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Heat, Lecture notes of Technical Writing

ABOUT HEAT

Typology: Lecture notes

2014/2015

Uploaded on 07/01/2015

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Heat
Heat is defined as a form of energy that is transferred across the
boundary of the system by virtue of temperature difference.
The temperature difference is called thermal potential and the
heat transfer is called the flux.
The direction of heat transfer is taken from the high temperature
system to the low temperature system.
The different modes of heat transfer are:
Conduction
Convection
Radiation
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Heat

  • (^) Heat is defined as a form of energy that is transferred across the boundary of the system by virtue of temperature difference.
  • (^) The temperature difference is called thermal potential and the heat transfer is called the flux.
  • (^) The direction of heat transfer is taken from the high temperature system to the low temperature system.
  • (^) The different modes of heat transfer are:
    • (^) Conduction
    • (^) Convection
    • (^) Radiation

Heat

  • (^) Symbol Q is used for heat transfer the quantity of heat transferred with in a certain time.
  • (^) Heat transfer into the system is taken as positive.
  • (^) Heat going out of the system is taken as negative.
  • (^) Heat is form of energy in transit and it is a boundary phenomena and it occurs only at boundary of a system.
  • (^) Energy transfer by virtue of temperature difference is called as heat transfer. All other energy interactions may be termed as work transfer. For suggestions or queries mail to drkrk@kluniversity.in

Heat

  • (^) A process in which no heat crosses the boundary of the system is called adiabatic process. In this process, there may be work transfer also.
  • (^) The wall which is impermeable to heat flow is called adiabatic wall, where as the wall which is permeable to heat flow is called diathermic wall.
  • (^) Heat can be expressed as J (or) kJ in SI system.
  • (^) Rate of heat transfer can be expressed as W (or) kW in SI system. For suggestions or queries mail to drkrk@kluniversity.in

Representation of work transfer and heat transfer in

quasi static processes on p-v and T-x coordinates

For suggestions or queries mail to drkrk@kluniversity.in

  • (^) Hence is inexact differential.
  • (^) Where X is an extensive property and dX is an exact differential.
  • (^) To make inexact differential integrable it has to be multiplied with an integrating factor which is in this case it is 1/T.
  • (^) Here the extensive property is yet to be defined and it is entropy For suggestions or queries mail to drkrk@kluniversity.in

Types of heat

  • (^) Specific heat of a substance can be defined as the amount of heat transfer required to raise a unit mass of the substance through a unit rise in the temperature. It can be expressed as J/kg-K (or) kJ/kg-K in SI system.
  • (^) The product of mass and specific heat is termed as total heat capacity. It can be expressed as J/K (or) kJ/K in SI system.
  • (^) Latent heat is the amount of heat transfer required to change the phase of the substance at constant pressure and constant temperature.
  • (^) Various forms of latent heat are:
    • (^) Latent heat of fusion (solidliquid or liquidsolid)
    • (^) Latent heat of vaporization (liquidvapour or vapourliquid )
    • (^) Latent heat of sublimation (solidvapour or vapoursolid)

Work vs Heat

SIMILARITIES:

1. Both are path functions and inexact differentials.

2. Both are boundary phenomena (recognized at the

boundaries of the system as they cross them).

3. Both are associated with a process , not a state. Unlike

properties, work or heat has no meaning at state.

4. Systems possess energy, but not work or heat.

Work vs Heat

DISSIMILARITIES:

1. In heat transfer, temperature difference is required.

2. In a stable system , there cannot be work transfer;

however, there is no restriction for the transfer of heat.

3. The sole effect on things external to the system could be

reduced to the raising of a weight , but in the case of

heat transfer, other effects are also considered.

4. Heat is a low grade energy whereas work is a high grade

energy.