Re: [Phys-L] Energy & Bonds

[Richard Tarara <rtarara@saintmarys.edu>]

Let me suggest that a major stumbling block for HS and other 
introductory students, is the idea of NEGATIVE ENERGY.   The 
'conceptual' idea of kinetic energy is pretty easy to deal with, but 
that can only have positive values. We talk about energy being neither 
created nor destroyed--OK, but what about this negative energy?  _WE_  
understand (pretty well) that we are dealing with POTENTIAL ENERGIES 
where we can set the zero-point wherever we want.  A first example might 
be considering the table top as the zero of gravitational PE where the 
floor then takes on a negative value.  This is not a bad example since 
we can talk about the fact that we would have to do work (add energy) to 
raise an object (for me usually a bowling ball) from the floor up to the 
table top, while the ball easily would move from the table top to the 
floor without doing much (just a nudge).  BUT...taking this into the 
realm of electrons, atoms, and molecules is much less intuitive and 
harder to get across.  Negative energy wells make sense to the schooled 
scientist, but it is a much harder sell to new students.

other comments:

I'm also not sure one gains anything by saying that a deuteron is NOT a 
proton and a neutron.  As an experimental nuclear physicist who used 
deuterons as projectiles and measure the energies of deuterons knocked 
out of nuclei, the deuteron has always been a bound proton and neutron.  ;-)

Not sure, as someone said, that the negative PE is due to the fact that 
the charges involved in the calculation are different. Certainly this 
isn't the case for calculating the potential. OTOH, the actual 
integration from infinity into r always confuses me in that seemingly 
you can get three negative signs in there. Most textbooks seem to 
obfuscate all the sign considerations! ;-(

rwt

On 11/14/2013 11:37 AM, Jeffrey Schnick wrote:
> > -----Original Message-----
> > From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Paul Lulai
> > Sent: Wednesday, November 13, 2013 9:37 AM
> > To: Phys-L@Phys-L.org
> > Subject: Re: [Phys-L] Energy & Bonds
> >
> > First, thanks for all of the help.  I am beginning to improve my understanding.
> > George, Jeff, and others have helpfully pointed out that binding energy is a
> > misnomer.  I am now thinking of binding energy as being similar to a work
> > function value.  I think that is a fair analogy.  I am open to being corrected
> > there.
> >
> > I am still having issues with one (at least) concept.  Endo-thermic graphs (like
> > this one: http://bit.ly/184Ow3E (full url below))  show the products with
> > more energy than the reactants. This makes me wonder a couple of things.
> > First, what 'energy' is being plotted on the y-axis? Second, why is the product
> > (which for an endothermic process has a higher 'energy' than the reactants)
> > the more likely substance?
> To me what makes this diagram
> http://bit.ly/184Ow3E
> confusing is the position of the horizontal axis of the graph.  It suggests that the 0 of energy is below the curve.  I recognize that the energy in question is only defined up to an additive constant, but I see no point in implying a zero of energy that confuses the issue.  The system is bound both before and after so it makes more sense to depict the energy as being negative for both the reactants and the products.  In fact, I would move the horizontal axis to a position above the entire curve, perhaps tangent to the curve at the top of the curve.  Then the whole thing makes sense.  The reactants are strongly bound.  We add some energy to raise the energy of the particles to about zero and then some energy (less energy than the amount originally added) is given off as they drop down to a weakly bound state.
>
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@phys-l.org
> http://www.phys-l.org/mailman/listinfo/phys-l


--
Richard Tarara
Professor of Physics
Saint Mary's College

free Physics educational software
www.saintmarys.edu/~rtarara/software.html