 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 3 ] Next Last |
The Seebeck effect is the conversion of heat differences directly into electricity.
This effect was first discovered, accidentally, by the German physicist Thomas Seebeck in 1821.
He discovered that a voltage existed between two ends of a metal bar, when a temperature gradient ∇T existed in the bar.
He also discovered that a compass needle would be deflected when a closed loop was formed of two metals, with a temperature difference between the junctions. This is because the metals respond differently to the heat difference, which creates a current loop, which produces a magnetic field.
A voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This usually causes a continuous current to flow in the conductors. The voltage created is on the order of several μV per degree ( Celsius or kelvin) of difference.
In the circuit:
(which can be in several different configurations and be governed by the same equations), the voltage developed can be derived from:
SA and SB are the Seebeck coefficients (also called thermoelectric power or thermopower) of the metals A and B, and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:
Thus, a thermocouple works by measuring the difference in potential caused by the dissimilar wires. It can be used to measure a temperature difference directly, or to measure an absolute temperature, by setting one end to a known temperature. Several thermocouples in series are called a thermopile.
This is also the principle at work behind thermal diodes, thermoelectric generator s which are used for creating power from heat differentials.
This is due to two effects: charge carrier diffusion and phonon drag.
If the temperature difference between the two nodes is small,
and a voltage ΔV is seen at the terminals, then the thermopower of the entire thermocouple is defined as:
This can also be written in relation to the electric field E and the temperature gradient ∇T, by the equation
In semiconductors the sign of the thermopower is used to decide whether the charge carriers are electrons or holes.
Charge carriers in the materials (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) will diffuse when one end of a conductor is at a different temperature than the other. Hot carriers diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor. Cold carriers diffuse from the cold end to the hot end for the same reason.
If the conductor were left to reach equilibriumFor the 2002 science fiction movie see Equilibrium (2002 movie Equilibrium or balance is any of a number of related phenomena in the natural and social sciences. In general, a system is said to be in a state of equilibrium if all influences on the system, this process would result in heat being distributed evenly throughout the conductor (see heat transferHeat transfer or heat flow is the process whereby heat flows from regions of higher to regions of lower temperature. The flow of heat from one region to another is called a heat current. The rate of heat flow depends on the mode of heat transfer and on th). The movement of heat (in the form of hot charge carriers) from one end to the other is called a heat currentA heat current is a kinetic exchange rate between molecules, relative to the material in which the kinesis occurs. A specific pattern of collisions among molecules, similiar to a river-bed effect on the direction of water travelling therein.. As charge carriers are moving, it is also an electrical current.
In a system where both ends are kept at a constant temperature relative to each other (a constant heat current flows from one end to the other), there is a constant diffusion of carriers. If the rate of diffusion of hot and cold carriers were equal, there would be no net change in charge. However, the diffusing charges are scatteredIn physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles. In astronomy and optics scattering is deflection of photons by either macroscopic surfaces such as an asteroid or by small particles as in by impurities, imperfections, and lattice vibrations ( phononA phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because they contribute to many of the physical properties of materials). If the scattering is energy dependent, the hot and cold carriers will diffuse at different rates. This will create a higher density of carriers at one end of the material, and the distance between the positive and negative charges produces a potential difference; an electrostatic voltage.
This electric field, however, will oppose the uneven scattering of carriers, and an equilibrium will be reached where the net number of carriers diffusing in one direction is canceled by the net number of carriers moving in the opposite direction from the electrostatic field. This means the thermopower of a material depends greatly on impurities, imperfections, and structural changes (which often vary themselves with temperature and electric field), and the thermopower of a material is a collection of many different effects.
| Politics | Business | Finance |
 |
 |
 |
|