Heat is transferred from one body to another in three possible ways, CONDUCTION, CONVECTION, and RADIATION.
Conduction:
CONDUCTION (more accurately, thermal conduction) is the flow of heat from a hot part of a body to a cooler part, without transfer of matter.
Conduction can also take place from one body to another, provided the two bodies are in contact, and a temperature difference exists between them. For example, a pot on a hot plate is heated by conduction from the stove surface, via the underside of the pot.
Conduction is the energy transfer from the more energetic to the less energetic particles of a substance due to interaction between them, a microscopic activity.
Convection:
CONVECTION of heat takes place in fluids (i.e., liquids and gases), and involves the movement of matter from hot regions to cooler regions. This takes place because the hot regions of the fluid are less dense than the cooler regions, and so will tend to rise. As the warm fluid rises, it is replaced by cooler liquid from above. A so-called convection current is set up.
In the photograph on the right, convection causes the warm regions of the liquid, coloured with a dye, to rise towards the top.
Convection of air masses is responsible for weather. We also use the principle of convection in the heaters that warm our homes.
Convection is the energy transfer due to random molecular motion a long with the macroscopic motion of the fluid particles.
Radiation:
RADIATION of heat is the transfer of heat energy in the form of waves in the infrared region of the electromagnetic spectrum. This process can take place in a vacuum, and is in fact the way in which heat from the sun reaches the earth across 150 million km of empty space. All objects above absolute zero radiate heat energy to a greater or lesser extent.
Special equipment enables one to visualize the heat energy radiated by objects. This is called THERMAL IMAGING. The picture on the left is that of a warship photographed with equipment sensitive to infrared radiation.
The amount of heat radiated onto unit area obeys the inverse square law, so, the further an object is from the source of heat, the less the heat which is available on a unit area.
It is the energy emitted by matter which is at finite temperature. All forms of matter emit radiation attributed to changes m the electron configuration of the constituent atoms or molecules. The transfer of energy by conduction and convection requires the presence of a material medium whereas radiation does not. In fact radiation transfer is most efficient in vacuum.
Thermal conductivity:
The rate at which heat is transferred in a body (Q/t) is proportional to the cross-sectional area, A, of the body, and the temperature gradient (ΔT/L) along the direction of heat flow (L is the length of the body in the direction of heat flow, and ΔT is the temperature difference between the ends of the body):
Q/t = κA ΔT/L
The constant κ (Greek letter kappa) is the Thermal conductivity of the material: κ = QL/AtΔT
The units of thermal conductivity are J.s-1.m-1.K-1.2
The inverse square law:
Consider a heat source which radiates heat onto a 1 m2 square target placed perpendicularly 1 m away from the source.
Let the quantity of heat falling on the surface in unit time be Q J.s-1.
If the target is placed 2 m away from the source, the quantity of heat falling on it in unit time will be ¼Q J.s-1.
In general, if the target is placed a distance d from the source, the quantity of heat radiated onto it per second will be Q/d2 J.s-1.2
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