Current Electricity and Electrical InstrumentVery Hard
Question
The thermo emf of a thermocouple varies with the temperature θ of the hot junction as E = a θ + bθ2 in volts where the ratio a/b is 700oC. If the cold junction is kept at 0oC, then the neutral temperature is
Options
A.700oC
B.350oC
C.1400oC
D.no neutral temperature is possible for this thermocouple.
Solution
E = aθ + bθ2
At neutral temperature dE/dθ = 0
∴
= a + 2bθn = 0; θn = - 
Now
= 700oC(given)
θn = - 700/2 = - 350oC
Now θc = 0oC
So, θn > 0oC
But mathematically θn < 0oC
At neutral temperature dE/dθ = 0
∴
= a + 2bθn = 0; θn = - Now
= 700oC(given)θn = - 700/2 = - 350oC
Now θc = 0oC
So, θn > 0oC
But mathematically θn < 0oC
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