AT Any Time Padhai Academy JEE Mains Special Voltmeter
VOLTMETER A high resistance is put in series with galvanometer. It is used to measure potential difference across a resistor in a circuit. Whole system is known as voltmeter I, Ra O B O B For maximum potential difference V = IG-R + IG RG R RG IG For measuring the potential difference a voltmeter is connected across that element. (parallel to that element it measures the potential difference that appears between terminals 'A' and 'B'.) For calculation it is simply a resistance A O O B
Resistance of voltmeter Rv RG + RR | o . R 0 Ideal voltmeter. A good voltmeter has high value of resistance. Ideal voltmeter which has high value of resistance Note: . For calculation purposes the current through the ideal voltmeter is zero v-V x 100 e Percentage error in measuring the potential difference by a voltmeter is s
Example A galvanometer has a resistance of G ohm and range of V volt. Calculate the resistance to be used in series with it to extend its range to nV volt. Full scale current i,-- to change its range Solution :
Example Find potential difference across the resistance 300 in A and B. 200 2 200 2 +100 V (>600 300 300 100 V Solution n (A Potential difference- x 300 60 volt 200+300 100 300x600 50 volt In (B): Potential difference x 600 300 600 300 600 200+ We see that by connecting voltmeter the voltage which was to be measured has changed. Such voltmeters are not good. If its resistance had been very large than 300 S2 then it would not have affected the voltage by much amount.
Example 43. A galvanometer with a scale divided into 100 equal divisions, has a current sensitivity of 10 division per mA and voltage sensitivity of 2 division per mV. What adoptions are required to use it (a) to read 5A full scale and (b) 1 division per volt?
Example 43. A galvanometer with a scale divided into 100 equal divisions, has a current sensitivity of 10 division per mA and voltage sensitivity of 2 division per mV. What adoptions are required to use it (a) to read 5A full scale and (b) 1 division per volt? 100 Solution Full scale deflection current i, mA = 10 mA cs 10 Full scale deflection voltage Vg VS = mv= 50 mv So galvanometer resistance G9- (a) to convert the galvanometer into an ammeter of range 5A, a resistance of value SS2 is connected in parallel with it such that (5-0.01) S 0.01 x 5 S 0.01 Ans. 499 (b) To convert the galvanometer into a voltmeter which reads 1 division per volt, i.e. of range 100 V, V-ig (R+ G) 100 = 10 x 10" (R + 5) R 10000 5
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