# How to find internal energy

Internal energy of a body consists of set of kinetic and potential energy of molecules of a body. The person has no devices capable to measure this size directly. He can only calculate it, knowing the body weight and its temperature.

## It is required to you

• thermometer, scales, Mendeleyev's table.

## Instruction

1. As to find potential energy of interaction of molecules of a body very difficult, authentically it is possible to calculate this size only for gas in which molecules practically do not interact, and it means that the potential energy of their interaction is equal to zero.

2. Define a chemical formula of gas which internal energy you measure. After that according to the periodic table of Mendeleyev find his molar weight. For this purpose find the mass of all atoms of which the gas molecule consists in cells of the corresponding elements. Put the received mass of atoms - the mass of a molecule which is in number equal to the molar mass of substance in grams on mol will be result. After that measure the mass of gas. For this purpose cool or heat it to normal conditions (0ºС with a pressure of 760 mm Hg), measure its volume which is equal to the volume of a vessel or the room in which it is also density according to the special table then receive value of weight, having increased gas density by its volume.

3. If it cannot be made, take a tight cylinder, extort from it all gas and find its weight on scales. Then begin to rock in it some mass of gas and again weigh. The difference of an empty and full cylinder will also be equal to the mass of gas. In all cases you take measurement of weight in grams. Take gas temperature by means of the thermometer. As the majority of thermometers has scales in degrees Celsius, transfer its value to kelvins. For this purpose to the received result add number 273.

4. To receive value of internal energy of gas the mass of gas divide into molar weight, increase result by number 8.31 (universal gas constant), temperature of gas and divide into 2. If a gas molecule one-atomic, increase result by 3 if diatomic - on 5 if triatomic - on 6. Internal energy of gas in Joules will be result.

Author: «MirrorInfo» Dream Team