## Heat Transfer

• INTRODUCTION
• Heat is energy in transit which flows due to temperature difference; from a body at higher temperature to a body
at lower temperature. This transfer of heat from one body to the other takes place through three routes

• If the temperature of a cross-section at any position x in the above slab remains constant
with time (remember, it does vary with position x), the slab is said to be in steady state.
Remember steady-state is distinct from thermal equilibrium for which temperature at any
position (x) in the slab must be same

• THERMAL RESISTANCE TO CONDUCTION
• If you are interested in insulating your house from cold weather or for that matter
keeping the meal hot in your tiffin-box, you are more interested in poor heat conductors,
rather than good conductors. For this reason, the concept of thermal
resistance R has been introduced

• SLABS IN PARALLEL AND SERIES
• Slabs in series (in steady state)
Consider a composite slab consisting of two materials having different thicknesses

• Slabs in parallel
• Slabs in parallel

• Can you now see how the following facts can be explained by thermal conduction ?
• Can you now see how the following facts
can be explained by thermal conduction ?

• CONVECTION
• When heat is transferred from one point to the other through actual movement of heated
particles, the process of heat transfer is called convection. In liquids and gases

• The process of the transfer of heat from one place to another place without heating the
intervening medium is called radiation. The term radiation used here is another word for
electromagnetic waves. These waves are formed due to the superposition of electric and
magnetic fields perpendicular to each other and carry energy.

• PREVOST THEORY OF EXCHANGE
unit time depends on the nature of the emitting surface, its area
and its temperature. The rate is faster at higher temperatures.

• PERFECTLY BLACK BODY AND BLACK BODY RADIATION
• PERFECTLY BLACK BODY AND BLACK BODY RADIATION

• ABSORPTION, REFLECTION AND EMISSION OF RADIATIONS
• ABSORPTION, REFLECTION AND EMISSION OF RADIATIONS

• KIRCHOFF'S LAW
• The ratio of the emissive power to the absorptive power for
the radiation of a given wavelength is same for all substances
at the same temperature and is equal to the emissive power
of a perfectly black body for the same wavelength and
temperature.

• NATURE OF THERMAL RADIATIONS : (WIEN'S DISPLACEMENT LAW)

• STEFAN-BOLTZMANNâ€™S LAW
• According to this law, the amount of radiation emitted per unit time
from an area A of a black body at absolute temperature T is directly
proportional to the fourth power of the temperature

• NEWTON'S LAW OF COOLING
• NEWTON'S LAW OF COOLING

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