National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”

Faculty of Physics and Mathematics

Physics 1.

Developers of the work program: Shtofel Olha, assistant of the Department of General Physics and Solid State Physics

PV = ν RT=

CHAPTER 2. MOLECULAR PHYSICS AND THERMODYNAMICS

Work and amount of heat The internal energy of the system can be changed, work is

performed on the system by external forces, or by heat transfer.

Work is performed on the system and is accompanied by moving external bodies (for example, a piston in a cylinder with gas)

The ‘-’ sign in the equation indicates that work performed over the gas by external bodies is negative. Work, which performs gas over external bodies is equal in modulus and opposite in sign.

To calculate the total work of gas on external bodies it is necessary to integrate the expression

Graphically, the work is defined as the area of the figure under the graph of the function in the coordinates p, V

The work is not a function of the system, but depends on the form of transition of the system from one position to another.

The transfer of heat to the macrosystem is not related to the movement of external bodies. The amount of heat given to the system, as well as the work depends from how this system passes from the initial to the final state. It is considered that the amount of heat provided to the system is positive and that taken into the system is negative.

Heat capacity of an ideal gas

The heat capacity of the body (gas) is the amount of heat that must be provided to the gas to increase its temperature by one kelvin:

Gas can have different masses, and therefore different number of molecules, then we will use specific and molar heat capacities:

The heat capacity of an ideal gas also depends on the process of transition of the system from one state to another.

The first law of thermodynamics

It is possible to change the state of the system in two ways: either to perform work on the system, or to provide (take away) heat from it. In the general case, the transition of the system from one state to another is associated with both the performance of work on the system and the provision of heat to the system.

The law of conservation of energy it is formulated taking into account the form of energy transfer in the form of

heat transfer, is a fundamental law of physics and is called the first law of thermodynamics:

The amount of heat provided to the system (Q) goes to the increase of internal energy (ΔU) and to the performance of the system work on external bodies (A):

All quantities included in the equation are algebraic, ie they can be both positive and negative. If Q

We can talk about the increase in internal energy ΔU, but we can not talk about the increase in work and heat.

Heat is provided to the system, and work on the system is performed. In differential form, the first law of thermodynamics is as follows:

If we consider the first law of thermodynamics for each isoprocess, we will have the following relations