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Re: [Phys-L] widget rate puzzle ... reasoning, scaling, et cetera



We collect data on resistors and light bulbs and plot them. We take a different approach. We ask what range (that we can test safely) do they behave linearly.this is the range in which they are ohmic.
A subtle difference. Maybe not terribly significant.


.:. Sent from a touchscreen .:.
Paul Lulai



-------- Original message --------
From: Philip Keller
Date:01/02/2015 6:09 PM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] widget rate puzzle ... reasoning, scaling, et cetera

I would start at the other end of the process. After we define
resistance, I give my students a two-part question:

1. We apply 1 volt across a device and measure a current of .2 A. Find
the resistance.

That usually goes fine.

2. We increase the applied voltage to 3 volts. Find whatever you can.

I let them work on that for a while, arguing in groups. Then we discuss
what answers they have come up with. There are usually some who guess
that resistance stays constant. Others guess that current stays
constant. And some just give up, feeling (correctly, as it turns out)
that they don't have enough information.

By this time of the year, they know that I would LIKE for them to think
about proportions and ratios, but they don't know what stays constant.
To settle the question, we do an experiment. First I use a resistor,
but then I use a little light bulb. The resistor behaves nicely -- the
current triples. The light bulb not so much. So on the first day that
they hear the word "ohmic" they discover that it is a property that some
devices have and others don't. I don't even like to call it "Ohm's
Law". I call it "the Ohmic property".

But once it is given that something is ohmic, then yes, this is another
setting to practice with ratio thinking.



On 1/2/2015 5:39 PM, Richard Tarara wrote:
On 1/2/2015 4:21 PM, Robert Cohen wrote:

It is quite likely that students having problems with this are too
focused on the algebraic equation...Ohm's Law...without much
understanding of what the three quantities in that equation actually are.

I'd be careful to introduce and discuss current, resistance, and
voltage thoroughly before introducing Ohm's Law. I will have my
classes come up with an equation for the resistance of an object
through analogy and simple reasoning. They will even come up with
the idea of temperature dependency (which we then say we usually
ignore!) But at this point, sans the temperature component, the
resistance of a given object (yes, between two specified points) is
fixed by its geometry and material.

With that idea firmly in place, students may be less likely to try and
solve for the resistance as V/I and then be befuddled because they
don't know that the current is.

To be honest though, I've never actually used the question below in
quizzing, so not sure how my groups would have fared.

rwt

How about the following? Students frequently have trouble with it. An
ohmic resistor of resistance 10 ohms allows 1 A of current to flow
when a voltage of 10 V is applied across it. What is the resistance
of the SAME resistor when a voltage of 100 V is applied across it?
How would you help students who have difficulty with this question?
Philip Keller wrote:
They are already functioning at the edge, not completely convinced
that thinking is a way to find things out.
When formulating your answer, keep PK's quote in mind.

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