Magic capacitors!

[markp]

Hey, I took two years of college physics, and got As. But as an
engineer, it's not prudent to say that a capacitor has zero charge
when it would actually knock you dead if you touched it.

Whatever term a physicist uses for "the differential charge on a
capacitor" or "the integral of all the current that has ever gone
through a capacitor" or "the charge on one plate", circuit designers
just call "charge", which happens to be C*V, in coulombs. I have no
idea how a typical physicist describes this in everyday English. The
few physicists I know wouldn't correct me for saying that a 15 pF cap
charged to 4 volts stores 60 picocoulombs.

I'm afraid a real physicist would, as far as their interpretation of what
'electrical charge' is, because to them this would require more charged
particles to be present on the capacitor than were before. If they know you
are using the electrical convention of 'charge stored on a capacitor', where
q=CV and the plates have +/-q on them, then maybe not. What has happened in
reality is you have taken charge (in the form of electrons) from one side of
the plate to the other, via the external circuit, in the process doing work.
The total number of electrons is the same before as it is afterwards. The
net storage of electrical charge in a capacitor is therefore zero. What you
have done, though, is created an electric field between the plates, and it
is the electric field that stores the energy (equal to the work done needed
to move the electrons from one plate to the other in the first place) - the
belt you get is due to that energy discharging (and hence causing a current
to flow, which moves the electrons in the plate with the abundance of
electrons back into the plate with the depletions of electrons).

Given that this is how EEs design electronics, one must be careful
about basing conclusions on conservation of C*V. That's all I said.

In the case of the ancient "connecting the capacitors" riddle, the
explanation almost always includes the phrase "since charge is
conserved..." and uses C*V as the definition of "charge." It works in
this circuit. In some circuits it doesn't. Using an inductor, I can
transfer all the energy from one cap into another of a different C
value, and C*V will not be conserved. No electrons will be created or
destroyed.

Don't apologize. Just recognize that we use the word "charge" in a way
than a physicist might get legal-picky about. (Unless that physicist
designs circuits, too.)

Well that's my point, a physicist *will* get picky, unless they understand
you are using the electrical convention of q=CV, and they (and you!)
understand that the capacitor has +/-q on its plates.

 

[Grant]

....

I'm afraid a real physicist would, as far as their interpretation of what
'electrical charge' is, because to them this would require more charged
particles to be present on the capacitor than were before. If they know you
are using the electrical convention of 'charge stored on a capacitor', where
q=CV and the plates have +/-q on them, then maybe not. What has happened in
reality is you have taken charge (in the form of electrons) from one side of
the plate to the other, via the external circuit, in the process doing work.
The total number of electrons is the same before as it is afterwards. The
net storage of electrical charge in a capacitor is therefore zero. What you
have done, though, is created an electric field between the plates, and it
is the electric field that stores the energy

Okay, so what do you call that electric field? In my mind, that's the
charge? I accept your argument about the equal but opposite quantity
of electrons, but in moving 'charge' over to define that is what loses
the point of energy stored, no?

(equal to the work done needed
to move the electrons from one plate to the other in the first place) - the
belt you get is due to that energy discharging (and hence causing a current

And here you quite happily talk about 'discharging', you cannot have it
both ways?

to flow, which moves the electrons in the plate with the abundance of
electrons back into the plate with the depletions of electrons).


Well that's my point, a physicist *will* get picky, unless they understand
you are using the electrical convention of q=CV, and they (and you!)
understand that the capacitor has +/-q on its plates.

Sure, a capacitor's not going to do much with the other plate isolated,
but it does do a little, because a body may carry charge relative to
the environment.

Grant.

 

[JosephKK]

Lol.

Nah, I just choose to recognize that "time integral of I" is a concept
distinct from "charge stored on one capacitor plate." Even though
their values are the same, and even though the former is the cause of
the latter, they aren't measured the same. Why try to count the
excess charges on a capacitor's internal plates when we can just
measure the current in the lead wires?

Besides endless Newsgroup fights, really this stuff is only important
when teaching basic physics/electronics to newbies and when writing
electronics textbooks. And further, it's only important if we've
decided to avoid filling students' heads with misconceptions like
single-wire capacitors or "capacitors store charge."

Some educators say things like "what's good enough for me is good
enough for them," and so proceed to infect their students with their
own muddled thinking. This might work for most purposes, and the
misconceptions in question might not act as very large learning
barriers. But over decades and generations it's a "game of
telephone." It's a recipe for filling textbooks and classrooms with
increasing mistakes. Why not instead reverse the trend and turn your
students into physicists who happen to specialize in electronics?
It's not that hard ...just identify common misconceptions which
violate basic physics rules, then avoid spreading those misconceptions
to students. "Capacitors store charge" makes no sense to a student
with a gut-level understanding of charge conservation. Or
conversely, any student who truly believes that capacitors store
charge, might forever afterward have troubles with basic physics.
Remove the contradiction, and "Aha!" everything suddenly connects
together in your brain and makes perfect sense. Ideas like
"capacitors store charge" are bad because they prevent the wonderful
Aha. That's why they need to be taken seriously as errors, and not
just labeled as "nitpicking."




And you're certain that it doesn't just take the time integral of the
current in the capacitor leads?

Case well presented. Much appreciated.

 

[JosephKK]

Sorry, JosephKK, But it does. Shame :-(


Do the math JosephKK. You are a disappointment :-(

Newbies. If you think differently, go into sales ;-)

...Jim Thompson

Had to grind it a few cases. Ding Dang it, take some piano wire and
hang it.

My Apologies Don. The resistor always eats half the energy.

 

[Spehro Pefhany]

For Pete's sake, we all took physics. We understand this stuff. But we
need to talk about these things quickly... you should hear some of the
sessions around here.

The "we" I'm talking about is little kids and grandmothers. [Move
this thread to SEB instead of SED? ] If we want to explain
basic circuitry to the general public, we first need to go over the
traditional explanations with a fine tooth comb. Get rid of
contradictions, and suddenly E&M stops resembling a black art.

Explain electronics to the general public? You've got to be kidding.
The GP drives cars but hardly any understand Newton's laws, much less
the rotational versions of same.

When our building was being wired up, I showed one of the electricians
a sketch of a triangle with a center-tap on one leg, the basic
"stinger" diagram. He said "we don't do that theory stuff." But he
sure could bend conduit beautifully; he was an artist.

Teach them COE and some simple mechanics first. My wife wants me to
put a windmill on our roof.

John

Exellent idea. Be sure to put a nice induction motor on there so that
it can be spinning like mad even when it's dead calm. You'll be the
envy of your neighbors.

 

[Martin Brown]

For Pete's sake, we all took physics. We understand this stuff. But we
need to talk about these things quickly... you should hear some of the
sessions around here.

The "we" I'm talking about is little kids and grandmothers. [Move
this thread to SEB instead of SED? ] If we want to explain
basic circuitry to the general public, we first need to go over the
traditional explanations with a fine tooth comb. Get rid of
contradictions, and suddenly E&M stops resembling a black art.

Explain electronics to the general public? You've got to be kidding.
The GP drives cars but hardly any understand Newton's laws, much less
the rotational versions of same.

When our building was being wired up, I showed one of the electricians
a sketch of a triangle with a center-tap on one leg, the basic
"stinger" diagram. He said "we don't do that theory stuff." But he
sure could bend conduit beautifully; he was an artist.

Does he bother doing the load capacity and balancing calculations?

Teach them COE and some simple mechanics first. My wife wants me to
put a windmill on our roof.

You really don't want it on the roof the noise will drive you crazy and
the local turbulence will not make the turbine work well either. Our
present UK Prime Minister had one on his home or was going to until they
realised it would be a total disaster when monitored.

A lot of commercial wind turbine in the UK are installed to farm the
grants rather than the wind. Few places in lowland England have anything
like a strong enough wind to be worthwhile sites.

Snag is that generating capacity scales with the cube of windspeed and
they have to feather if it gets too windy. I think the vertical axis
Darius rotors are about the nicest looking.

A 6kW unit in a region with average windspeed of 6m/s or more will
payback on almost commercial terms depending on future electricity
prices and subsidies - toy ones for small yatchs are everywhere now.

A low tech one using a sawn in half 45 gallon drum, alternator and
bearings from a car was published around 1970. Offer to build her one of
those if pressed....here's a page with details in those arcane "English"
units you insist on using.

http://mb-soft.com/public/wind.html

Regards,
Martin Brown