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[Phys-l] g & E fields



--- Craig & Margaret Lucanus <lucanus@iinet.net.au>
wrote:

Attributing energy to the field rather than to an
object influenced by the
field is something worthwhile which I can pass on to
my students. So if
there is nothing under the influence of a field,
regardless of field
strength, there is no energy stored in the field.
Thanks JB, this is
something I must have missed in text books, or at
least it wasn't put that
way. This a helpful list. SNIP

Actually the thanks goes to Greg Swackhammer for his
talks on energy with less work (given at many Modeling
workshops). If you can ignore the W 4-letter word and
instead think of workING - a process by which the
energy of a system is changed by the application of a
force across the system boundary. Step 1 - define the
system (which objects are in, which are out) Step 2 -
just focus on whether the energy of the system
changes, remembering that energy is stored in the
field. If the system energy rises due to appln of an
external force, it's b/c something's been working on
it (OK, doing work). If the system energy falls b/c
something in the system exerts a force on the
surroundings, the system's been working on the
surroundings (or, UGH, negative work is done by the
surroundings - a torturous way to put it if you ask
me!).

One mo' ting - you can think of the term "gh" as
gravitational potential measured in J/kg. The higher
we move our point of interest in a g field, the
greater the gravitational potential. If we move a mass
up to the point of interest, now there is a change in
the energy stored in the field. So,
g in units of N/kg (field strength)
gh in units of J/kg (grav potential)
mgh in units of J (energy)

This is a HUGE help to students when you get to
electric fields b/c now there is a very nice bridge
from a familiar system to one that is much more
abstract (to HS students). Just change the kg to C and
you can paint a very nice picture. Moving a + test
charge from the - plate towards the + plate is very
much like lifting a mass in a g field. This treatment
really helps students distinguish between electric
potential and electric potential energy.

John Barrere
Fresno Unified


----- Original Message -----
From: "John Barrer" <forcejb@yahoo.com>
To: "Forum for Physics Educators"
<phys-l@carnot.physics.buffalo.edu>
Sent: Sunday, December 02, 2007 1:12 AM
Subject: Re: [Phys-l] EM, is it energy



--- Craig & Margaret Lucanus
<lucanus@iinet.net.au>
wrote: SNIP

or when a book on a
shelf falls and loses PE,

This seems to imply that the book "has" PE. Not
so. In
the book/Earth system (one always needs to define
the
system when analyzing energy changes), the work
done
by raising the book to the shelf results in an
increase of energy stored in the Earth's
gravitational
field.

It may also be helpful to remember that since
computation of system energy is always reference
frame-dependent, the only thing that really
matters is
CHANGE in the energy of a system.

John Barrere
Fresno Unified School District
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