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Re: Giancoli Problem

Peter, R1 and R2 are not in parallel. For two resistors to be in parallel,
both ends must be joined. R1 and R2 have one end joined but their other ends
have C between them. Same for R3 and R4. Use Kirchhoff's rules to solve the
problem.

poj

----- Original Message -----
From: "Peter Schoch" <pschoch@NAC.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, 07 March, 2002 17:10
Subject: Giancoli Problem


OK, I'm stumped and need a bit of help. The problem is in Physics for
Scientists and Engineers by Gaincoli (the latest ed.), Chapter 26 number
83.

They give a circuit with 4 resistors and 1 capacitor, something like
this:

-----------|------------|
R2 R1
| |
-----C------
| |
R4 R3
----------|------------|

They then connect this to a 12V battery and ask what the voltage across
the C will be when it is fully charged. Then they ask what the time
will be for the C to discharge to 0.05 of its max. value when
disconneted. (At least that is a quick recap.)

So, here's my question(s): aren't R1 and R2 in parallel and R3 and R4
are in parallel? So, shouldn't you combine them first to get an
equivalent resistance and then treat them in series with the C? Then,
for large time the I (current) through the capacitor is 0, and you
should be able to relate the V through the capacitor to the voltage
drops across the two equivalent resistors you just found, correct?

But if I work the problem that way, I get nowhere near the "book
answer". So, I'd appreciate anyone who can give me an insight as to
where my logic is wrong.

Thanks,
Peter Schoch