Heat Transfer Concept Page - 4

Example

Thermal resistance of a continuous body with varying cross-sectional area.

The rate of heat flow through a cross-section of the rod shown in figure is _______. 
( θ2>θ1${\displaystyle {\theta }_2>{\theta }_1}$ and thermal conductivity of the material of the rod is K$K$.here the cross section of the tube is having two different radii,
Now we know that rate of heat flow is given as Q=KAΔTL$Q={\displaystyle \frac{KA\mathrm{\Delta }T}{L}}$
where areaA=πr2effective$A=\pi {r^2}_{effective}$
reffective=r1r2−−−−√$r_{effective}=\sqrt{r_1{r}_2}$}
so Qt=K(πr1r2)(θ1−θ2)L${\displaystyle \frac{Q}{t}}={\displaystyle \frac{K(\pi r_1{r}_2)({\theta }_1-{\theta }_2)}{L}}$

Definition

Good and Bad conductors of heat

Most metals are good conductors of heat; silver and copper are exceptionally good. On the other hand, substances such as cork, wood, cotton and wool are bad conductors. Both good and bad conductors have their uses. The bit of a soldering iron is made of copper, so that when its tip is cooled through contact with the work, heat is rapidly conducted from the body of the bit to restore the temperature of the tip and maintain it above the melting-point of solder.
 
Bad conductors have a very wide application. Beginning with our own personal comfort, we prevent loss of heat from ourselves by covering of poorly conducting material. Textiles are bad conductors of heat, since they are full of tiny pockets of air enclosed by the fibres of the material. Air, in common with all gases, is a very bad conductor of heat. It is usual to say that wool is warmer than cotton. Technically, of course, we imply that it has a lower thermal conductivity than cotton. Double glazing in windows reduces heat loss from a building owing to the poorly conducting layer of air between the two sheets of glass. 

Definition

Heat conduction using Kinetic model

Thermal conduction is the transfer of heat (internal energy) by microscopic collisions of particles and movement of electrons within a body. The microscopically colliding objects, that include molecules, atoms, and electrons, transfer disorganized microscopic kinetic and potential energy.In solids, conduction is mediated by the combination of vibrations and collisions of molecules, of propagation and collisions of phonons, and of diffusion and collisions of free electrons. In gases and liquids, conduction is due to the collisions and diffusion of molecules during their random motion.

Definition

Convection

Convection is the mode of heat transfer in fluids, i.e., liquids and gases. Water gets heated in spite of being a poor conductor of heat due to convection. Water at the bottom of the pot gets heated when put on a flame. Hot water being lighter moves up and cold water moves down. The process of mixing continues until the whole mass of water has a constant temperature.

Land and sea breezes are caused due to convection currents in air. Water in a pot boils also due to convection.

Example

Uses of convection

The following are the uses of convection:

1. Car engines are cooled by convection currents in the water pipes. Water is a very good substance to carry the unwanted heat away from the engine to the radiator.
2. Land and sea breezes are caused due to convection currents.
3. Rising air over the land are convection currents and are used by glider pilots to keep their gliders in the sky.
4. Air conditioners are installed near the ceiling of the room, to allow the setting up of convection currents. The air-conditioner releases cool dry air into the room. As cool air is denser, it sinks. The warm air, being less dense, will rise. The air circulated and the temperature of the air will eventually fall to the desired value.

Result

Direction of heat flow

Heat flows from a body at higher temperature to a body at lower temperature.

Definition

Heat insulation in a house

Thermal insulation in buildings is an important factor that reduces unwanted heat loss or gain by the usage  insulation materials employed to heat loss, such as: cellulose, glass wool, rock wool, polystyrene, urethane foam, vermiculite, perlite, wood fibre, plant fibre (cannabis, flax, cotton, cork, etc.), recycled cotton denim, plant straw, animal fibre (sheep's wool), cement, and earth or soil. It can decrease the energy demands of heating and cooling systems. Many of the materials mentioned above deal with heat conduction and convection by the simple expedient of trapping large amounts of air (or other gas) in a way that results in a material that employs the low thermal conductivity of small pockets of gas, rather than the much higher conductivity of typical solids.

Definition

Radiation

Radiation is the process of transfer of heat from a hot body to a cold body directly without heating the space between the two bodies. This process of heat transfer does not require any medium. The energy so radiated is called radiated energy.
The sun gives light and also heat. Heat transferred by the sun is via radiation. During cold winter nights people gather around a fire to keep warm. This heat is transferred by radiation.

Definition

Properties of thermal radiation

• Thermal radiation once radiated behaves like an electromagnetic wave.
• It also obeys the laws of optics on account of its wavelength being comparatively small to obstacles.
• The radiation emitted by a body is a mixture of waves off different wavelengths. However, only a small range of wavelength has significant contribution to the total radiation. 
• The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases.

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