# Day 157: WB’ing The Mother of all Ohms Law Labs and This one is a Mystery

Today, we WB’ed the Mother of All Ohm’s Law experiments.  It’s such a simple experiment that is so incredibly rich.  Here is some sample data: As one can see, the two spools only have a different gauge (which we know relates to the cross-sectional area).  There are also spools in the set that have the same gauge but different lengths.  Here are the concepts we developed as we WB’ed this experiment:

1.  Resistance (of course): It is the slope of the linear graph. We build it from looking at the units on the slope; volts/1.0 amp. So it is the electrical potential difference that gives a current of 1.0A for each spool. We discuss how with the two spools there is ‘something’ opposing the flow of charge, especially for the thinner, #30 gauge wire. This is seen because a greater potential difference is needed for the same current when compared to the #28 gauge spool. The ‘something’ that is opposing the flow of charge is defined as electrical resistance.

2.  Ohm’s Law — The EOL for the linear graph.

3.  Using a DMM — We debate which group was correct with the slope value (each spool was done independently by two groups).  Eventually a student with some background will suggest measuring it with an ohmmeter. So we learn how to use the DMM.

4. The relationship between the thickness of the wire and the resistance (slope).  This is easily seen from the data. It shows a thinner wire will have more resistance that a shorter wire (of the same material and length).

5.  The relationship between the length of the wire and the resistance.  Again easily seen from the class data.  The longer the wire the greater the resistance.  I wish I had snapped a picture of the board that displayed the 80cm and 160 cm spools, almost exactly 1/2 the resistance for the 80cm spool.

6.  Using the above two relationships, it is a short transition to how the material affects the resistance.  Through discussion, we develop the concept of resistivity.  One could use the spools to experimentally develop this concept, if one had the time.

7.  Electrical Power– I ask the student to predict the shape of the graph of potential difference vs current graph if I had assigned each group a given potential difference that could not be changed, and they measured the current through the various spools.  The students do not have much trouble predicting a horizontal graph (well the ‘could not be changed’ gives it away).  The area trapped is shown (with units) to be the electrical power, P=ΔV*I. We then use some substitution to write the other common models for electrical power.

And there you have it… the mother of all Ohm’s Law experiments.

General Physics:

We started by using the Light Bar demo discussed in a previous post, but then move on to an activity that provides more practice on determining circuit types and schematic diagrams based on observations (data).  Here are two short clips from the activity:

The Mystery Boxes were constructed by a colleague that used to teach the Advanced Physics course. He made two sets of seven boxes. I really, really like them!  I have also used them as an assessment… pick a box, gather some data, draw the schematic and the EPD.