Science Buddies: "Ask an Expert"

I am assisting my 5th grade son in designing a science experiment in which he will study the electrical conductivity of certain metals.

We proposed that he will investigate different material types, lengths, and diameters.

We are considering using different diameters of copper wire, steel wire (actually varying amounts of steel wool strands), and aluminum.

The only problem I foresee is how to verify quantitatively that one conductor is better than another.

If we connect a multimeter in series with a circuit that consists of a battery, lightbulb, and the conductor under test, will we be able to see measurable differences by changing one of the parameters (material, diameter, length)?

Just looking for any suggestions on how we can demonstrate in a report that he learned something from this study.

Thanks!

It's not likely you're going to see any significant differences on a cheap meter using reasonable lengths of wire. You may see an effect if you use a long length of wire or something weird like steel wool.

For example, let's say you have a bulb as large as a car headlight. If you power it from a car battery and include a 1 foot length of wire, you will see some voltage drop across the length of that wire. According to the calculator at http://www.calculator.net/voltage-drop-calculator.html, 1 foot of 16 AWG wire (standard medium gauge), you'll see a 0.04 drop in voltage due to the wire. If went to a big hefty wire like a 6 AWG wire you might find attached to the battery, you only see a 0.004 volt drop. If your meter can tell the difference between 0.040 and 0.004 volts, then yes, you can measure that difference. If instead of changing the diameter you changed from copper to aluminum, you might see a change from 0.040 to 0.063 volt drop. It's measurable and significant but it requires a really accurate meter.

To make that measurement, connect the black lead of your meter to one end of the test material closest to the battery ground. The red lead goes to the other end of the test material. You'll be measuring in millivolts.

To define "better", you need to keep everything constant except for one thing. You can't really compare a length of copper wire to a biscuit of steel wool without noting that the geometry as well as the material is vastly different. Also, if you do use a higher voltage like 9 or 12 volts and a higher amp load to increase the effect, be very careful you don't burn yourself - the test material ,might become rather hot.

Howard