## Heat and temperature

Heat and temperature are related as:

### Heat

Heat is a form of energy, which produces in us the sensation of warmth. The thermal energy in matter is present in the form of translation, rotational and vibration energy of its atoms/molecules.

### Temperature

Temperature of a body is a measure of degree of hotness/coldness of the body. This macroscopic property determines the direction of flow of heat, when the given body is placed in contact with some other body. Heat flows from a body at higher temperature to a body at lower temperature the flow continues till temperatures of the two bodies become equal. The state of two bodies/systems in which their temperatures become equal and consequently no net heat flows between them is called the state of thermal equilibrium. In this state, all the parts of the system are at the same temperature.

### Zero of the scale of temperature

It is taken as
1) Melting point of ice, when temperatures are measured in ° C.
2) Triple point of water when temps are measured on Kelvin scale (K). Triple point of water is the temperature at which three phases of water (ice, water and vapours) coexist in the thermal equilibrium. Its value is 273.16K.

### Latent heat

Latent heat of a substance is the amount of heat required to change the state of unit mass of the substance without changing its temperature. Its unit is cal/g or joule/kg. Latent heat of fusion of ice = 80 cal./g. Latent heat of vaporisation of water = 80 cal./g. Note that a solid melts into liquid only at melting point of the solid. Again, a liquid changes into vapours only at boiling point of the liquid. There is no change in temperature whatsoever during the change of state of entire substance.

### Specific heat

Specific heat of any substance is the amount of heat required to raise the temperature of one gram of the substance through 1 °C. It is measured in cal/g/°C. The specific heat of an ideal /perfect gas does not depend upon temperature. By definition, specific heat of water is 1 cal/g/°C or 4.2 joule/g/°C or 4200j/kg/°C.

### Two principle specific heats of a gas

1) Specific heat at constant volume= amount of heat required to raise the temperature of 1gram of gas through 1°C at constant volume.
2) Specific heat at constant pressure= amount of heat required to raise the temperature of 1gram of gas through 1°C at constant pressure.

### Internal Energy of the gas

The energy possessed by the molecules of a gas by virtue of their particular configuration and molecular is called an internal energy of the gas. It is of two types.
1) Internal potential energy- Which is due to molecular configuration i.e. due to mutual interaction of atoms/molecules.
2) Internal kinetic energy : Which is due to motion of the molecules of the gas. Thus U=Up+Uk Internal energy of gas depends upon on volume of the gas and also on temperature of the gas. In an ideal gas, where intermolecular forces are assumed to be zero, Up=0. U=Uk. Therefore, internal energy of an ideal gas depends only on the temperature of the gas.