Physics Practical viva-voce Questions-2018

Potentiometer (Internal Resistance and comparison of Cells)

(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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