Re: [Phys-l] Conservation of energy in nuclear reactions

[John Denker <jsd@av8n.com>]

On 05/01/2009 08:20 AM, Robert Cohen wrote:
> I periodically encounter a source that states that energy is conserved
> except during nuclear reactions. 

Yecchhhhh.

> It is my understanding that energy is always conserved, even in nuclear
> reactions. 

Yes.

>  The mass-energy equivalence is really between the
> "potential" energy and the mass, so it is the *kinetic* energy that is
> not conserved during nuclear reactions, not the *total* energy. 

I'm not sure I would have said it quite that way.

Energy is energy (E).

Mass is mass (m)./1/

Part of the energy is mc^2.  We call this the _rest energy_.
I suppose you could define potential energy to be equal to the
rest energy, but not everybody does.  Also there are issues 
with the gauge-dependence of typical notions of potential energy.

Let's be clear:  The equation E=mc^2 applies in the rest frame
of the particle.  This is Einstein's original meaning of the
equation, and also the meaning that has been conventional among
those who do relativity for a living, and has been for several
decades that I know of.  (I am aware that from time to time there
have been attempts to define some notion of "relativistic mass",
but this is a Bad Idea./2/)  Mass (m) means rest mass, so it is 
redundant but harmless to call it "rest" mass.  

Bottom line:  Energy is conserved.  Period.  There are no known
exceptions.  Of course to make this work you need to include all
forms of energy, including the rest energy mc^2.

1.  http://www.av8n.com/physics/mass.htm

2.  http://www.av8n.com/physics/spacetime-acceleration.htm