Aim : To compare EMF of two given primary cells using potentiometer
Apparatus : potentiometer , a leclanche cell , a daniel cell , an ammeter , a voltmeter , a galvanometer , a battery , (battery eleminator), a rheostat , of low resistance , a resistance box , a one way key , a two way key , a jockey , a set square , connecting wire , a piece of sand paper .
When we keep key (K1) closed and (K2) open , let the null point found be lAJ1
When we keep K1 open and K2 closed , let null point obtained by lAJ2 .
E1 / E2 = KlAJ1 / KlAj2
Where E1 and E2 are the emf of two given cells .
- Arrange the apparatus as shown in the circuit diagram .
- Connect the positive poles of the cells to the terminal and the negative poles to the terminal a and b of the two way key .
- Insert the plug in the key K and also in between the terminals a and c of the two way key .
- Slide the jockey gently over the potentiometer wires until you obtain a point of no deflection .
- Note the length l1 at the point.
- Repeat this with E2 by disconnecting E1 and inserting plug into gap a and c of two way key .
- Record l2 at null point .
- Repeat this different resistance .
|Balancing Lengths||E1 / E2 = lAJ1 / lAj2|
|L1 for cell E1 (cm)||L2 for cell E2 (cm)|
The ratio of emf E1/E2 ~0.87
- The connections should be neat, clean and tight.
- The plugs should be introduced in the keys only when the observations taken.
- The positive poles of the battery E and cells Ex and E2 should, all be connected to terminal at the zero of the wires.
- The jockey key should not be rubbed along the wire. It should touch the
- The e.m.f. of the battery should be greater than the e.m.f.’s of the either cells.
- Some high resistance plug should always be taken out from resistant the jockey is moved along the wire.
Sources of error
- The auxiliary battery may not be fully charged.
- The potentiometer wire may not be of uniform cross-section and material. throughout its length.
- End resistances may not be zero.
What is emf of a cell?
Electromotive force of a cell is the potential difference across the terminals of the cells when it is in open circuit i.e. no current is drawn from the circuit .
What is potentiometer ?
An instrument to measure potentiometer difference or emf of a cell .
What is the principle of potentiometer ?
For a constant current , fall of potentiometer along a uniform wire is directly proportional to its length .
What is preferred for making potentiometer wire ?
Magnum , due to its low temperature coefficient of resistance and high resistivity .
What will you conclude if the deflection of the galvanometer is in the same direction ?
The positive terminals aren’t connected at one point
Potential difference between wires ends is less than emf of cells to be measured .
Emf of driving cells is less than that of cells to be measured .