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Re: RC circuit and oscilloscopes



Hi Yanai,
I guess the first thing is to relate what they see on the screen to
the charge on the capacitor: the screen shows the potential difference
across the capacitor and that potential difference is proportional to
(and so a measure of) the charge, So what they see on the screen is a
measure of how much charge is on the capacitor.

Then I think, it;s easier to explain the discharge process. When the
ends of the capacitor are connected the charge will die away. There
are two main controls: The more charge on the capacitor, the faster it
will run away but that rate of discharge falls off (exponentially) as
the charge gets less and less (that is, as time goes by). You are
talking to them about one of the most common changes in nature: the
exponential decay. How soon you want to use the word" exponential',
how much, if any, maths you want to bring into it and how many other
natural exponentially decreasing processes you want to talk about is
up to you and how elastic your 5 minutes is.

The other control will be the size of the resistor through which the
discharge is occurring. The idea that the greater the resistance, the
slower the decay, should not be too hard to deal with: there are the
two extremes, a short circuit, where the discharge will occur so
quickly that they will have to wind the time-base to its extreme to
try to see something and the open circuit, which corresponds to
infinite resistance and no (immediate) discharge will occur.

Once the students have got the idea of the exponential decay, you
should be able to get them to suggest to you how the reverse process
will go: how, for charging, it will be easy when there is no charge on
the capacitor, how it will become more difficult as the capacitor
becomes more and more charged an, thus, how to explain what they see
on the screen.

When the screen shows the potential difference across the resistor
(swap a lead or two around), that potential difference is proportional
to the current through the resistor - get them to tell you why -
and use the fact that the total potential difference around the
complete circuit - the capacitor and the resistor - must be zero -
conservation of energy - to get them thinking and talking about the
linkage between the two potential differences (and between the wave
forms for the charge on the capacitor and the current through the
resistor.

Have fun. the students will too, as long as you don't drown them in
maths but head for conceptual understanding.

Brian mcInnes
----------
From: Yanai Krutman <yanai@BGUMAIL.BGU.AC.IL>
To: PHYS-L@LISTS.NAU.EDU
Subject: RC circuit and oscilloscopes
Date: Thu, 13 May 1999 7:32 PM


Hi, phys-l-netters,

We are preparing a three hours lab-class for a 1st year
eng. students at our department on scopes and want to show
the RC-circuit as one of the scope applications.

Anybody has a way to explain to first year students ( only mech
theory they have ), how a capacitor is charged and discharged,
in terms of I(t) or Q(t)??
I need to talk on this may be 5 min!?
Suggestions are wellcome.

Yanai

====================================================
Yanai Krutman, Students Laboratory Instructor
Ben-Gurion University of the Negev
POB 653, Be'er Sheva, 84105, Israel
phone:+972-7-6403593/6400682 fax:+972-7-6232336
yanai@bgumail.bgu.ac.il
=====================================================