This page contains outline notes on thermal effects associated with electricity. The notes include
the Seebeck Effect, The Peltier Effect and the Thomson Effect. The notes are outline in
nature to provide basic understanding of the principles for mechanical engineers.
The Effect of Temperature on Resistance
The resistance of metals increases with the temperature in accordance with the formula>
R = Rref ( 1 + ρ(t-tref))
R = resistance at temperature t. R ref = resistivity at temperature t ref C and ρ is the temperature coefficient of resistance.
Table of Resistance in Ohms per m cube and Temp coefficents C-(per cent)
Note: The Resistivity is effectively Ohms . m2 /m length and therefore has the units of Ohms.m..
Important Note: In completing this table I have used a number of reference sources. The values provided by these sources varies a little for each element. I could have shown a range for each element /alloy but I decided to show typical values. If accurate values are needed please consult more reliable references . (see links below)
The Seebeck Effect
When two wires of different materials are joined together to make a complete circuit and one of the junctions is heated then a current will flow as shown. This effect is called the Seebeck effect after the discoverer Thomas Seebeck...
The arrangement is called a thermocouple. The value of the e.m.f generating this current (Seebeck current) is dependent on the following:
a) The materials of the hot and cold junction
If a third material conductor is linked into one of the thermocouple
components, as shown, the overall thermocouple effect is not changed.
The relationship can be approximately represented by the formula
E = At + Bt2
The main applications which utilise this effect involve the use of semiconductors which provide by far the greatest e.m.f's in relation to the thermal gradients.
A and B are constants
The Peltier Effect
When a current flows around a circuit made up of two dissimilar metals heat is absorbed at one junction and is rejected from the other. This effect is shown in the figure below:
The effect is completely reversible in that if the e.m.f. is reversed the emmitting junction
will become the absorbing junction and the absorbing junction will become the radiating junction.
The Thomson Effect
It has been proved that the total Seebeck effect is due to the sum of the Peltier effect and the