Author Topic: 2 Plate capacitor explanation!  (Read 3649 times)

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2 Plate capacitor explanation!
« on: September 08, 2008, 16:02:50 pm »
Hi,

I found this explanation towards holding energy in capacitors and not a charge.
I re-read it many times ans it makes much sense.
So i share this with you..

br
Steve



On Fri, 13 Feb 1998, Rauber, Joel              Phys wrote:

> BTW I also think that saying the capacitor "stores charge " is reasonable;
> for two reasons.
>
> (1) the silly trivial reason, it is storing a paritcular configuration of
> charge; for example a neutral spherical conductor is storing charge.
> Admittedly as much negative as positive charge.

But (1) has unwanted implications.  Should we say that a neutral hydrogen
atom, when its electron is pumped up from the ground state, has "stored
some charge"?  But the atom's still neutral!  A particular configuration
of charge has been stored, but a configuration of charge is not "electric
charge".  When I stretch a rubber band, I have stored a particular
configuration of rubber, but I have not injected any rubber into it.  When
a hydrogen atom absorbs a photon, we do not say that it has absorbed
charge, and we should maintain the same consistent treatment for (2-plate)
capacitors.

In addition, if we focus always on the charge phenomena regarding
capacitors, we may forget to describe capacitors from another standpoint:
capacitors store energy, they are "charged" with EM energy, they release
EM energy during "discharge", and EM energy is a very different animal
than electric charge.  Insisting that capacitors store charge can confound
the roles of energy and charge in the minds of students.  (This certainly
is what happened to me as a student.)  It's analogous to stating that
springs store steel.  Very confusing.  Springs store energy in a
particular *configuration* of steel, but a configuration of steel is not
steel.  When a spring is "charged" or compressed, no steel is injected
into it, but energy is.  When a capacitor is "charged", no charge is
injected into it, but energy is.  At least springs are visible, so some
sorts of misconception are impossible.  Not so with invisible charge-flow
and invisible EM energy.

 (Another aside.  Once I realized that capacitors do not store charge, I
 discovered a very strange notion: capacitors actually are "atomic energy"
 devices.  Not nuclear, but atomic.  They are excellent macroscopic
 analogs for atoms.  A capacitor absorbs a slug of electromagnetic energy
 from a battery, and the energy is stored as a reconfiguration of the
 capacitor's electrons.  The battery separates the electrons from protons
 in one plate, transports the electrons across a small physical space, and
 leaves protons separated from electrons afterwards.  It's the macroscopic
 version of absorbtion of quanta by atoms, with the electrons being
 pushed to a greater distance from the protons.  COOL!  Two disparate
 physical phenomena are really the same thing!  Capacitors are giant
 atoms, and atoms are little bitty capacitors!  No wonder e-fields are
 weird, they are sub-atomic phenomena ballooning up into our macroscopic
 world!  In Leyden jars, early scientists harnessed the secrets of the
 atom without even realizing it!  'Discharging' a capacitor is a great
 analogy for flourescence!  The energy states of electron shells of are
 measured in volts for a an understandable reason!  The veil over my eyes
 is stripped away!  Now if I could just get past the obscuring effects of
 all these exclamation points!


 
> (2) What is a misconception with the following statement, regarding ordinary
> two conductor capacitors (ordinary, meaning what we see in intro physics,
> e.g. the parallel plate capacitor)
>
> " A capacitor is a device, that stores charge; the charge stored by the
> capacitor is defined to be the net charge on the conductor that has a net
> positive charge on it."

Again, the implications following from this might not be desirable.
Essentially you are instructing students to ignore the negative plate.
Pretend the negative plate doesn't count?  Then, using similar
justification (or just staying consistent)  we can also ignore the
*current* going to the negative plate.  If we want to define capacitors as
being charge-storage devices, then what's wrong with also pretending that
a 2-plate capacitor is not an element of a complete circuit, and therefor
defining "charge" and "discharge" as being a net flow of charge into and
out of the device through a *single* wire?  It looks like I'm being silly
here, but I'm serious.  It might be traditional to define "stored charge"
by erasing the negative plate, but I think it's as misleading as erasing
and ignoring one of the capacitor leads.

I think that students would certainly forget this specialized
re-definition of "charge", and start applying the normal meaning of
"charge" to capacitors.  As a result, they would forever be trying to
imagine capacitors as being "buckets" for charge which really should have
just one plate and one connecting wire.  But capacitors are buckets which
store charge in the same way that springs are buckets which store steel.

I like the analogy which someone brought up earlier: two water-filled cups
fastened mouth-to-mouth with a rubber membrane separating them.  Does this
device store water?  When "charged," has water been injected into it?
Yes, but only if we define "water" in weird ways.  And once we have
redefined the meaning of "water", then we no longer can explain why the
weight of this cups-device never changes as it is "filled" and "emptied"
of water.

All this leads me to realize something.  When taking undergrad physics, my
instructor used H&R, and I believe that I was taught the definition in
Joel's #2 above.  As a result I did not form a proper mental picture of a
capacitor.  Instead I formed a distored picture, where a 2-plate capacitor
is like an isolated metal sphere, and where a 2-plate capacitor can only
be understood if one plate is connected to ground and ignored.

As I said before, I'm convinced that many electrical engineers have this
same difficulty, as well as the majority of electronics technicians.  They
want to think of capacitors as charge storage devices similar to a
single-plate device.  They can deal with capacitors with one grounded
plate, but shy away from more complex circuits (or hide behind
formula-guessing techniques or CAD programs.)  Odd circuits with networks
of floating capacitors and switching elements come to seem like a "black
art" which can be analyzed by CAD programs, and can only be designed by
experts.  But in this case, "experts" are simply people who don't suffer
from the misconception which has a univeral grip on everyone else.
Without the "capacitors store charge" misconception, a beginner might be
able to design these "black art" circuits. But under the grip of the
misconception, an engineer with decades of experience is made incompetent.


> There is verbage that follows this, i.e. the verbage of examples and how
> charge the thing up with an emf, etc etc.  It is important verbage as it is
> the discussion and explanation of the words mean and is analogous to all the
> verbage that follows the definition of velocity and acceleration that one
> uses to explain what those words mean.

Beware, students may assume that "capacitors store charge" is the short
version of the complicated verbiage, and then never take time to
understand.

>
> Perhaps there are a better set of words to describe the situation, but the
> words along with their definitions do not constitute a misstatement of the
> facts and are therefore not a misconception of the same type as is "the
> higher temperatures in a green house are due to the 'Green House effect' ".

So you really cannot accept that #36 is a misconception?  Here it is
below.  Perhaps it could be clarified, I've added other possibilities.

 36) 2-plate capacitors are charged with energy, not with electrical
     charge. A charged capacitor contains just as many + and - charged
     particles as a discharged capacitor.


    WHEN A CAPACITOR IS CHARGED BY A BATTERY, NO CHARGE BUILDS UP
    IN THE CAPACITOR.  THIS IS BECAUSE EQUAL QUANTITIES OF CHARGE
    FLOW INTO ONE CAPACITOR PLATE AND OUT OF THE OTHER PLATE.

    WHEN A CAPACITOR IS DISCHARGED BY A RESISTOR, THE CAPACITOR
    AS A WHOLE LOSES NO CHARGE, SINCE FOR EVERY ELECTRON THAT
    LEAVES ONE PLATE, AN ELECTRON ENTERS THE OTHER PLATE.

    WHEN A CAPACITOR IS DISCHARGED BY A RESISTOR, CHARGE FLOWS IN
    THE RESISTOR.  ARE THE CHARGES FLOWING THROUGH THE RESISTOR SUPPLIED
    BY THE CAPACITOR? NO. THEY ARE SUPPLIED BY THE SUBSTANCE OF THE
    CONNECTING WIRES AND OF THE RESISTOR ITSELF.  THE CAPACITOR CAN
    PUMP CHARGES INTO MOTION, BUT IT DOES NOT SUPPLY CHARGES.

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Re: 2 Plate capacitor explanation!
« Reply #1 on: September 08, 2008, 16:19:09 pm »
Thanks, great post.