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Re: The Capacitor problem - once more



I disagree with Jack Uretsky's analysis that the person connecting the
capacitors absorbs the energy. In his experimental description he may
be correct, but he has changed the experiment drastically.

I think my spring pendulum analogy can help us here.

In the original case, simply connecting the two charged capacitors
together is like letting loose of a stretched spring pendulum. The
spring pendulum either oscillates forever (no friction) or dissipates
kinetic energy into thermal energy over some period of time. Likewise
the suddenly-connected capacitors oscillate forever (E-energy to
B-energy and vice-versa) or else energy is radiated via some
combination of thermal energy or as E&M waves.

The process Jack describes is not the same. Rather, his process is
like slowly letting go of the spring pendulum. Rather than simply
letting go, keep holding onto the spring-pendulum mass and move it
toward equilibrium, and only let go once it is at equilibrium. In this
case the person has "absorbed" all the energy that had been stored in
the spring. There is no oscillation and there is no generation of
thermal energy (other than in the person).

My point... Jack's analysis describes a completely different pathway.

Basically what we are doing here is starting in one state with a
particular energy in the system (the two separated capacitors) and
moving to a second state with lower energy in the system (the two
joined capacitors). Often in this type of situation there is more than
one way to get from state-1 to state-2. Appropriate selection of
capacitor size and wire resistance might push the predominant
energy-loss mechanism toward E&M waves. But selecting larger wire
resistance and larger capacitance can push the predominant energy-loss
mechanism toward thermal energy. Jack has described yet another
interesting pathway in which a third party removes the energy
difference between state-1 and state-2.

Back to the spring pendulum... put it in a vacuum and the primary
energy-loss mechanism will be friction within the spring itself. Put
it in water and the primary energy-loss mechanism will be friction with
the water. Move it slowly to equilibrium with your hand and the
primary energy-loss mechanism is you. Make the spring pendulum be an
HCl molecule in an excited state and the energy-loss mechanism will be
IR-light.

In the original capacitor description we simply close a switch. If
we're hung up on whatever energy that takes, don't think of a
knife-blade switch... think of a field-effect transistor switch.
Assuming usage of wires and capacitors commonly found in the
electronics shop, the primary energy loss is going to be radiative
(thermal and E&M).

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817