A high resistance voltmeter is connected to a dry cell battery. The voltmeter reads 10 volts. When a 20 ohm resistor is connected across the terminals of the battery the voltmeter reading becomes 8 volts. What is the internal resistance of the battery?

Dear Vidisha

If voltmeter resistance is high then it practically no current through it.

in first case it shows the emf, E of the cell

E=10 volts

in second case I=E/R+r  , R=20 and r is internal resistance

E-Ir=8

E-Er/(R+r) = 8

put value of E=10 and R=20

10 - 10r/20+r = 8

solving we get r=5 ohm

All the best.                                                           

AKASH GOYAL

AskiitiansExpert-IIT Delhi

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