IIT JEE Physics Formula Revision 33. Thermal and Chemical Effects of Electric Current

1. Joule's laws of heating

Work done by the electric field on the free electrons in time t is

W = potential difference * charge
= V(it)
= (iR)it = i²Rt

2. If θ_c, θ_n, θ_i are temperature of the cold junction, neutral temperature and inversion temperature respectively
_c

θ_n – θ_c = θ_i - θ_n

3. Thermo emf depends on temperature with the relation

Є_AB = a_ABθ + ½ bsub>ABθ²

Where a_AB and b_AB are constants for a pair of metals A and B.

This gives dЄ_AB/dθ = a_AB + bsub>ABθ

The quantity dЄ_AB/dθ is called thermoelectric power at temperature θ.

5. Law of intermediate Metal

Suppose Є_AB, Є_AC, Є_BC are emfs from thermocoupes AB,AC, and BC.

If hot junctions and cold junctions of the three thermocouples are at the same temperature,
Then,

Є_AB = Є_AC - Є_BC

6. Law of intermediate temperature

Let Є _{θ1, θ2}, represent the thermo-emf of a given thermocouple when the temperatures of junctions are maintained at θ1and θ2. Then

Є _{θ1, θ2} = Є _{θ1, θ3} + Є _{θ3, θ2}

7. The actual emf produced in a thermocouple loop is the algebraic sum of the net Peltier effect and the net Thomspon effect.

So emf in a thermocouple loop = Є_AB =
(Π_AB)_T - (Π _AB ) _T0 +(T – T0) (σ_A - σ_A)

(More explanation is needed for the formula.)