To determine internal resistance of a given primary cell using potentiometer | Class 12 | Physics Practical Copy | Handwritten with observations ~ Class K Notes

Aim: To determine internal resistance of a given primary cell using potentiometer.

Principal:
When a resistance R is connected across a cell of emf E and internal resistance r, then the current I in the circuit is I =
E

R + r

[eq.1] V = IR [eq.2]

01

By [eq.1] and [eq.2] the potential difference V across the two terminals of the cell is given by

V =
E

R + r
R
r = (
E

V
- 1) R
[eq.3]

02

If l_o and l are the distances of the balance null point from end A of the potentiometer for an open and a closed circuit, then E is proportional to l_o and V is proportional to l.
E

V
=
l_o

l

[eq.4]
By equation [eq.3] and [eq.4] we get:
r = (
l₀ - l

l
)R

03

Procedure:

Connect all the electrical components as shown in the circuit. After checking the circuit connections, close the key K₁.

Open keys K₂ and K₃ and a protective high resistance P from the R_{BOX 2}, find the position of the balance point. For final reading, short circuit the resistance P by closing the key K₃ and find the balance length l_o.

04

Take R = 10Ωfrom the resistance box, close the key K₂ and measure the new balance length l. Open K₂ after this has been done.

Keep the ammeter readings constant while taking the above observations.

Reduce the value of R in equal steps of 1Ω and for each value of R obtain the balance length l.

At the end of the experiment, open key K₂ and repeat step 2 to find l0 again.

Principal:

When a resistance R is connected across a cell of emf E and internal resistance r, then the current I in the circuit is I =
E

R + r

[eq.1] V = IR [eq.2]

By [eq.1] and [eq.2] the potential difference V across the two terminals of the cell is given by
V =
E

R + r
R
r = (
E

V
- 1) R

[eq.3]

If l_o and l are the distances of the balance null point from end A of the potentiometer for an open and a closed circuit, then E is proportional to l_o and V is proportional to l.
E

V
=

l_o

[eq.4]

By equation [eq.3] and [eq.4] we get:
r = (
l₀ - l

l
)R

03 Procedure:

Connect all the electrical components as shown in the circuit. After checking the circuit connections, close the key K₁.
Open keys K₂ and K₃ and a protective high resistance P from the R_{BOX 2}, find the position of the balance point. For final reading, short circuit the resistance P by closing the key K₃ and find the balance length l_o.

04
Take R = 10Ωfrom the resistance box, close the key K₂ and measure the new balance length l. Open K₂ after this has been done.
Keep the ammeter readings constant while thking the above observations.
Reduce the value of R in equal steps of 1Ω and for each value of R obtain the balance length l.
At the end of the experiment, open key K₂ and repeat step 2 to find l0 again.