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line of the probes would have a very much smaller effect. This difference allows some localization and measurement of the size of discontinuities. 343 Electromagnetic Techniques for Material Identification
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Though not strictly electromagnetic, another useful approach to material sorting involves thermoelectric effects. There are three thermoelectric effects: the seebeck effect, the peltier effect and the thompson effect. The seebeck effect is observed when two conductors are joined: if the junctions are maintained at different temperatures, an electromotive force (EMF) will be developed around the closed loop. This force is the thermocouple voltage or seebeck voltage θ . It is easily measured by inserting a voltmeter in the loop. The peltier effect is the evolution (or absorption) of heat when an electric current passes from one material to a different material at the same temperature. The rate at which heat is evolved at the junction when a current J is passing from material 1 to material 2 is π 12 J where π 12 is the peltier coefficient. The thompson effect is the evolution (or absorption) of heat when an electric current passes from a material at one temperature to the same material at a different temperature (in a material with a temperature gradient imposed). The rate at which heat is evolved per unit temperature difference is μ· J where μ is the thompson coefficient and J the current flowing from the higher to the lower temperature. The thompson heat like the peltier heat is reversible and is in addition to any (irreversible) joule heat. The mechanisms can be visualized that give rise to thermoelectricity and they can be loosely described as the tendency of heat to drag along electricity. The tendency of electricity to drag along heat could also be described but is of no particular interest here because most commercially available instruments use the first approach. Imagine a piece of material as a long box of electrons with a uniform concentration of charge carriers. Make one end hot and the other end cold. The electrons at the hot end will diffuse more rapidly than those at the cold end, so they will move around a little more and more of the hot electrons will move to the cold end. This flow of electric current would keep up indefinitely if the piece of material under consideration were not electrically insulated. In this case the initial current produces a pile up of electrons at one end, which gives rise to a back electromotive force that prevents further flow of charge. In the equilibrium situation, hot electrons keep flowing to the cold end while cold electrons flow to the hot end because of the voltage gradient. Thus, there is no net flow of particles or charge. There is however a flow of energy, a heat current. Under the influence of a temperature gradient, there exists then a heat flow together with a tendency for electricity to flow. This tendency is balanced by the back electromotive force.
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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