Science Buddies: "Ask an Expert"

My 3rd grader and I set up a circuit using 2 6V batteries, alligator clips, and a little 1.5" fan (in lieu of an indicator light). His hypothesis was to show that under certain conditions, insulators can be conductors. We introduced into the circuit 5 household items: clothes pin, pencil, plastic stick, wax candle, and piece of rubber glove. They all registered as insulators. We then put all of them in a bucket of water for a few minutes. Then we re-tested all of them to see which ones turned into conductors. None of them turned into conductors!
I thought that maybe the pencil wasn't absorbing the water enough to be a conductor (maybe because of the paint?) so I sharpened both ends and let it soak for over 30 min.....still no electrical current through it. We then tried a toothpick, thinking that the pencil was just too big and dense...but still it didn't not register as a conductor (turn the fan on).

Everything would be okay if I didn't happened to know the fact that water does indeed conduct electricity. I was especially looking forward to claiming in the Application Section that a valuable safety lesson was learned concerning electricity, BUT it's showing that water does NOT conduct electricity....Can you please help us?

Thanks, Kerri

Hi,

Actually, very pure water is a vert good insulator. It only becomes a fairly good conductor when ions are dissolved in it.

Here is a science buddies project at about your son’s grade level for measuring conductivity:

https://www.sciencebuddies.org/science- ... g&from=TSW

Your motor is a very insensitive instrument for measuring conductivity. You would get far more accurate and precise information if you used a multimeter to measure the resistance of your samples. Inexpensive multimeters are available from many suppliers including Radio Shack, Home Depot and Amazon for less than $20. A multimeter would probably show lower resistance (increased conductivity) when you made your test objects wet.

Here is a Science Buddes page on multimeters:

https://www.sciencebuddies.org/science- ... eter.shtml

Here is an explanation of the conductance of water. Look at Figure 4, way down the page, for a graph of conductivity of various concentration ionic solutions:

http://www.wileywater.com/Contributor/Sample_2.htm

I hope this helps answer your questions. Good luck with your project!

Best regards,

Barrett L Tomlinson

Thank You! That was very helpful! I love the suggested experiment, too.

It was late, but an electrician-friend ended up stopping by. He brought his OHM Meter with him to try and help us. We ended up taking the fan out of the circuit. It seems that there was current passing through the wet items, but not enough so that the fan would turn on. So, we just logged the amount of voltage that the different items were able to conduct. For example, we took a small dried twig from the yard. Dry: it carried no voltage. Wet: it carried 4 of the 6 possible volts. Yeah!

Thanks again for the help and quick reply!

Kerri

kerrihorton - You are on the right track using a multimeter as barretttomlinson suggested, but just reading voltage in this setup doesn't really convey the conductance of your circuit or the materials you are using. In this case, you are reading out the voltage seen across the input impedence of the multimeter. It would be more accurate to measure the DC current flow in the battery powered setup and use that value to calculate the resistance of the material being tested.

Rick Marz