NEET Physics Notes Mechanics-Transfer of heat-Calorimetry
The measurement of heat exchanges between two or more bodies is termed as calorimetry. When two or more bodies at different temperatures and/or in different states come in thermal contact, then heat exchange between different bodies takes place in order to attain thermal equilibrium.
According to the principle of calorimetry, heat supplied by few of the bodies = heat consumed by the remaining bodies, i.e. heat lost = heat gained If temperature changes, heat exchanged is given by
As temperature of the body increases, it means heat is taken by the body, otherwise given out by the body.
In case of phase change, heat is consumed during melting and boiling while released during freezing and condensation.
The heat required to change the phase of a system is proportional to the mass of the system i.e.
where, L is the latent heat, which is defined as the amount of heat required to change the phase of a substance at given temperature.
Vaporisation of Liquids
When liquid phase is converted to gaseous phase at boiling point. The process is termed as vaporisation. Latent heat in the vaporisation process is termed as latent heat of vaporisation Lv.
where, Q is the amount of heat required to vaporise a liquid of mass m.
It is a reversible process just like melting. Reverse of the vaporisation process is termed as condensation.
A substance can sometimes change directly from solid to gaseous phase, this process is termed as sublimation. Corresponding latent heat is termed as latent heat of sublimation, Ls. The reverse process can also occur.
- Very pure water can be cooled several degrees
below the freezing temperature without freezing, the resulting unstable state is described as supercooled. When this supercooled water is disturbed (either by dropping dust particles etc), it crystallises within a second or less. ‘
- A liquid can sometimes be superheated above its normal boiling temperature. Any small disturbance such as agitation causes local boiling with bubble formation.
Water Equivalent of a Substance
Water equivalent of certain amount of substance is defined as the amount of water, which when replaced by the substance requires the same amount of heat for the same rise in temperature