Question

a voltmeter is connected in parallel with a variable resistance R which is in series with an ammeter and a cell as shown in figure for one of the are the metre reads 0.3 ampere and 0.9 volt for another value of are the readings are 0.25 empire and 1 volt what is the internal resistance of the cell

Answer 1

The internal resistance of the cell is 2 ohms.

Example :

Let E be the emf of the cell and r be it's internal resistance

In the circuit the voltage equation can be written as,

E - Ir - IR = 0, where I = ammeter reading

but IR = V = voltmeter reading

=> E - Ir - V = 0

=> E = Ir + V

from case-I

E = 0.3r + 0.9

from case-II

E = 0.25r + 1

equating both the equation,

0.3r + 0.9 = 0.25 r + 1

=> 0.05r = 0.1

=> r = 0.1/0.05 = 2 ohms

Hence the internal resistance of the cell is 2 ohms.