NEET Physics Notes Mechanics-Properties of Matter-Heat
Heat is a form of energy which characterises the thermal state of matter. It is transferred from one body to the other due to temperature difference between them.
Heat is a scalar quantity with dimensions [ML2T~2] and SI unit is joule (J) while practical unit is calorie (cal)
1 cal = 4.18 J
If mechanical energy (work) is converted into heat then, the ratio of work done (W) to heat produced (Q) always remains the same and constant.
The constant J is called mechanical equivalent of heat.
Heat is not a state function and thus it is path dependent and is taken to be positive, if the system absorbs it and negative, if it releases it.
- The devices which are used to measure the temperature are termed as thermometers, while the science related to measurement of temperature is termed as thermometry.
- For construction of a thermometer, two fixed reference points are chosen : ice point (equilibrium temperature of a mixture of ice and water at one atmospheric pressure) and steam point (equilibrium temperature of mixture of steam and water at one atmospheric pressure).
Three most common scales are Celsius scale or Centigrade scale, Fahrenheit scale and Kelvin scale (Absolute scale).
When we supply energy to a body, then its temperature may rise. This increases the molecular kinetic energy which results in an increase in the intermolecular separation and hence, the body expands which we call as thermal expansion. As the intermolecular forces are weakest in gases, expansion is maximum in gases