Transfer of heat, takes place from high temperature to low temperature. There are three modes of heat transfer: (i) conduction, (ii) convection, and (iii) radiation. In solids, conduction is common. In liquids and gases, convection takes place. For radiation, no medium is required.
Conduction:
In conduction heat is transferred without actual movement of molecules. The molecules that are in contact with the heat sources, take heat from the source directly and transfer it to the surrounding molecules by increasing the amplitude of their vibrations during collision between the adjacent atoms. So, region of increasing temperature extends it along the length.
Assume a slab of thickness x and cross-sectional
area A. Let the temperature of its two faces be T1 and T2, so that T1 > T2. The quantity of heat (∆Q) transmitted from hot face (with temp. T1) through cold face in time ∆t is directly proportional to time (∆t), the area of cross-section (A) and the difference in temperature (T1 - T2); and inversely proportional to thickness (x).
\(\Rightarrow\ \Delta Q\propto\frac{A(T_1 - T_2)}{x}\Delta t\)
or, \(\Delta Q = \frac{KA(T_1 - T_2)\Delta\ t}{x}\)
where K is constant of propotionality and is called coefficient of thermal conductivity.
Thermal current: The flow of rate of heat is known as thermal current.
\(H = \frac{\Delta Q}{\Delta t}\)
\(H = \frac{KA(T_1 - T_2)\ \Delta t}{\Delta t\ x}\)
\(H = \frac{KA\Delta T}{x}\)
The term \(\frac{T_1 - T_2}{x} = \frac{\Delta T}{x}\) is called temperature gradient. Eqn (16) is called Fourier's law and it defines as the coefficient of thermal conductivity K as:
Hence the coefficient of thermal conductivity is equal to the amount of heat when the temperature gradient, area of cross section and time are take as unity.
The unit of K is \(\frac{Watt}{meter\times kelvin}\)
Thermal resistance: Thermal resistance is defined as the temperature difference per unit thermal current:
Convection:
The liquids and gases transmit large fraction of heat by a process in which a part of the hot medium moves from one place to another place. This type of transfer of heat is called convection. Within the hot medium, the movement takes place in the form of currents. Hot current of gas or liquid moves upwards, and its place is taken by cold current of gas or liquid (it moves downwards).
Two Kinds of Convection
1. Forced Convection: Convection in which heated substance is forced to move.
2. Natural Convection: Convection in which heated substance moves without any help of external agency, because of the difference in density of hot and
cold material.
The rate of flow of heat due to convection is,
H = hA(Tsurface - Tfluid)
Here A → area
(Tsurface - Tfluid) → Temperature difference
h → Convective heat transfer constant.
Fluids (liquids and gases) are heated mainly by the process of convection in which buoyancy and gravity play on important role. As shown in figure, when a fluid is heated from below, the hot position at the bottom expands and becomes less dense. Because of buoyancy, this lighter portion rises up. The denser colder fluid takes its place by moving downwards. Thus convection current is set up in the fluid. The actual movement of a liquid can be seen by colouring the liquid with potassium permanigarate crystals placed at the bottom of the vessel.
1. In maintaining comfortable room temperature in cold countries: In cold during winter, the outside temperature is much below 0°C while the room temperature is comfortably maintained around 20°C. However, the inside air close to the glass window is cooler than 20°C while the outside close to the window is warmer than the chilling temperature of the atmosphere. Thus heat is continuously transferred from the room to the outside by convection of air inside the room. Conduction across the glass pane and again convection of air outside. But this heat loss is compensated by the heating system provided in the room.
These are local convection currents. Specific heat of water is higher than that of soil. So land and breeze air above it is heated faster in summer during day time than air above the sea. The air above land expands and rises and its place is taken up by the colder air from sea to land and is called sea breeze. At night the land gets cooled faster than water. So, colder air flows from land to sea and is called land breeze.
Radiation:
The process of the transfer of heat from one place to another place without heating the intervening medium is called radiation. Heat from the sun reaches the earth through radiation mode.
Thermal radiation or heat radiation: Everybody emits energy in the form of waves (i.e. electromagnetic waves) due to its temperature. These waves are known as thermal radiation or heat radiation.
When a body emits heat radiation it loses heat. On the other hand, the body gains heat whenever it absorbs thermal radiation. Thermal radiation have wavelength ranging from 8 x 10-7 m to 4 x 10-4 m. This wavelength range lies in the infra-red region of electromagnetiic spectrum. Therefore, thermal radiations are also known as infra-red radiations.
According to Prevost all bodies whose temperature is above 0 K or -273°C emit thermal radiations continuously.
Thermal radiations require no material medium for their propogation, so they can travel in vacuum also. They travel with speed of light. They travel in straight lines. They can be reflected and refracted like light waves. They do not affect the intervening medium through which they pass.
