# As internal energy of ideal gas changes

Change of internal energy of ideal gas is a basis of the first beginning of thermodynamics. This postulate states two main possible ways of change of internal energy.

## It is required to you

• The textbook in physics, a ball pen, the sheet of paper.

## Instruction

1. Read in the textbook in physics of the tenth class a formulation of the first beginning of thermodynamics. It is known that it defines ways of change of internal energy of ideal gas both in case of an open system, and in case of the closed system. Under this law, the amount of heat which is failed to a thermodynamic system leads to change of its internal energy and to commission by the system of work against external forces.

2. Pay attention that the first beginning of thermodynamics can be treated differently, having moved composed commission of work in the left member of equation. In this case on the one side of the equation there will be a change of internal energy, and with another – the difference of the amount of heat transferred to a system and the perfect system of work. Thus, this equation says that it is possible to change internal energy in two ways. The first way consists in transmissions of energy to a system from the outside, and the second – in commission by the system of work.

3. Write down the received ratio of the first beginning of thermodynamics. Notice that the member designating commission by the system of work is faced by a minus sign. It means that when the system itself makes work against external forces, that is positive work, internal energy of a system decreases. It would be quite fair to put a plus before the member of change of work, but then it would need to be treated differently, namely as commission of work as the system. That is work in this case has to go not against external forces, and at the expense of them. Then internal energy of a body increases. It corresponds to a case when, for example, gas is compressed by means of the piston. In this experiment if adiabatically to isolate gas, perfect work completely will leave on change of internal energy of gas.

4. Do not forget that the above ways of change of internal energy belong only to a case of the closed isolated systems. In case the system open, internal energy can change also due to change of quantity of particles of substance. Each particle of gas or liquid introduces own contribution to the general energy of all substance. Respectively, loss of a particle means loss of a part of internal energy. In this case the first beginning of thermodynamics is modified by emergence of additional substance of a system composed, proportional to change of particles and to its chemical potential expressing the internal energy falling on one particle.

Author: «MirrorInfo» Dream Team