Question

2 resistor R1 IS 2500 and R2 IS 4000 are joined in sseries and connected to 100 V, voltage drop across R1 and R2  are measured successively by a voltemeter having resistance of 50,000ohm find the sum of two readings?

Answer 1

Resistance R1 = 2.5 k ohms and voltmeter resistance Rv = 50 k ohms are in parallel. This is because voltmeter is connected in parallel with a resistance to measure the voltage across the resistance.
So equivalent resistance = 1/R  =  1/R1 + 1/Rv  or  R = R1 *Rv /(R1+Rv)


Equivalent resistance of R1 and Voltmeter = 2.5 * 50 / (50+2.5) = 2.38 k ohm
Voltage shown by voltmeter = reading1 = 2.38 * 100 / [2.38+4]  = 37.3 volts

equivalent resistance of R2 and voltmeter = 4 * 50 / (50+4) = 3.7 k ohms
Voltage reading across R2 = 3.7 * 100 / [ 2.5+3.7] = 59.68 Volts

Sum of the readings = 96.98 volts