Current Thermal Effects
Joule's law of heating heat produced in a conductor when current I passes through it for a time t is H = I2Rt where R is resistance of the conductor. Heat procured in the conductor is independent of direction of the current.
Seebeck effect when two metallic strips made of two different metals and joined at the ends to from a loop as shown in is called thermocouple. If two junctions of a thermocouple are kept at different temperature, an electric current is induced in the loop. This effect is called see back effect and emf developed is known as see beck emf or thermo emf.
The magnitude and the direction of the emf depend upon the metals and the temperatures of the hot and cold junctions.
Where ∅ is temperature difference between hot and cold junction (if ∅c = o then ∅ is temperature of jot junction note the curve is parabolic.
∅N is neutral temperature at which the emf is maximum moreover, at ∅ = ∅N
Dε / d∅ = o or ∅N = a / β
Note that ∅N depends upon the nature of materials which form junction ∅ is the inversion temperature at which the emf changes sign.
Form ∅I - ∅N = ∅N - ∅c
Or ∅N = ∅I - ∅c / 2
If ∅c = 0 then ∅c = 2∅N
There is a series of metals called thermoelectric series. The first and the last element of the series if used to form a thermocouple give maximum emf. The series is
Sb, Fe, Zn, Ag, Au, Mo, Cr, Sn, Ob, Hg, Mn, Cu, Co, Ni, Bi
If hot and cold junctions are interchanged then the direction of emf changes,
Peltier effect:- it is converse of see back effect. If current is passed through a the mpcouple, one of the junction becomes hot and the other gets cold. The heat liberated or absorbed at one of the junctions is proportional to charge transferred.
Peltier emf π = ΔH / Δ Q
Peltier coefficient is the amount of heat liberated or absorbed per second when I A of current is passed through the thermocouple. The hot and cold junction will interchange if the direction of current is revered. π = TS = Tdε / d∅ where S is see beck coefficient
Thomson effect emf is developed between two parts of a single conductor if they are at different temperatures. This effect is called Thomson effect.
If dV is the potential difference between two points of a conductor then Thomson coefficient
σ = dV / dθ = - T d2 ε / d∅2 = - T dS / d∅
See back's coefficient S = dε / d∅
If 0ne part of conductor is at different potential then the other or the current is flowing' a temperature difference d∅ will be developed across the two ends.
Applications of thermal effects
Measurement of temperature (thermocouple thermometer and platinum resistance thermometer)]
Detection of heat radiation
Power generation (thermopile)
P = I2 R, use this formula when devices are in series
= V2 / R use this formula when devices are in parallel
= V. I, when potential drop across the device and current through it are known.
The SI unit of power is watt, (W)
Kilo watt-hour (k Wh) or board of trade unit or simple called unit = 3.6 x 106 j .
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