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| Origin
of Thermo E.M.F. |
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If two dissimilar metals are in contact , more free electrons
diffuse from one metal (heaving lower work function) to the
other (having higher work function). Thus one metal become positively
charged and the other negatively charged. This continues until
the difference developed is known as contact e.m.f. |
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The direction and magnitude of contact e.m.f. depend upon what
metals are used and one the temperature of the junction. When
copper is brought in contact with iron, more free electrons
diffuse from iron to copper, thus making iron positive w.r.t.
copper as shown in fig (a). It is clear that direction of contact
e.m.f at Fe-Cu junction will be as indicated. |
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Fig (a)
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When
the junction of the metals forming the thermocouple are at
the same temperature .the two contact e.m.fs (E1 & E2)
are equal in magnitude and of opposite polarity. However,
if the two junction of the thermocouple are at different temperature
see the fig (b)., the contact e.m.f. at junction of the thermocouple
are at different temperatures as shown in fig (b) ,the contact e.m.f. at the hot junction is “more than at the cold
junction (i.e. E1 > E2 ). |
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Fig (b)
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Thus, there is a net e.m.f. in the thermocouple and thermoelectric
current flow in the direction as shown in fig (b). It may e
noted that contact e.m.f. depends only on work function of the
metals, not on size or area of contact. |
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