Science Buddies: "Ask an Expert"

Me and another person are working on the "Cool Junctions" experiment, and the test for the Peltier Effect isn't working. We set it up to the exact diagram shown, and it is in a room at room temperature, but the wire junctions stay at room temperature. Can you offer advice about what you think is wrong?

Sincerely,

Matt and Andres

Hello ereisenauer,

While the Peltier effect can produce large temperature differences, the total power produced or absorbed at one of the junctions is limited to less than the electric power supplied to it.

Be sure the junction you are testing is not touching something that would conduct heat into or out of the junction. For example it should not be lying on a metal plate or immersed in a glass of water.

Be sure current is flowing through the wires. You can use the multimeter to measure the current. It should be about 9 milliamps. See this figure for the hookup.
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If the current is not not flowing properly, check all your connections to make sure they are tightly coupled.

I hope one of these suggestions helps.

Good luck, WW

Thank you so much! We will try this. We did not set it up through a multimeter, but will do that to ensure the current is flowing! Hopefully, we'll experience success:)

Hello Again....Another couple of questions. The boys set up the connections as you showed. The multimeter indicates that electricity is flowing, however the temperature of the junctions has not changed.

How long should we expect to wait to measure a temperature change at the junctions?

Is a 9 volt battery strong enough to effect this temperature change?

Should we put electrical tape around the junctions to tightly couple them....or will this prevent us from measuring the temperature?


Thanks!

The usual way to make the junctions is to twist them together tightly with pliers. Be sure your wires are clean and have no insulation on them. If in doubt, scrape the wires before twisting them to get a clean metal surface. If you clean and twist tightly you won't need tape, and the tape would hamper your ability to measure the temperature.

I think the temperature of the junctions should take only a fraction of a minute to settle down.

The experiment description indicates it should be enough. Be sure your battery is fresh and that you get the 9 milliamp current expected from Ohm's law: I = E/R = 9v / 1000 ohms = 9 milliamps.

Hello emreisenauer,

After my last reply to you I continued to think about the problems you have had with observing the Peltier effect. I had not done the experiment as described in the Cool Junctions project description, and I just assumed, as I'm sure you did, that the effect was easy to observe if set up correctly. Last night I did it and confirmed my suspicions that the effect produced by the suggested setup is very small.

The effect is not large enough for nearly any inexperienced person to observe. Using a chromel-alumel junction with the 9 milliamp current that is recommended, I was able to measure at most a temperature change when the current started flowing of 0.2 degrees Fahrenheit. This was achieved only by wrapping the junction in insulating foam, waiting three minutes for it to equilibrate, using a sensitive thermocouple meter to measure the temperature, making multiple measurements, and taking the largest result (not a reliable technique).

Tests with currents of 38 milliamps and 80 milliamps gave about the same temperature reduction when the current was flowing to produce cooling but higher temperature rises when the current was reversed as resistive heating began to overcome the Peltier effect.

I think this result is consistent with what one would expect. The electrical power applied to the junction is no more than a few milliwatts. The cooling power has to be much less than the supplied power since metal-metal contacts are not efficient thermoelectric coolers.

The procedure described for observing the Seebeck effect has a better chance of producing observable results, but you should be advised about the very small magnitude of the likely result. Experimenters are advised to try common metals available at hardware stores. Thus, a copper-iron (or steel) junction is a very likely one to be tried. That junction has a Seebeck coefficient of about 13 microvolts per degree Kelvin. If one tries boiling water versus ice water (as suggested) he will encounter a voltage of only 1.3 millivolts. If you have a modern multimeter with a most-sensitive range of 200 mV, you will be just able to see this reading. If you heat one junction with a flame, you'll see a bigger effect.

I apologize for your unnecessary troubles with this experiment. Science Buddies has a policy of testing the suggested projects, but I am told that this one has been around for a long time and likely missed the reviews now in place because of its age.

You can very easily observe the Peltier effect if you get a thermoelectric module such as at http://www.parts-express.com/pe/showdet ... er=320-253 or at http://www.amazon.com/Peltier-Junction- ... -1-catcorr. Such a module is built using semiconductor materials that create a much larger Peltier effect than do metals, and the module combines a large number of junctions to get a significant power rating. You can read more about thermoelectric cooling at http://en.wikipedia.org/wiki/Thermoelectric.

Again I apologize for the unhelpful advice given earlier and thank you for posing the questions that led to discovering this problem with one of the project descriptions.

Best regards, WW