The cup experiment from tutorial is shown at right Points A

The cup experiment from tutorial is shown at right. Points A and B are within the foil on the cup. The rod has been in place lor a long time. Consider the system consisting only of the charged rod. Is the potential difference from point A to point B in this system positive, negative, or zero? Explain. Consider the system consisting only of the charges in the cup. Is the potential difference from point A to point B in this system positive, negative, or zero? Explain. Now consider the entire system of both the rod and the cup. Is the net potential difference from point A to point B in this system positive, negative, or zero? Explain. Now suppose the rod is suddenly removed, and consider an instant in time before the charges in the cup have begun to move. Is the net potential difference from point A to point B in this system positive, negative, or zero? Explain. What can you conclude about the net potential difference inside of conductors in equilibrium? What about in conductors not in equilibrium? Explain.

Solution

(a)The rod is Teflon. So it has negative charge. Due to the induction, positive charge is acquired towards the rod or at the point B and the point A on the ring has negative charge.

Therefore, the potential difference between the points A and B is negative.

(b)When the cup has only charge, then it has a uniform charge density. Hence, the potential at point A and B is same. Therefore, the potential difference between the points A and B is zero.

(c)Due to the negative charge on the rod, the charge on the point B is positive and the point on A is negative. Hence, the potential difference between the points A and B is negative.

(d)When the rod is removed, before the charges are moved to neutralize, then the net potential difference between the point A and B is also negative.

(e)The net potential difference inside the conductor at the equilibrium is zero, since there is no electric field inside the conductor.

The net potential difference inside the conductor at the non equilibrium is nonzero.