Question 1: How would the potential energy change in case of situation a when two blocks of similar charges move towards each other, as described in the figure?
A. The energy of the system would increase as the blocks are moved toward each other
B. The system’s energy would decrease as the blocks are moved toward each other
C. There would be no change in the energy of the system as the blocks are moved toward each other
D. There is insufficient information to determine the response
Answer Analysis
The correct answer is A.
The system’s potential energy increases as the blocks move toward each other. If you bring two positive charges or two negative charges closer, you have to do positive work on the system, which raises their potential energy. Since potential energy is proportional to 1/r, the potential energy goes up when r goes down between two positive or two negative charges.
Question 2: How would the potential energy change in case of situation b when two oppositely charged block move away from each other, as described in the figure?
A. The energy of the system would increase as the blocks are moved away from each other
B. The system’s energy would decrease as the blocks are moved toward each other
C. There would be no change in the energy of the system as the blocks are moved toward each other
D. There is insufficient information to determine the response
Answer Analysis
The correct answer is A.
The system’s energy would increase as the blocks are moved away from each other. Work must be done to separate the two opposite charges. The amount of work done equals the increase in potential energy.
Question 3: How would the potential energy change if the oppositely charged blocks were moved towards each other?
A. The energy of the system would increase as the blocks are moved toward each other
B. The system’s energy would decrease as the blocks are moved toward each other
C. There would be no change in the energy of the system as the blocks are moved toward each other
D. There is insufficient information to determine the response
Answer Analysis
The correct answer is B.
The system’s energy would decrease as the blocks are moved toward each other. The total energy is conserved. The total energy is the kinetic one plus the potential one. When the oppositely charged particles get closer, they accelerate, so the kinetic energy increases. Because the total energy must stay constant, the potential energy has to go down.