Question

CLASS WORK A voltmeter is connected across a variable resistance R which is in series with an ammeter and a cell. The cell is of emf E and internal resistance r. For a certain value of R, the ammeter reads 1 A and the voltmeter 3 V. When R is adjusted to another value, the ammeter reads 1.5 A and the voltmeter 2.5 V. i) Draw a circuit diagram of the set up described above; ii) Calculate the values of E and r, neglecting the resistance of the meters

Answer 1

Answer:2OHM

Explanation:

ANSWER

Let E be the emf of the cell emf r be its internal resistance.

In the circuit the voltage equation can be written as,

E−I

r

​

−I

R

​

=0, where I=ammeter reading

but I

R

​

=v=voltmeter reading

⇒E−I

r

​

−V=0

⇒E=I

r

​

+V

From case i,

E=0.3r+0.9

From case ii,

E=0.25r+1

Equating both the equation,

0.3r+0.9=0.25r+1

⇒0.05r=0.1

⇒r=

0.05

0.1

​

=2ohms

Hence the internal resistance of the cell is 2 ohms.