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Re: Circuit Question



Sorry Tina if you took my reply as too brusque, but this is exactly how I
approach basically the same question with my students. The first E&M lab
that we do (before we have covered anything in class) is a simple 'bulbs &
batteries' lab. Actually, we use a $300 regulated and protected power
supply. After adjusting the voltage to light a single light bulb, they are
forbidden from changing that voltage. Then they are asked to light three
bulbs at the same brightness as the first. Invariably, the first thing they
try is the series circuit, which lights the bulbs but not brightly enough.
Eventually they get the parallel circuit built. They draw schematics of
each. They then go on to other circuits.

A couple of weeks later we do an Ohm's Law lab. At that point I have the
class go back to their series and parallel circuits from the earlier lab,
and with the assumption that the light bulbs work (basically) like
resistors, and that their brightness is a function of the current, now they
must EXPLAIN the behavior of these circuits using Ohm's Law. Thinking it
through is really more useful than just being told--as long as you know
where to start.

For your question, you did need the info that John Denker provided--that the
battery is more or less a constant voltage device (hence 9 Volt or 1.5 Volt
batteries), but from there, Ohm's law should provide you enough to
understand. Unlike Bob S., I prefer to look at Ohm's law in the form I =
V/R as the prescription for determining the amount of current. Anyway, the
different wattage bulbs will draw different currents from the same battery
and Ohm's Law will tell you that the more current the lower the resistance
of the bulb.

Rick

*************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

FREE PHYSICS INSTRUCTIONAL SOFTWARE
www.saintmarys.edu/~rtarara/software.html
NEW: Photo Realistic Laboratory Simulations
**********************************************************

----- Original Message -----
From: "Tina Fanetti" <fanettt@QUEST.WITCC.CC.IA.US>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, February 07, 2002 6:29 PM
Subject: Re: Circuit Question


Thank you to all who answered.

However, is it really vaild to answer a question about a concept by saying
look at this or that law.

I mean really. I can solve for Ohm's law in a variety of situations. ANd
I can look at Ohm's law and know if I have a circuit this and that is going
to happen.

Now I have a masters degree. I am teaching people who can barely solve
algebraic problems.

If I told them to look at Ohm's law and think real hard about it they
would revolt. The students would not get it and quit.

Yes I can say look at Ohm's law and show them via Ohm's law but that
doesn't help us.

I am now afraid to ask any more questions on this list for fear they are
too "simple" for the people on this list.
It seems like this list is geared more towards topics at the advanced
level and grad level and not to the intro topics that most must teach.

Tina