NEET Physics Notes Mechanics-Transfer of heat-Heat Transfer
The heat can be transferred within the body and from one body to the other body, through the following modes
Conduction of heat is that mode of heat transfer in which molecules of the body transfer heat from a place at higher temperature to a place at a lower temperature without actual transfer of the material. Particles of the body simply oscillate but do not move from their place. Heat is transferred through the coussions of bounded molecules with each other.
The amount of heat transmitted through a conductor is given by
Formation and Growth of Ice on a Lake
Time required for the thickness of the layer of ice to increase from dj to d2 will be
According to the widemann-Franz law, the ratio of thermal and electrical conductivities is the same for the metals at a particular temperature and is proportional to the absolute temperature of the metal.
Convection is the mode of heat transfer by actual motion of the matter (or particles of matter). Convection is not possible in solids and can take place in fluids only.
In natural convection gravity plays an important role. When a fluid is heated, the hot part expands and becomes less dense. Consequently it rises and the upper colder part replaces it. This again gets hot, rises up and is replaced by the colder part of the fluid. In this way, heat is transferred from the bottom to top.
In a forced convection the material is forced to move up by a pump or by some other physical means. Common examples of forced convection are human circulatory system, cooling system of an automobile engine and forced air heating system in offices, etc.
Radiation is that mode of heat transfer which needs no medium. Radiant energy travels in the form of electromagnetic waves with a speed of 3 x 108 ms-1 in free space.
When radiant energy Q is incident on a body, a part of it Qa is absorbed, another part Qr is reflected back and yet another part Qt is transmitted such that