MHT-CET : Physics Entrance Exam

### MHT - CET : Physics - Current Electricity Page 2

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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.

 VAP µ lAP Where VAP = potential difference between points A and P. lAP = length of the wire between points A and P.

•

• Where L = total length of the wire
VAB = p .d. between end points A and B of the wire.

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.

 E1 = ( VAB ) l1 L

Where l1 = l (AP) = balancing length for cell E1
L = Total length of the wire.

Comparsion of EMFs

 E1 = l1 E2 l2

 E1 = l3 + l4 E2 l3 - l4

Where
l1= l (AP) = balancing length for E1

Where
l3 = balancing length for (E1 + E2) position

l2 = balancing length for E2

l4 = balancing length for (E1 - E2) position.

Difference between potentiometer and voltmeter:

 Potentiometer 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 ( l1 - 1 ) l2

OR

 r = R ( l1 - l2 ) 12

 Where r = internal resistance of cell E1 R = value of resistance taken out from the resistance box. l1 = balancing length when K2 is open. l2 = balancing length when K1 is closed and resistance R is taken out from the resistance box.

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