Thermo Couple

Location:

Concepts Shown:

semiconduction, fermi energy vs. temperature

Equipment:

3.5" piece of Alumel wire, 3.5" piece of Chromel wire, 2.5" piece of insulating material to separate the 2 wires, Ammeter, cigarette lighter. Both wires are available wherever a thermocouple can be purchased. After pre-assembly, the only necessary tools for demonstration are the assembled unit and a lighter. Cost less than $20.

Procedure:

Setup:

Connect each wire to one input of the ammeter and then run them through the insulator (do not let them touch) leaving about0.75" sticking out of the end of the insulator. Twist the ends of the two wires together at the tips. [eq]. Note: Electrical tape may be used to insulate the wires, however, several layers should be used around each wires and then wrapped around both to hold the wires together. Again leave about 0.75" of each wire sticking out of the end of the insulation.

Presentation:

  1. Show the assembly to the class, explaining that the ammeter will measure any current that might flow through the system.
  2. Heat the ends of the twisted wires with the lighter and observe that the ammeter begins to read a current flowing through the small circuit.
  3. Remove flame and watch the current slowly return to zero.
  4. Discuss the science behind the current that is caused by how each materials fermi energy reacts differently vs. temperature
  5. Pass the assembly around.
Note: If after assembly, when the wires are heated up and a negative current is read on the ammeter, switch the wires around.
Science:

The reason this works is because different metals have different Fermi energy levels. The equation for Fermi energy, F(e) vs. temp is: F(e) = exp(-(E-Ef)/(kT)) Because F(e) is exponentially related to temperature, when the two metals are heated up, the F(e) of one increases much faster than the F(e) of the other. This difference creates a potential difference between the two metals. In a sense, the two wires become like the two ends of a battery where one end has a higher potential than the other. And just like a batter, when connected in a circuit, a current will flow. The current can then be measured with an ammeter. The hotter the wires get, the greater the potential difference between the two metals, and the stronger the current that is produced. The applications are not so much in a source of producing a current as it is a way of measuring temperature electrically. [eq].

References:

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