The south poles of two bar magnets are 10 cm from each other. The poles are brought to 5 cm apart and then moved back to their original positions. How does the energy of the system change?

Question

Responses

The potential energy of the system first decreases and then increases.

Both the potential and kinetic energy of the system decrease.

The total change in the energy of the system is zero.

The kinetic energy of the system decreases because some of it is transferred to thermal energy.

Answer 1

To analyze the situation, we need to consider the interaction between the two bar magnets when their south poles are brought closer together and then moved back to their original position.

1. Bringing the magnets closer together (from 10 cm to 5 cm): When you move the south poles of the magnets closer together, they experience a repulsive force due to like poles repelling each other. As they are brought closer together, the potential energy of the system decreases because work is done against this repulsive force. Hence, the potential energy is reduced.

2. Returning the magnets to their original position (from 5 cm back to 10 cm): When you move the magnets back to their original position, you are allowing the potential energy to return to its initial value. The work done to increase the distance once again transfers that energy back into the system.

Considering these points, the changes in energy can be summarized as follows:

• When the magnets are moved to 5 cm apart, potential energy decreases.
• When the magnets are returned to 10 cm apart, potential energy returns to its original value.

In conclusion, the energy of the system in total ends up unchanged (it returns to its initial state), there are no unaccounted transformations into kinetic energy or thermal energy during this idealized movement.

The most accurate statement about the energy change in the system is:
The total change in the energy of the system is zero.