cylinder is mounted within the rectangular coil frame This makes the lines of force pointing along the radii of a circle. It produces eddy current which helps galvanometer comes at rest soon. 4. A pointer or needle.: 4. A pointer or needle. A pointer or a needle is connected with moving coil which moves with movement of coil . Working: Working When a current passes through the galvanometer coil, it experiences a magnetic deflecting torque, which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension strip. The twist in the strip produces an electric restoring torque. The coil rotates until the elastic restoring torque due to the strip does not equal and cancels the deflecting magnetic torque, then it attains equilibrium and stops rotating any furthers. Slide 13: Let B be the strength of the magnetic field, n be the number of turns in the coil, A be the area of the coil and I be the current flowing through the coil. The deflecting couple produced in the coil = BIAn. Due to this deflecting couple, the coil rotates and this couple is opposed by the torsional couple produced in the phosphor bronze wire. The restoring couple produced in the coil =Cθ Slide 14: Where, C is couple per unit twist of the wire and θ is the angle of twist which can be measured by means of a lamp and scale arrangement. At equilibrium, BIAn = Cθ i.e., I = (C/BAn) * θ since, C,B,A and n are constants, the defelction in the coil is directional in the coil is directly proportional to the current through the coil. Current sensitivity: Current sensitivity A sensitive galvanometer should have a long deflection for small current through it. A sensitive galvanometer should have a long deflection for small current through it. Voltage sensitivity: Voltage sensitivity Voltage sensitivity is the deflection produced for a unit voltage applied across the two terminals of the galvanometer Conversion of Galvanometer into Ammeter: Conversion of Galvanometer into Ammeter A galvanometer can be converted into an ammeter (device measuring the current flowing through a conductor) by connecting a low resistance (called shunt resistance) in parallel to the galvanometer as shown in the figure. Let Rg represent the resistance of the galvanometer, ig the current which produces full scale deflection in the galvanometer. Since the shunt is connected in parallel to the galvanometer, the potential difference across galvanometer = potential difference across shunt. : Let R g represent the resistance of the galvanometer, i g the current which produces full scale deflection in the galvanometer. Since the shunt is connected in parallel to the galvanometer, the potential difference across galvanometer = potential difference across shunt . Where as S is the shunt resistance. 'Ideal Ammeters have zero resistance '. Conversion of Galvanometer to Voltmeter : Conversion of Galvanometer to Voltmeter A Galvanometer can be converted into a voltmeter by connecting a high resistance in series with a galvanometer as shown. In series connection the current through the galvanometer is same as that due to the resistance. The total resistance of voltmeter: In series connection the current through the galvanometer is same as that due to the resistance. The total resistance of voltmeter This works as a voltmeter of range 0 to V volt. Since the value of R is high, the effective resistance also has a higher value. Thus voltmeters have high resistance. 'Ideal Voltmeter has infinite resistance'.