Thermoelectricity

Prepared by

Md. Kabir Ahamed

Student of

Department of Applied Physics & Electronics

Bangabandhu Sheikh Mujibur Rahman Science &

Technology University, Gopalganj – 8100

Thermal ElectromotiveForce

What is Thermal Electromotive Force (EMF)?

Thermal EMF

Reference: http://www.pidtechinsights.com/2012/03/13/thermal-electromotive-force-emf/

Thermal electromotive force (EMF) is generated when the dissimilar metals that make up terminals, contact springs and contacts touch. This electromotive force, measured in micro volts, could be very small or large and can create undesirable noise during electrical measurements.

What do you understand by thermoelectric effect?

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa.

Seebeck Effect

The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.

Seebeck Effect

Reference: http://www.pidtechinsights.com/2012/03/13/thermal-electromotive-force-emf/

In 1821 German physicist Thomas Johann Seebeck discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field.

Seebeck Effect

Reference: http://searchnetworking.techtarget.com/definition/Seebeck-effect/

The voltage produced is proportional to the temperature difference between the two junctions. The proportionality constant (a) is known as the Seebeck coefficient, and often referred to as the thermoelectric power or thermopower.

The Seebeck voltage does not depend on the distribution of temperature along the metals between the junctions. This is the physical basis for a thermocouple, which is used often for temperature measurement.

E = a (Th - Tc)

Here,

E = Generated EMF

a = Seebeck Constant

Th = Temperature on hot junction

Tc = Temperature on cold junction

Seebeck Effect

Reference: http://searchnetworking.techtarget.com/definition/Seebeck-effect/

The voltages produced by Seebeck effect are small, usually only a few microvolts (millionths of a volt) per kelvin of temperature difference at the junction. If the temperature difference is large enough, some Seebeck-effect devices can produce a few millivolts. Numerous such devices can be connected in series to increase the output voltage or in parallel to increase the maximum deliverable current. Large arrays of Seebeck-effect devices can provide useful, small-scale electrical power if a large temperature difference is maintained across the junctions.

In 2008, physicists discovered what they are calling the spin Seebeck effect. The spin Seebeck effect is seen when heat is applied to a magnetized metal. As a result, electrons rearrange themselves according to their spin. Unlike ordinary electron movement, this rearrangement does not create heat as a waste product. The spin Seebeck effect could lead to the development of smaller, faster and more energy-efficient microchips as well as spintronics devices.

Peltier Effect

The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors.

Peltier Effect

Reference: http://en.wikipedia.org/wiki/Thermoelectric_effect/

When a current is made to flow through a junction between two conductors A and B, heat may be generated (or removed) at the junction. The Peltier heat generated at the junction per unit time, is equal to,

Peltier Effect

Reference: http://en.wikipedia.org/wiki/Thermoelectric_effect/

where ПA and ПB is the Peltier coefficient of conductor A and B respectively, and I is the electric current (from A to B).

The Peltier coefficients represent how much heat is carried per unit charge. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ПA and ПB are different.

= (ПA – ПB) I

The End