Heat and Thermodynamics
Thermal Resistance
(i) R = 
= 
= 
Greater the K, smaller will be the thermal resistance.
(ii) Unit of R = 
= 
(iii) Dimensions of R = 
.
Where 
= dimensions of kelvin temperature.
= 
.
Note: K denotes thermal conductivity of material , 
denotes dimensions of keliven temperature. is chosen to avoid confusion between conductivity-coefficient and kelvin temperature.
Thermal diffusivity/thermometric conductivity:
(i) This quantity is respectively by h.
h = 
h = 
= 
where
= density of material , s = specific heat of material
K = coefficient of thermal conductivity
(ii) Unit of h 
= 
.
(iii) Dimensions of h = 
= 
= 
= 
.
Ingen Hausz experiments/apparatus:
Several rods are coated with wax.
K = thermal conductivity of material of rod.
l = length of rod upto which wax melts.
= constant.
= 
= 
= 
Accretion/formation of ice on lakes :
(i) t = time taken, x = thickness of ice formed
= density of ice, L = latent heat of ice
K = coefficient of thermal conducitivity
= temperature above lake
t = 
(iii) t = 
= time taken in increasing thickness of ice from 
to 
.
Process of heat conduction:
- Molecules from higher temperature zone collide with molecules of lower temperature zone.
- In metals, presence of free electrons contribute to conduction.
- Silver is the best conductor while copper is the second best donductor of heat (also electricity).
- Mercury is the only liquid which is a good conductor of heat.
Convection of heat:
- During convection, medium moves to transfer heat on account of difference of densities between hotter and colder parts.
- During convection, the temperature gradient exists in vertical direction and not in horizontal direction.
- Liquids and gases are heated by convection.
- Convection is of two types:
- Forced or induced convection.
Newton's law of cooling is applicable. - For natural convection
Rate of cooling
.
- Forced or induced convection.
- Gravity has a role to play in convection.
- Speed of convection is greater than that of conduction but much less than that of radiation.
- Most of the heat transfer on earth is by convection.
- The contribution due to conduction and radiation is very small.
Radiation of heat:
It is the process in which transmission of heat occurs in the form of electromagnetic waves of wavelength from about 8000 Å 
to about 4 mm. Intervening medium is not required for radiation. Heat radiations are actually covered under infrared waves. In vacuum, the speed of radiation is 
m/s. This is equal to velocity of light or velocity of electromagnetic radiations.