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

[Richard Tarara <rtarara@saintmarys.edu>]

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.
>
> _______________________________________________

--
Richard Tarara
Professor Emeritus
Saint Mary's College

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