‹‹ Previous | Page 1 | Page 2 | Next ››

6.

Potentiometer

Principle: The potential difference between any two points on the wire of a potentiometer is directly proportional to the length of the wire between these two points.

V_{AP}

µ l_{AP}

Where V_{AP} =

potential difference between points A and P.

l_{AP} =

length of the wire between points A and P.

Potential gradient =

V_{AB}

L

To measure e.m.f. of a cell using potentiometer Circuit diagram

A cell whose e.m.f. is to be measured is always connected in the secondary circuit.

E_{1} =

(

)

l_{1}

Where l_{1} = l (AP) = balancing length for cell E_{1} L = Total length of the wire.

Comparsion of EMFs

E_{1}

=

E_{2}

l_{2}

l_{3} + l_{4}

l_{3} - l_{4}

Where l_{1}= l (AP) = balancing length for E1

Where l_{3 }= balancing length for (E_{1} + E_{2)} position

l_{2} = balancing length for E_{2}

l_{4} = balancing length for (E_{1} - E_{2}) position.

Difference between potentiometer and voltmeter:

Voltmeter

1. Potentiometer measures e.m.f.

1. Voltmeter measures terminal potential difference which is slightly less than e.m.f.

2. Potentiometer measures even small e.m.f. accurately.

2. Voltmeter cannot measure small e.m.f. accurately.

3. Not portable.

3. Portable.

7.

To find the internal resistance of a cell using potentiometer

A cell whose internal resistance is to be determined is connected in the secondary circuit.

r = R

- 1

OR

l_{1} - l_{2}

1_{2}

Where r =

internal resistance of cell E_{1}

R =

value of resistance taken out from the resistance box.

l_{1} =

balancing length when K_{2} is open.

l_{2} =

balancing length when K_{1} is closed and resistance R is taken out from the resistance box.