An Exemplary Viva Voce (CBSE Class XII Physics Practical)
(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.
E: OK, Tell me the principle of a potentiometer
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.
E: Good. What is internal resistance?
S: The resistance offered by the electrodes and electrolyte of a cell is called internal resistance.
E: What are the factors affecting internal resistance?
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.
E: Good; How does the internal resistance change if we increase the area of electrodes?
S: Sir, the internal resistance will decrease if we increase the area of electrodes.
E: What will happen to internal resistance if we increase the temperature?
S: The internal resistance of the cell will decrease with increase in temperature.
E: How does the Resistance of a conductor vary with temperature?
S: For a conductor the resistance increases with increase in temperature due the decrease in relaxation time.
E: Oh! What is relaxation time?
S: The average time interval between two successive collisions of electrons is called relaxation time.
E: Why does the relaxation time decrease with an increase in temperature?
S: Sir, as temperature increases, the collisions become more frequent and therefore the average time interval between collisions decreases.
E: Define potential gradient of a potentiometer.
S: The potential drop per unit length of the potentiometer wire is called potential gradient.
E: How does the sensitivity of a potentiometer vary with potential gradient?
S: The sensitivity of the potentiometer decreases with an increase in potential gradient.
E: Can you explain why?
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.
(Thereafter the examiner may go on to questions based on the activities and demonstration experiments performed by the student.)
Hope that the above post gave you an idea of what happens in a viva voce. These were just some of the typical questions which could be asked. Depending upon the creative mind of the examiner and the response of the students, the style of asking and the nature of questions may vary.
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