Cool Junctions

Abstract

Objective

The goal of this project is to investigate thermoelectricity. How much voltage can be generated between two junctions made of different conductive materials held at different temperatures? Can you create a temperature difference between two junctions made of different conductive materials by passing a current through them?

Credits

By Akram Salman 

Edited by Andrew Olson, Ph.D., Science Buddies

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Introduction

Thermal energy (heat) is one of the oldest forms of energy known to mankind. Thermal energy is usually a byproduct of other forms of energy such as chemical energy, mechanical energy, and electrical energy. The process in which electrical energy is transformed into thermal energy is called Joule heating. This is what causes wires to heat up when current runs through them, and is the basis for electric stoves, toasters, etc.

Transforming thermal energy into electrical energy is known as the Seebeck effect, discovered by J.T. Seebeck in 1821. Seebeck discovered that making one end of a metal bar hotter or colder than the other produced an electric voltage between the two ends. Seebeck experimented with junctions (simple mechanical connections) made between different conducting materials. He found that if he created a temperature difference between two electrically connected junctions (e.g., heating one of the junctions and cooling the other) the wire connecting the two junctions would cause a compass needle to deflect. He thought that he had discovered a way to transform thermal energy into a magnetic field. Later it was discovered that he had created a simple electric current loop, which produced a magnetic field. (See the Science Buddies project idea: Using a Magnet as an Electric Current Detector.)

The magnitude of the voltage produced between two junctions depends on the materials used to create the junctions and on the temperature difference between them. The diagram in Figure 1 shows how you can measure the voltage that is produced. The red and black lines represent wires made of different materials. For example, let's say the black line is an iron wire, and the red lines are copper wires. The wires are twisted together at the points where they touch, forming a junction. One of the junctions is heated (that's a candle, on it's side, heating the junction with it's flame), and the other is cooled (on a block of ice). The multimeter measures the electrical potential (voltage) between the two junctions.

Figure 1. Diagram of experimental setup for measuring the Seebeck effect.

The reverse of the Seebeck effect is also possible: by passing a current through two junctions, you can create a temperature difference. This process was discovered in 1834 by scientist named Peltier, and thus it is called the Peltier effect. This may sound similar to Joule heating described above, but in fact it is not. In Joule heating the current is only increasing the temperature in the material in which it flows. In Peltier effect devices, a temperature difference is created: one junction becomes cooler and one junction becomes hotter. Although Peltier coolers are not as efficient as some other types of cooling devices, they are accurate, easy to control, and easy to adjust. Peltier effect devices are used coolers for microelectronic devices such as microcontrollers and computer CPUs. This use is very common among computer hobbyists to help them in over-clocking the microprocessors for more speed without causing the CPU to overheat and break in the process.

In this starter kit we will describe how to create an experiment to demonstrate the Seebeck effect.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• current,
• voltage,
• Peltier effect,
• Seebeck effect,
• Joule heating, and
• thermocouple.

Bibliography

• To learn about Seebeck effect check the following websites:
  • Wikipedia contributors, 2006. "Thermoelectric Effect," Wikipedia, The Free Encyclopedia [accessed May 22, 2006] http://en.wikipedia.org/w/index.php?title=Thermoelectric_effect&oldid=53657667.
  • Kuphaldt, T.R., 2003. "Thermoelectricity," All About Circuits: Volume VI—Experiments [accessed May 22, 2006] http://www.allaboutcircuits.com/vol_6/chpt_3/10.html.
  • Thermoelectrics.com, 2005. "Introduction to Thermoelectrics," Thermoelectrics.com [accessed May 22, 2006] http://www.thermoelectrics.com/introduction.htm.
• For learning about the Peltier effect check the following website:
  Steinbrecher, T., 2005. "The Heatsink Guide: Peltier Cooler Information," Heatsink-Guide.com [accessed May 22, 2006] http://www.heatsink-guide.com/peltier.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Multimeter, such as the Equus 3320 Auto-Ranging Digital, available online at Amazon.com,
• 9 V battery,
• test leads with alligator clips,
• a beaker or cup to hold ice or cold water,
• candle or other heat source,
• lengths of wire made of different metals, e.g.,
  • iron,
  • copper,
  • constantan,
  • aluminum.
  The wire should be available at your local hardware store. If not, you can obtain suitable wire from: http://www.omega.com/toc_asp/sectionSC.asp?section=H&book=temperature.

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Experimental Procedure

Before starting the experiment, do your background research so that you are knowledgeable about the terms and concepts above.

Measuring the Seebeck Effect

1. Create two junctions between two different materials (as shown in Figure 1, above) by twisting wires firmly together. As shown in the diagram, you'll need one length of the first material, and two lengths of the second material.
2. Set your multimeter to DC Volts.
3. Attach the multimeter leads to the two free ends (as shown in Figure 1, above.) The test leads with alligator clips will be useful for this.
4. Measure and record the voltage with both junctions at room temperature.

   Tip: When you connect the multimeter and circuit as shown, you may get a negative reading on your multimeter. If you prefer to have a positive reading, simply switch the multimeter leads around. But remember you are measuring the difference in voltage between the two different junctions. One junction will always be more positively charged than the other regardless of how the multimeter is connected.

5. Insert one junction in a cold liquid or place it against an ice block and measure and record the voltage again (leave the other junction at room temperature).
6. Insert the other junction in hot liquid or put it in the flame of a candle. Measure and record the voltage again. Be careful with this step! Avoid touching the heated wires!
7. Repeat the experiment using different pairs of materials to create the junctions.
8. Make a graph of the voltage vs. temperature difference for each kind of junction.
9. Which pair of materials gives you the best results (i.e., highest voltage measured for the same temperature difference)?

Measuring the Peltier Effect

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Variations

• The Seebeck effect experiment can be expanded to create a real temperature sensor. You will need an independent means of measuring the temperature difference between the two junctions to calibrate your device.
• For a more advanced Peltier effect experiment, you can vary the current and measure the temperature difference created. You'll need to figure out a method for measuring the temperature of each junction. Use different resistors to change the current. Use Ohm's Law (http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html) to calculate how much current will flow in the circuit. Also calculate how much power will be dissipated in the resistor (and be sure to use a resistor with sufficient wattage rating). Plot the temperature difference vs. current for each type of junction.
• Advanced. You can get a commercial Peltier effect device and study its temperature vs. current characteristics.

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• Science Fair Project Guide
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