Re: [Phys-l] g & E fields

[John Barrer <forcejb@yahoo.com>]

One more thing to add. The analogy is clearly not
perfect. There is most definitely energy stored in a
charged capacitor regardless of whether or not a test
charge is moved about. Not the case for a g field. But
the energy stored in the field of the capacitor IS
increased ever so slightly by moving a + test charge
"uphill" towards the + plate, just like moving a mass
upwards in a g field.

Barrere

--- John Barrer <forcejb@yahoo.com> wrote:

> --- 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
> > > _______________________________________________
> > > Forum for Physics Educators
> > > Phys-l@carnot.physics.buffalo.edu
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>
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