Re: [Phys-l] EM, is it energy

[John Denker <jsd@av8n.com>]

On 11/24/2007 06:35 PM, Craig & Margaret Lucanus asked 
whether EM is energy.

Answer:  No.



We don't say a book on a high shelf "is" energy.  It _has_ energy,
i.e. more energy that it would on a low shelf.

We don't say a speeding train "is" energy.  It _has_ energy,
i.e. more energy that it would at rest.

We don't say an EM field "is" energy.  It _has_ energy, i.e. more 
energy that it would if the field-strength were less.

Energy is an abstract thing.  It is more abstract than books,
more abstract than trains, and even more abstract than EM
fields.  It is a perfectly real, well-behaved thing, just a 
little bit abstract.

> In discussion with my students about different forms of energy , a student 
> asked whether light (EM) is a form of energy at all, or whether it is really 
> just a mode of transmission of potential energy. 

Potential energy?  What's "potential" got to do with it?
 -- One does not transmit potential energy, just energy.
 -- Potential energy is not conserved;  energy is.

>   Heck, what is light? 

1) And God said

                1
       ∇ F = ----- J
              c є0

and there was light.

2) Light is the name we give to a certain subset of the things an
EM field can do.  Basically EM waves with a wavelength between 
380 and 700 nm.