Re: Energy density; the correct one

[Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

Good examples! I tried to come up with an even simpler analogy from general
physics. Stand on a skateboard next to a wall. Push on the wall with your hand -
the wall pushes back with an equal and opposite force. You accelerate away from
the wall and end up with a certain kinetic energy. Did the the wall do work
while keeping your hand in place? As we all know, the answer is no. The energy
comes from work done by the muscles in your arm, which do have motion. The arm
is kind of an analogy to the field acting on the the moving charge.

Bob at PC

Robert Cohen wrote:

> Situation 1. Suppose I hold two equally positively-charged particles, q1 and
> q2, a distance r12 apart.  If I keep hold of q1 and release q2, q2 flies off
> to a place infinitely far away and gains kinetic energy equal to E2.  I then
> release q1, which stays put since q2 is now infinitely far away.  The total
> energy of the system is E2.
>
> Situation 2. Suppose I hold two equally positively-charged particles, q1 and
> q2, a distance r12 apart.  If I release both particles at the same time,
> they fly away from each other.  When they are infinitely far away from each
> other, each has an energy E12.  The total energy of the system is 2*E12.
>
> In situation 1, I did no work on either q1 or q2.  I admit that my muscles
> converted chemical energy to heat energy in the process of holding onto q1,
> but I still did no work *on q1*.  q1's electric field did work on q2.  The
> work done by the q1's electric field on q2 equals the kinetic energy gained
> by q2, E2.
>
> In situation 2, I did no work on either q1 or q2.  Is E1 equal to 2*E12?