Re: [Phys-l] Mass and Energy (Binding)

["John M Clement" <clement@hal-pc.org>]

There are many issues here with respect to student understanding.

There are some books which actually state that bonds store energy, so 
one must be aware of these.  In addition some teacher repeat this so 
that students may have actually been taught the misconception. 
Actually there are a large number of teachers who have the 
misconception.  Some treat this statement as shorthand for what is 
really going on.

Now to get down to some problems with pedagogy.  The idea of negative 
energy is very confusing to many students so just making the quantity 
negative will not solve the problem.  This is bound up in how well 
students understand negative, positive and zero.  Most teachers in HS 
and many in college do not confront the idea that we pick which 
direction is positive.  Indeed math in HS takes a very rigid view of 
this and even implies that positive is always either up or to the 
right.  They have even put arrows on both ends of the axes to tell the 
student the line goes on.  This eviscerates the traditional usage of 
the arrow to show the positive direction.  It also causes problems in 
understanding vectors.

Some HS books such as Minds on Physics explicitly confront the issue 
of positive and negative.  Now once students have acquired a good view 
of the sign of numbers, the idea of negative energy will be a little 
more understandable.  Notice that we have just picked the positive 
direction for binding energy.

Then there is the problem of how do you get energy.  The texts all say 
that food contains energy.  But if you seal it into a vacuum packed 
baggie, you can not get that energy out.  So where is the energy?  It 
is in the food-Oxygen system, not in the food or in the oxygen. 
Notice that as long as this misconception is not confronted students 
will say that bonds have to have energy.

Finally there is the usage of words.  We say bonds break, but that 
conjures up what happens when a rubber band breaks.  Again this gives 
students the wrong image.  When a rubber band breaks it snaps and 
releases energy.  They need to think of a bond like being something 
that merely stretches and thins until it is effectively gone.  At this 
point there is the logical puzzle of where the energy went.  Again to 
locate it you need to say it is in the system, but not in the 
individual atoms, nucleons ...

So confronting this misconception requires a lot of attention to 
detail.  Yes, lots of graphs can be helpful, if students actually 
understand graphs.  This of course depends on the student audience. 
Elementary school teachers will require a different treatment from 
physics students.  But it is valuable because students can graduate 
from college with a degree in biology and still have this 
misconception.  I daresay the majority of batchelor's degree students 
in biology may have this misconception.  I was told by one that the 
college biology books even repeat this misconception.

John M. Clement
Houston, TX

> Maybe we can do the same for "bond energy".  That way, students may 
> be
> less inclined to think that breaking a bond always "releases" 
> energy.
>
> > Since "binding energy" is (always?) defined as a positive 
> > quantity, I find it confusing. We can define it as
> >
> > 1) The minimum energy needed to separate the particles (which 
> > makes it, in my mind, the unbinding energy) and/or
> >
> > 2) As you say, the minimum energy released when the system is 
> > formed.
> >
> > I think that the "binding energy" should actually be a 
> > negative quantity, but we are not going to change tradition.
> >
> > Tom Sandin