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CBSE Class 12-science Answered

A voltmeter reads 8V at full-scale deflection and is graded according to its resistance per volt at full-scale deflection as 5000 ohms/volt. How will you convert it into a voltmeter that reads 20V at full-scale deflection? Will it still be graded as 5000 ohms / volt.? Will you prefer this voltmeter to one that is graded ats 2000 ohms /volt? PLEASE ANSWER IT EARLIEST
Asked by modi72879 | 16 Jan, 2018, 07:28: PM
answered-by-expert Expert Answer
Voltmeter consists of a moving coil galvanometer and a high resistance conneced in series as shown in figure. The given voltmeter(actually moving coil galvanometer) graded as 5000Ω /volt means the galvanometer maximum current handling is 200μA. Hence to measure 8V full scale 40kΩ resistor should be connected in series.( 5kΩ × 8 = 40kΩ).
 
Let us assume 40kΩ is connected to galvanometer and is given as 8V full scale voltmeter. If we need to use this voltmeter for 20V full scale, then we need to connect 60kΩ resistor . ( 5kΩ × 20 = 100kΩ, 40kΩ already connected).  
 
The gradinjg 5000Ω /volt remains same even after this modification.
 
if we use a moving coil galvanometer graded 2000 Ω/volt, then the maximum current limit for galvanometer is 500 μA. for 8V full scale needed resistor is 16kΩ and for 20V full scale needed resistor is 40kΩ.
 
It is preferable to use a voltmeter that draws less current. Hence the voltmeter graded 5000 Ω/volt that draws current 200 μA  is preferable to use.

Answered by Thiyagarajan K | 16 Jan, 2018, 10:27: PM

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