Electromagnetism - Energy

A system consists of two charged particles of equal mass. Initially the particles are far apart, have zero potential energy, and one particle has nonzero speed. If radiation is neglected, which of the following is true of the total energy of the system?

1. It zero and remains zero
2. It is negative and constant
3. It is positive and constant
4. It is constant, but the sign cannot be determined unless the initial velocities of both particles are known.
5. It cannot be a constant of the motion because the particles exert force on each other.

(GR8677 #78)

Solution:

Particles have zero potential energy → PE = 0

One particle has nonzero speed → v 0 → KE 0

→ Total energy: E = PE + KE = 0 + (KE 0) → E  0

No radiation → no energy loss → Energy constant.

→ Energy is positive and constant.

Answer: C