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(Assuming that the student was assigned the experiment “To determine the internal resistance of a cell using potentiometer“)
Examiner (E):
What was the experiment allotted to you?
Students(S):
Sir, I was assigned the experiment to determine the internal resistance of a primary cell using potentiometer.
Examiner (E):
OK, Tell me the principle of a potentiometer
Students(S):
Sir. The principle of a potentiometer is that the potential drop across any length of a wire of uniform cross section and composition and carrying a constant current is directly proportional to the length.
Examiner (E):
Good. What is internal resistance?
Students(S):
The resistance offered by the electrodes and electrolyte of a cell is called internal resistance.
Examiner (E):
What are the factors affecting internal resistance?
Students(S):
Sir. The internal resistance of a cell depends on the nature of electrodes and electrolyte, the temperature of electrolyte, the area of electrodes, the concentration of electrolyte and the distance between electrodes.
Examiner (E):
Good; how does the internal resistance change if we increase the area of electrodes?
Students(S):
Sir, the internal resistance will decrease if we increase the area of electrodes.
Examiner (E):
What will happen to internal resistance if we increase the temperature?
Students(S):
The internal resistance of the cell will decrease with increase in temperature.
Examiner (E):
How does the Resistance of a conductor vary with temperature?
Students(S):
For a conductor the resistance increases with increase in temperature due the decrease in relaxation time.
Examiner (E):
Oh! What is relaxation time?
Students(S):
The average time interval between two successive collisions of electrons is called relaxation time.
Examiner (E):
Why does the relaxation time decrease with an increase in temperature?
Students(S):
Sir, as temperature increases, the collisions become more frequent and therefore the average time interval between collisions decreases.
Examiner (E):
Define potential gradient of a potentiometer.
Students(S):
The potential drop per unit length of the potentiometer wire is called potential gradient.
Examiner (E):
Can you explain why?
Students(S):
As the potential gradient increases, greater potential difference is obtained for a small change in length of the wire. Or the length of the potentiometer for a given change in potential will be less. The potentiometer is more sensitive if we get a considerably larger change in length for a given change in potential. Therefore, with an increase in potential gradient, the sensitivity decreases.
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