Determine the internal resistance of a cell using a potentiometer.

Apparatus:

• It can be to determine the emf of a cell.

• It can be used to compare the emf of two cells.

• It can be used as a continuously by potential divider.

• It can be used to measure the internal resistance of the cell.

Q9. What is meant by internal resistance of the cell?
Ans. It is the resistance offered by the electrolyte in the cell to the flow of current through it.
Q10. Does the internal resistance of the cell remain constant?
Ans. No, internal resistance changes with the change of current
Q11. What is cell?
Ans. It is a device in which converts the chemical energy into to electrical energy.
Q12. Is there any difference between a battery and a cell?
Ans. Yes, a cell cannot be recharged after it is used once, but a battery can be recharged after it is discharged.
Q13. Is internal resistance of cell is small or large?
Ans. The internal resistance is usually very small. It is of the order of o.1.
Q14. What is accumulator?
Ans. Accumulator is another name of recharged battery.
Q15. Why should the battery current be stopped every time after taking an observation of null point?
Ans. This is to avoid the heating effect of current as a result the resistance increases.

Q16. Why should diameter of potentiometer wire be uniform?

Ans. The diameter of potentiometer wire should be uniform so that its cross-sectional area and hence resistance per unit length may be uniform.

 

• Potentiometer

• Battery

• Ammeter

• Resistance box

• Rheostat

• Two keys

• Galvanometer

• Given cell

• Shunt wire and connecting wires            

Theory:
Potentiometer is an accurate device which is used to measure or compare the potentials without drawing any current from the circuit.

Principle:
When a steady current flows through a wire of uniform cross- sectional area then the potential difference across any length of wire is directly proportional to its length.The resistance offered by the electrolyte of a cell to the current through it is called the internal resistance of the cell. It depends on the size and separation of the plates of the cell.Consider a cell of electromotive force E and internal resistance connected to an external resistance R through a key k.A voltmeter V can measure the potential difference across the poles of the cell. If the plug is taken out of the key. The reading of the voltmeter given the electromotive force E of the cell. When the external resistance R is connected to the cell by inserting the plug in the key K, a current (say) / passes through it and by Ohm’s law we can write :
                E =1(R+r) = [R+] r …………. (i)
Here 1R is the external fall of potential across R. It is given by the close circuit reading v of the voltmeter which is little less than E. the E.M.F of the cell i.e.
               

The second term 1r of Eq (i) is the potential difference across the internal resistance r of the cell and it can not be measured by a voltmeter. Anyhow it is given by the difference of open circuit  
Reading E Eq (i) can be written as
             

Substituting the value of /from Eq (ii) we get
             
E and V can be measured by balancing them on a potentiometer wire. If /, is the length of the potentiometer wire which balances E and 1 is the length of the wire which balance v then
E α I₁ and v α I₂
Thus, the equ. (iv) becomes
             
E(V) gives the value of internal resistance of the cell.

Procedure

1. Draw a neat circuit as shown in Fig.7.
2. Clean the ends of connecting wires and arrange the apparatus as shown in Fig.8 where the battery E, key K, and rheostat R are connected in series with the wire AB of a potentiometer. The cell E_x whose internal resistance r is to be determined, is connected to resistance box R.B through a key K₂.The positive pole of the cell E is connected to the end A where the positive pole of the battery E _x  is also connected with end A. The negative pole of the cell E_xis connected to a jockey J through a galvanometer G.
3. Now introduce some resistance by the rheostat R_h in the potentiometer circuit. Close the key K_1... keeping the key k₂ open press the jockey on the potentiometer wire near its end A and then near its B and see that the deflection of the galvanometer in these two cases are in the opposite direction. Then the connection is correct otherwise not.
4. Close key K₁by keeping K₂ open, find the balance point on the wire. Note distance of balance point from end A as l, this value of l₁ remains same for rest of the experiment.
5. Introduce some suitable resistance R by the resistance box R. B. close the keys k₁ and k₂ A of the wire.Note that l₂ must be smaller than l₁.
6. Repeat the experiment thrice by slightly changing the resistance R and calculate internal resistance of cell using the relation.         
              R= (l₁-l₂/l₂) R
   
   Observation and calculations:
   Distance of balance point from end A (when k₂ is open) = l_{1= ___________ cm}
   
   Precaution:
   All the connection should be clean and tight.
7. The current indicated by the ammeter should remain constant for one set of observation.
8. After each reading keys must be opened for some time to allow the wires to cool.
9. The positive pole of the battery and that of the cell should be connected to the terminal on the zero end of the potentiometer wire.
10. Other precautions are the same as given in the experiment seven.

                                                                                         VIVA VOCE
Q1. what is meant by electric potential?
Ans. Electric potential is analogous to level in hydrodynamics. It gives the state of a charge and indicates the direction in which a positive charge will flow when free to move. The potential at a point is measured by the work done in bringing a unit positive charge from infinity to the point.
Q2. Define EMF?
Ans. EMF (electromotive force) of a cell is the difference of potential set up between the two poles of a cell because of the chemical action taking place inside the cell and is responsible for driving the current through the circuit. It is equal to open circuit potential difference across the poles of the cell.
Q3. What is meant by short circuiting of a cell?
Ans. When the two poles of a cell are connected by a wire of negligible resistance, the cell is said to be short circuited. This must be avoided because a very large current begins to flow and the cell is likely to be damaged.
Q4. Under what condition does a battery give the maximum current?
Ans. A battery gives maximum current when the external resistance is equal to the internal resistance of the battery.
Q5. What is potentiometer?
Ans. It is an apparatus used for measuring and comparing potential.
Q6. What is the working principle of potentiometer?
Ans. When a current passes through a potentiometer wire, the potential difference across any length of wire is directly proportional to the length i.e., VI
Q7. Can we measure the emf of a cell with potentiometer accurately?
Ans. Yes, we can measure the emf of a cell from potentiometer without drawing any current.
Q7. What are uses of potentiometer?
Ans.

• It can be to determine the emf of a cell.

• It can be used to compare the emf of two cells.

• It can be used as a continuously by potential divider.

• It can be used to measure the internal resistance of the cell.

Q9. What is meant by internal resistance of the cell?
Ans. It is the resistance offered by the electrolyte in the cell to the flow of current through it.
Q10. Does the internal resistance of the cell remain constant?
Ans. No, internal resistance changes with the change of current
Q11. What is cell?
Ans. It is a device in which converts the chemical energy into to electrical energy.
Q12. Is there any difference between a battery and a cell?
Ans. Yes, a cell cannot be recharged after it is used once, but a battery can be recharged after it is discharged.
Q13. Is internal resistance of cell is small or large?
Ans. The internal resistance is usually very small. It is of the order of o.1.
Q14. What is accumulator?
Ans. Accumulator is another name of recharged battery.
Q15. Why should the battery current be stopped every time after taking an observation of null point?
Ans. This is to avoid the heating effect of current as a result the resistance increases.

Q16. Why should diameter of potentiometer wire be uniform?

Ans. The diameter of potentiometer wire should be uniform so that its cross-sectional area and hence resistance per unit length may be uniform.