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

["Craig & Margaret Lucanus" <lucanus@iinet.net.au>]

The discussion went along the lines that EMR (whether it be visible or not ) 
is nothing until it is "received", whereupon an observable kind of energy is 
generated, such the electrical output of a photocell, or when IRR warms a 
physical object.

The PE idea comes from to the specific context of solar EMR (which I should 
have said). My students know that solar EMR comes from nuclear PE which is 
absorbed then re-emitted by atoms. But then I guess, even in this situation, 
light comes partly from KE because this a part of the energy state of an 
atomic electron (or that's where Shrodinger starts) and this changes as the 
electron changes quantum states upon emission.

.----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Sunday, November 25, 2007 9:20 AM
Subject: Re: [Phys-l] EM, is it energy


> 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.
>
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