Heat Transfer

Difference between Heat and Temperature

• Temperature is a measure of the amount of energy possessed by the molecules of a substance. It is a measure of hotness and coldness.
• Heat, on the other hand, is energy in transit. Heat is a form of energy which transfers from one system to another due to the temperature difference. 

Modern theory of Heat transfer

The kinetic energy of molecules of a substance is directly proportional to the absolute temperature.
The kinetic energy of molecules is sum of

1. Vibrational Energy
2. Rotational Energy
3. Translational Energy

Application Areas of Heat Transfer

Basic law of Thermodynamics

1) The First Law of Thermodynamics:

         Energy can not be created nor destroyed but can be changed from one form to another. It is also known as the law of conservation of energy.

2) The Second Law of Thermodynamics:

The change in the entropy in the universe can never be negative.

3) The Third Law of Thermodynamics:

     The entropy of a system approaches zero as the temperature approaches absolute zero.

4) The Zeroth Law:

    If two systems are in thermal equilibrium with the third system, they are equilibrium with each other.

Modes of Heat Transfer

1) Conduction:

    An energy transfer across a system boundary due to a temperature difference by the mechanism of molecular interaction.

Conduction is a mode of heat transfer that occurs in solid and fluids when there is no bulk movement of fluid.  

Fourier’s law of heat conduction: 
      The law of heat conduction, also known as Fourier's law, states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows
                               q = -kAdT/dx

2) Convection: 

The transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases

Newton’s law of cooling:

                              q = hA (Tw - T∞)

3) Radiation:

Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region.Radiation emitted by a body is a consequence of thermal agitation of its composing molecules.

Radiation required no medium for heat transfer.

Stefan-Boltzmann Law:

An ideal thermal radiator, or blackbody, will emit energy at a rate proportional to the fourth power of the absolute temperature of the body and directly proportional to its surface area

                              q emitted = σAT 4