35. The first law of thermodynamics

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. The first law is often formulated by stating that the change in theinternal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. Equivalently, perpetual motion machines of the first kind are impossible.

36. Internal energy of thermodynamic system

Internal energy - adopted in continuum physics, thermodynamics and statistical physics, the name for that part of the total energy of a thermodynamic system, which does not depend on the choice of reference and that in the framework of the problem, may change. That is, for equilibrium processes in the frame, relative to which the center of mass is considered a macroscopic object is at rest, and the change in the total internal energy is always the same. List the components of the total energy entering into the internal energy is not constant and depends on the task at hand. In other words, the internal energy - it is not a specific type of energy , and the set of the variable components of the total energy that should be considered in a particular situation.

The internal energy U of a given state of the system is determined relative to that of a standard state of the system, by adding up the macroscopic transfers of energy that accompany a change of state from the reference state to the given state:

37. Give definition of work. Work in isoprocess and cycles.

In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.

In thermodynamics, work performed by a system is the energy transferred by the system to another that is accounted for by changes in the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in physics.

Working gas in the thermodynamic processes is the integral of pressure over the volume. This immediately follows. that a prerequisite for doing work is a change in the volume:

1) isochoric process: A = 0.

2) isobaric process: A = p (V2-V1).

3) an isothermal process: A = pVln (V2 / V1).

4) The adiabatic (isentropic) Process: A = p1V1 / (k-1) (1- (V1 / v2) ^ (k-1)), where k = Cp / Cv - adiabatic index.

38.First law of thermodynamics to isoprocess.

The isochoric process (V = const) does not make gas work, A = 0. Therefore,

Q = ΔU = U (T2) - U (T1).

Here, U (T1) and U (T2) - the internal energy of the gas in the initial and final states. The internal energy of an ideal gas depends only on the temperature (Joule). At isochoric heating heat absorbed by the gas (Q> 0), and its internal energy increases. Upon cooling, the heat is given to external bodies (Q <0).

The isobaric process (p = const) the work done by the gas, expressed by the ratio

A = p (V2 - V1) = p ΔV.

The first law of thermodynamics for isobaric process provides:

Q = U (T2) - U (T1) + p (V2 - V1) = ΔU + p ΔV.

With isobaric expansion Q> 0 - heat absorbed by the gas, and the gas performs work. With isobaric compression Q <0 - heat given external bodies. In this case A <0. The temperature of the gas at reduced isobaric compression, T2 <T1; internal energy decreases, ΔU <0.

The isothermal process gas temperature does not change and is not changed and the internal energy of the gas, ΔU = 0.

The first law of thermodynamics for isothermal process is expressed by Q = A.