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Re: Energy density; the correct one



Good analogy. It could be fine tuned a little in one respect. The earth
also gains energy (kinetic) because the ball exerts a force on it and the
earth moves upward in the direction of that force. Both the ball and the
earth gain kinetic energy at the expense of the field. A similar thing
would happen with the charges as the proton and electron rush toward each
other.

bob at PC

"Carl E. Mungan" wrote:

Here's an analogous gravitational
case:

I drop a ball from rest from some height. (System=ball.) It gains KE.
Why? Well, because the earth pulled it downward while the ball was
moving downward and hence did positive work on it.

But work is an energy transfer from one object to another, right?
Therefore since the ball (adiabatically) gained energy, the earth
must have lost energy....

I think this is what John may mean by dubious energy accounting. The
problem is we forgot to include the gravitational field in our story;
it is the "third object" and it can gain/lose energy. Carl