Faculty of Science
Department of Physics
PHYS430: Advanced Lab
Topic: Thermoelectric Effect
Laith Marzouka ID:1160827
Instructor: Dr. Khaled Eid
Date: 23 March, 2020
Abstract
Both electricity and heat are essential elements in our daily life. Not long
ago, electricity was not that abundant and there were only a few known ways to
form a flowing current. Also, while heat was not a rare ingredient on earth thanks
to our solar system, cooling on the other hand was a dream that had to be
reached one way or another. Obviously, the refrigerator was the main motivation
for creating a source of cooling since it was related to our survival instinct which is
keeping food safe, but as all small inventions it led to an enormous fields of
applications in technology. It was a matter of luck when Seebeck discovered a
relation between heat and electricity, and opened a new era in technological
advancement. In this report, we will study what relations electricity and heat holds
together in two main effects called Seebeck and Peltier thermoelectric effects.
Introduction
Thermoelectric effect is where difference in temperature produces electrical
voltage, or an electrical voltage produces difference in temperature. This effect
enables us to generate electricity or heat/cool objects.
The effect of converting difference in temperature to electricity is called
Seebeck effect, and the effect of converting electricity to difference in
temperature is called Peltier effect.
1- Seebeck Effect
The phenomena observed by Seebeck is that when two dissimilar metals are
connected to form two junctions (called a thermocouple) one is heated while the
other is cold, a current flows in the circuit formed by the connection and the
voltage produced by the current is called the thermo electro-motive force (thermo
emf) see fig1.1
Fig1: Seebeck Effect
1 Please note that all figures in this report are drawn by me using Photoshop
2
The transfer of electrons is due to the charge-carrier diffusion property
which make the electrons diffuse from one end to another when the two ends are
held at different temperatures, i.e: electrons in the hot end will diffuse to the cold
end where lower density of excited electrons exist and vice versa. Since both
metals are connected they form a circuit, so by placing a load in the way of the
electrons traffic we can make use of the electricity flow to produce power or
thermo emf in that load. The system will reach equilibrium with time since the
same amount of electrons are transferring again to the hot end. To increase the
thermo emf, we must increase the temperature difference which will free more
electrons.
Also, If we reverse the hot/cold junctions the current flow will switch which
means that the effect is reversible.
The magnitude of the voltage difference is measured by the seedbeck
coefficient which is also called the thermopower 2. The unit of the thermopower is
(V/K) which means it measures the voltage induced over a temperature
difference.
The voltage is given by:
T2
V =∫ ( S B (T )−S A (T ))dT , where T 1 , T 2 are temperatures of the first and second
T1
junctions.
S A , S b are the Seebeck Coefficients of metals A and B.
The Seebeck Coefficients depend on the material’s temperature and crystal
structure. If they are constant under the range of temperatures used, then the
formula reduces to:
V =(S B −S A ).(T 2 −T 1 )
The thermopower of a material for small temperature difference can be
approximated to:
ΔV
S=
ΔT
Although, the thermo emf produced is very low (few microvolts for 1 kelvin
temperature difference) because metals have low thermopower due to the
cancellation of electrons and holes contributions to the thermoelectric voltage.
2 This term is misleading, since it has nothing to do with electric power.
3
Seebeck arranged metals in a series called thermoelectric series:
Fig2: Seebeck Series
We get higher thermo emf for larger gaps as seen in fig2.
The thermo emf can be measured by the equation:
1 2
V =S A ΔΤ + S B ΔΤ
2
It is preferable to use semiconductors since they can be doped with electrons and
holes making larger negative or positive thermopowers.
For example: if we take Si Crystal (4 valence electrons), dope it with Al (3 valence
electrons), and P (5 valence electrons):
Fig3: PN-Junction (doping)
4
Normally, a number of pn-junctions are connected in series to maximize the
thermo emf.
The seebeck effect is mostly used in thermocouple thermometers and in
thermoeletric generators.
2- Peltier Effect
This effect is the reverse of Seebeck effect which means it describes the
temperature difference generated by the emf. Peltier discovered this phenomena
13 years after Seebeck discovered his phenomena. Even though the two
discoveries are related and should be joined but since they were discovered
individually they are usually referred seperatrly.
When a current flow through the thermoucouple circuit, heat is absorbed at
the near junction and liberated at the other far junction. The heat absorbed by the
junction is given by:
Q̇=( Π a −Π b ) I , where Π a , Π a are the Paltier Coefficients of material a and b.
I is the current.
The peltier coefficient represents the heat current carried per unit charge through
the material.
At the first junction met by the current, temperature increases due to the
high density of electrons caused by the emf, so by diffusion they transfer to the
other junction where there is lower density and cool down, causing in lowering the
temperature, then they go back in the thermocouple to the complete the circuit to
get to the first junction again and so on.
As noticed, one junction can be used as a heater and the other as a cooler.
But mostly, peltier effect is used for cooling since there are more efficient ways for
heating.
The peltier effect is also reversible since changing the current flow direction
switches the hot/cold junctions.
5
As in the seebeck effect, we mostly use a large number of semiconductors
making a lot of pn-junctions and connecting them thermally in parallel while
electrically in series to maintain the same current.
The most common application for peltier effect is the refrigerator. But it is
also used in cooling electronic chips and Integrated circuits like the CPU, and used
as portable coolers as well.
6
Refernces
1- PHYS430 Manual
2- https://www.sciencedirect.com/topics/chemistry/thermoelectric-effect
3- https://www.newworldencyclopedia.org/entry/Thermoelectric_effect
4- https://waystoworld.wordpress.com/2014/04/18/thermoelectric-effect-seedbeck-peltier-thomson/
6- https://vlab.amrita.edu/?sub=1&brch=194&sim=351&cnt=1
7- https://www.scienceabc.com/pure-sciences/what-are-the-seebeck-effect-and-peltier-effect.html
8- https://nptel.ac.in/content/storage2/courses/122101002/downloads/lec-21.pdf
9- https://searchnetworking.techtarget.com/definition/Seebeck-effect
