Electric Potential Energy - General Physics - Solved Past Paper, Exams of Physics

Download Electric Potential Energy - General Physics - Solved Past Paper and more Exams Physics in PDF only on Docsity! 1. (25 pts) a) (5 pts) When computing the electric potential energy in a configuration of charges, why is the energy in the first charge always zero? Since there are no other charges anywhere in space, there is no electric field. Thus it requires no force to move the charge and hence no work. Another way to look at is that you could shift the coordinate system such that the present position of the first charge (before moving it) is its final position. Since the coordinate system is an imaginary construct, it takes no energy to shift it around. Either way works fine. b) (5 pts) If electric fields always point in the direction of decreasing electric potential (and they do, by definition), then why does the electric potential energy of an electron increase when moved in the same direction as the electric field vector? The electron is a negative charge. Thus the force we exert is in the opposite direction from what it would be if we were moving a positive charge. Therefore we do (positive) work when moving the electron in the same direction as the electric field. c) (5 pts) Explain why, when computing the electric potential of an extended non-point-like object, you do not need to break into components the contribution to the potential of each little piece of charge. The electric potential is a scalar and as such does not have components. d) (5 pts) Is it possible for the electric field at some point in space to be zero while the electric potential at that point is not zero? If not, explain why not. If so, give an example of an arrangement of charges that would produce this situation. It is certainly possible. Exactly half-way between two identical point charges will have an electric field of zero but an non-zero electric potential. If both charges are q and separated by a distance d, the electric potential at the mid-point between them is 4keq/d. Another possibility is within a charged conductor. The electric field is zero, but the electric potential may be non-zero (and constant). e) (5 pts) Someone charges a pair of conductors (thus forming a charged capacitor) and hands them to you after removing the battery. The amount of charge on the conductors is therefore fixed. You then bring the conductors closer together, thus increasing the capacitance. What happens to the electric potential difference between the two conductors when you do this? (Assume that you are wearing insulating gloves.) If Q remains constant while C is increased, then the potential will have to decrease.