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Re: series capacitors: more in sorrow than anger.



On Thu, 11 Sep 1997, brian whatcott wrote:

I am less than confident that physicists can execute this task after
reading their commentaries.
Care to try?

Sure; I love a good capacitor problem! I'll assume that these are
ideal capacitors behaving according to all the textbook rules.
You specify a "procedure" so I assume that the steps are carried
out in sequence. There is, unfortunately, some ambiguity in your
specification of the procedure so I will make my assumptions
explicit:

Cap 1; leads A, B Value 3 microfarad
Cap 2; leads C, D Value 6 microfarad
Battery 3; pos lead E, neg lead F 10 volts
Battery 4; pos lead G, neg lead H 30 volts

I'll assume that the capacitors are initially uncharged.

Procedure:
Connect cap 1 and 2 to battery 3.
Leads B and C to pos terminal
Leads A and D to neg terminal.

Q's ???

This is a simple parallel combination with both capacitors charged
to 10 V so Cap 1 attains a charge of 30 microcoulombs and Cap 2
attains a charge of 60 microcoulombs.

Then

By which I assume you mean "with the capacitors carrying the
charges attained in the first part."

Connect lead B to lead C

It already is.

Connect lead A to pos lead G of battery 4.
Connect lead D to neg lead H of battery 4.

This will simply short out battery 4 unless I first disconnect
leads A and D. So I'm *guessing* that you intended for me to
completely disassemble the first circuit before beginning this
part. Thus, I have formed a series circuit using Caps 1 and 2 and
battery 4 with Cap 1 precharged to 10 V and its (initially)
negative lead connected to the positive terminal of the battery
and with Cap 2 also precharged to 10 V and its negative lead
connected to the negative terminal of the battery.

Q's ???

After the charges redistribute we will find that Cap 1 still
carries 30 microcoulombs (although the polarity has changed) and
Cap 2 now carries 120 microcoulombs (with the same polarity.)

Voltage at leads B and C with respect to D ???

Leads B and C will be at a potential 20 V above that of lead D.

Unless I've really missed something, this problem is essentially
the same as one I usually give--albeit, I hope, less ambiguously
phrased--to my intro physics students. Many *do* find the second
part difficult, but it is essentially meat (or a soy substitute)
and potatoes.

John
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