Heat Transfer Concept Page - 5

Definition

Prevost theory of exchange

Prevost's Theory of Heat Exchanges 
Every material body, at any temperature above absolute zero, radiates heat to the surroundings and at the same time absorbs heat from the surroundings.

• The rate of emission of heat by a body depends upon its absolute temperature.
• The rate of emission of heat by a body does not depend upon the temperature of its surroundings.
• A body at a higher temperature than the surroundings radiates heat at a faster rate than it absorbs.
  ∴$\therefore$ It loses heat and its temperature falls.
• A body which is at a lower temperature than the surroundings absorbs heat at a faster rate than it radiates.
  ∴$\therefore$ It gains heat. Its temperature rises.
• In the case of thermal equilibrium, the process of radiation and absorption continue to take place. The rate of heat radiated in unit time by the body equals the rate of heat absorbed in unit time. Hence, there is no net loss or gain of heat. Its temperature is unchanged.

Law

Planck's law of radiation

Planck's law describes the spectral density of electromagnetic radiation. These radiations are emitted by blackbody when they remain in thermal equilibrium at a given temperature.
This law gives the spectral distribution of radiation from a blackbody.

Definition

Athermanous and diathermanous substances

A body which allows heat radiation to pass through it is called a Diathermanous Body(diathermic or diathermal bodies). Example: oxygen, quartz, hydrogen, iodine. Athermanous Body is a body which doesn't allow the passage of heat radiation through it.Example: Moist air, water, wood, etc.

Result

Good absorbers are good emitters

Black surfaces are better absorbers of heat. If we take two aluminium containers, one painted black and another with its original colours and fill them with water, the blackened container absorbs more heat than the other one.Now if the two containers are allowed to cool, the temperature in the blackened container falls more rapidly than the other one. This can be verified by keeping thermometers in the two containers and measuring the temperature differences.This confirms that good absorbers are good emitters.

Definition

Coefficient of reflection

Coefficient of Reflection: The ratio of the quantity of radiant energy reflected by a body per unit area per unit time to the total quantity of radiant energy incident upon it per unit area per unit time is called the coefficient of reflection.

Q$Q$ = quantity of incident radiant energy per unit area per unit time.Qr$Q_r$ = quantity of energy reflected by the body per unit area per unit time.

Coefficient of reflection r=QrQ$r={\displaystyle \frac{Q_r}{Q}}$

(or reflective power or reflectivity)

Definition

Transmission coefficient

Coefficient of Transmission: The ratio of the quantity of radiant energy transmitted by a body per unit area per unit time to the total quantity of radiant energy incident upon it per unit area per unit time is called the coefficient of transmission.

Q$Q$ = quantity of incident radiant energy per unit area per unit time.Qt$Q_t$ = quantity of energy transmitted by the body per unit area per unit time.

Coefficient of transmission t=QtQ$t={\displaystyle \frac{Q_t}{Q}}$

Definition

Relation between absorption,reflection and transmission

Let q be the total radiant energy incident on a body and qa,qr,qt$q_a,q_r,q_t$ be the energies absorbed, reflected and transmitted by the body, respectively.
Then qa+qr+qt=q$q_a+q_r+q_t=q$
∴qaq+qrq+qtq=1$\therefore {\displaystyle \frac{q_a}{q}}+{\displaystyle \frac{q_r}{q}}+{\displaystyle \frac{q_t}{q}}=1$∴a+r+t=1$\therefore a+r+t=1$
where a, r, t are the absorptivity, reflectivity and transmittivity of the body respectively.

Definition

Emissive Power

Definition: The energy of thermal radiation emitted in all directions per unit time from each unit area of a surface at any given temperature.

Definition

Emissivity

A body that does not absorb all incident radiation (sometimes known as a grey body) emits less total energy than a black body and is characterized by an emissivity which is its effectiveness in emitting energy as thermal radiation. ε<1$\epsilon <1$

Definition

Absorptive power

Absorptive power of a body is defined at a given temperature and wavelength as the ratio of the amount of heat energy absorbed to the amount of heat energy incident on it in a wavelength range.

BookMarks

Page 1  Page 2  Page 3  Page 4  Page 5  Page 6  Page 7  Page 8  Page 9  Page 10  Page 11
Page 12  Page 13