Re: [Phys-L] defining energy

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

Defining energy is certainly a problem.  The important idea is to have
students understand that energy is transferred from one place to another.
Bar charts help.  Also they have to understand that energy may be stored in
a system, and not in an individual object.  So gravitational energy of a
ball is stored in the Earth ball system.  Chemical energy is stored in the
gasoline/oxygen system and not in the gasoline, and never in the bonds.  You
can have energy stored in a field, and appealing to the idea of fields as
being like rubber bands might be helpful.  Then a stretched bond can store
energy.  Again there is the notion of breaking bonds which conjures up the
wrong image.  Students think of a rubber band breaking and releasing energy,
so the best term is "stretching bonds".  A good grasp on energy might be
helpful to them in other science classes.

The main problem with work is that students tend to think of work as a
"thing" they forget the word "do".  So Modeling has proposed the terminology
"working".  The idea of working as the means to transport energy is very
important, and they need to get out of the idea that work is a thing.  Money
may be helpful here.  When you transfer money from one person to another you
can talk about the transfer or the payment, but a payment is an action and
not a "thing".  The money is the tangible object.

In a sense you really can not tell someone what energy "is", but you can get
them to build an image of it once they have various examples and can relate
how it is always transferred.  The big problem is that energy can not be
seen and Lawson has shown that you need to be at a very high level of
thinking to deal with things you can not "see".  Students want to know what
something "is", but that is a question asking what does it "look like".  In
other words it is an impossible question to answer.

They need to see examples of energy, then the terms can be used to label
what they have seen.  Minds-On-Physics has some good examples of looking at
situations and then asking if there is energy.  They already have an
informal definition of the word, so you need to build on that to make it a
precise physics definition.

John M. Clement
Houston, TX


> I am going to be teaching this topic to my 11th graders soon. 
>  I have a question about the definition of energy.  I know 
> that the "ability to do work" definition runs into trouble 
> when you consider heat, engines and 2nd law issues.  But what 
> if I turn the definition around.  Instead of saying "energy 
> is the ability to do work", I want to say:
>
> Work is defined to be the product of force and displacement 
> (in the same direction).  Then, in different contexts, you 
> can show that work = delta (some quantity).  Any such 
> quantity is referred to as [blank] energy.  Fill in the blank 
> with an adjective that fits the context.
>
> So "energy" is not the ability to do work, but energies are 
> the quantities that are changed by work.
>
> Does this definition pass muster?