AT Any Time Padhai Academy JEE Mains Special Experiments
GALVANOMETER Galvanometer is represented as follow It consists of a pivoted coil placed in the magnetic field of a permanent magnet. Attached to the coil is a spring. In the equilibrium position, with no current in the coil, the pointer is at zero and spring is relaxed. When there is a current in the coil, the magnetic field exerts a torque on the coil that is proportional to current. As the coil turns, the spring exerts a restoring torque that is proportional to the angular displacement. Thus, the angular deflection of the coil and pointer is directly proportional to the coil current and the device can be calibrated to measure current. When coil rotates the spring is twisted and it exerts an opposing torque on the coil. There is a resistive torque also against motion to damp the motion. Finally in equilibrium magnetic-tspring- BINA sin But by making the magnetic field radial 0 90 . BINA-C 1 here B magnetic field I Current N Number of turns Current sensitivity A C Area of the coil torsional constant angle rotate by coil. The ratio of deflection to the current i.e. deflection per unit current is called current sensitivity (C.S.) of the galvanometer CSBNA
AMMETER A shunt (small resistance) is connected in parallel with galvanometer to convert it into ammeter; An ideal ammeter has zero resistance Ammeter is represented as follow - Whole system is known as Ammeter Ia, Ra If maximum value of current to be measured by ammeter is I then lo . RG = (1-1)S S- la Kg when I >> IG. where I = Maximum current that can be measured using the given ammeter For measuring the current the ammeter is connected is series. In calculation it is simply a resistance RGS RG +S Resistance of ammeter RA for S << RGRA S
what is the value of shunt which passes 10% of the main current through a galvanometer of 99 ohm? Example Solution n figure Rgl (1-Ig)S 10 10
Example Find the current in the circuit (a) & (b) and also determine percentage error in measuring the current through an ammeter. 2 2 0.5 10V Solution nA 5A 10 2 10 2.5 In BI4A i-i Percentage error is _-x100-20% Ans. Here we see that due to ammeter the current has reduced. A good ammeter has very low resistance as compared with other resistors, so that due to its presence in the circuit the current is not affected.
Example Find the reading of ammeter? Is this the current through 6 ? 62 18V 3x6+139 Solution: Req--+1_3 Current through battery I6 A So, current through ammeter 6x 4A No, it is not the current through the 6 resistor 3+6 18 3 Note: Ideal ammeter is equivalent to zero resistance wire for calculation potential difference across it is zero
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