M
markp
- Jan 1, 1970
- 0
John Larkin said: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.