2. Heat conduction process
• Heat conduction (or thermal conduction) is the movement of heat from one object to
another one that has different temperature when they are touching each other.
• Conduction is the process by which heat energy is transmitted through collisions between
neighboring atoms or molecules. Conduction occurs more readily in solids and liquids,
where the particles are closer together than in gases, where particles are further apart.
The rate of energy transfer by conduction is higher when there is a large temperature
difference between the substances that are in contact.
• The process of heat conduction depends on four basic factors:
1)temperature gradient
2)the cross section of the materials involved,
3)their path length, and
4)the properties of those materials.
3. Factor affecting heat conduction
1)Temperature gradient:- It is a physical quantity that describes in which direction and at what rate
the temperature changes in a specific location. Temperature always flows from the hottest to
coldest source, due to the fact that cold is nothing but the absence of heat energy. This transfer
between bodies continues until the temperature difference decays, and a state known as thermal
equilibrium occurs.
2)Cross-section and path length are also important factors:-The greater the size of the material
involved in the transfer, the more heat is needed to warm it. Also, the more surface area that is
exposed to open air, the greater likelihood for heat loss. So shorter objects with a smaller cross-
section are the best means of minimizing the loss of heat energy.
4. 3)Conduction on material properties
• These conductive properties are rated based on a “coefficient” which is measured relative to
silver. In this respect, silver has a coefficient of heat conduction of 100, whereas other materials
are ranked lower. These include copper (92), iron (11), water (0.12), and wood (0.03). At the
opposite end of the spectrum is a perfect vacuum, which is incapable of conducting heat, and is
therefore ranked at zero.
• Materials that are poor conductors of heat are called insulators. Air, which has a conduction
coefficient of .006, is an exceptional insulator because it is capable of being contained within an
enclosed space. This is why artificial insulators make use of air compartments, such as double-
pane glass windows which are used for cutting heating bills. Basically, they act as buffers against
heat loss.
Conduction, as
demonstrated by heating a
metal rod with a flame.
5. Heat transfer process in atmosphere
• Conduction, radiation, and convection
all play a role in moving heat between
Earth's surface and the atmosphere.
Since air is a poor conductor, most
energy transfer by conduction occurs
right near Earth's surface. Conduction
directly affects air temperature only a
few centimeters into the atmosphere
6. Law of heat conduction
• The law of heat conduction, also known as Fourier's law, means that the rate,
in time, of heat transfer through a material is proportional to the
negative gradient in the temperature and to the area at right angles, to that
gradient, through which the heat is flowing:
where:
Q is the amount of heat transferred, and
t is the time taken, and
k is the material's thermal conductivity' and
S is the area through which the heat is flowing, and
T is the temperature.
Thermal conductivity usually varies with temperature, but
the variation can be small, over a significant range of
temperatures, for some common materials
7. • General Heat Conduction Equation
• The heat conduction equation is a partial differential equation that describes the distribution of heat (or
the temperature field) in a given body over time. Detailed knowledge of the temperature field is very
important in thermal conduction through materials. Once this temperature distribution is known,
the conduction heat flux at any point in the material or on its surface may be computed from Fourier’s law.
• The heat equation is derived from Fourier’s law and conservation of energy. The Fourier’s law states that
the time rate of heat transfer through a material is proportional to the negative gradient in the
temperature and to the area, at right angles to that gradient, through which the heat flows.
This equation is also known as the Fourier-Biot equation, and provides the basic tool for heat
conduction analysis. From its solution, we can obtain the temperature field as a function of time.
