Topic 10.2 Thermodynamic Systems and Concepts Thermal Physics Topic 10.2 Thermodynamic Systems and Concepts

Thermodynamics Is the name given to the study of processes in which thermal energy is transferred as heat and as work. It had its foundations with engineers in the 19th century who wanted to know what were the limitations of the Laws of physics with regard to the operation of steam engines and other machines that generate mechanical energy.

Thermodynamics Treats thermal energy from the macroscopic point of view in that it deals with the thermodynamic variables pressure, volume, temperature And change in internal energy in determining the state of a system.

Heat Can be transferred between a system and its environment because of a temperature difference. Another way of transferring energy between a system and its environment is to do work on the system or allow work to be done by the system on the surroundings.

Definitions In order to distinguish between heat and work in thermodynamic processes Heat is defined as a process in which thermal energy is transferred due to a temperature difference. Work is defined as the process in which thermal energy is transferred by means that are independent of a temperature difference.

System In thermodynamics the word system is used often. A system is any object or sets of objects that is/are being investigated. The surroundings will then be everything in the Universe apart from the system.

For example When a volume of gas in a cylinder is compressed with a piston Then the system is the cylinder-gas-piston apparatus And the surroundings is everything else in the Universe.

A closed system Is one in which no mass enters or leaves the system. It is an isolated system if no energy of any kind enters or leaves the system. Most systems are open systems because of the natural dynamic processes that occur in the Universe.

Work Done by a Gas Consider a mass of gas with pressure p enclosed in a cylinder by a piston of cross-sectional area A The pressure, p, on the piston = force per unit area Therefore, the force on the piston, F, is given by F =pA

So that Suppose the piston is moved a distance l when the gas expands. Normally, if the gas expands, the volume increases and the pressure decreases, as was determined from Boyle's Law for ideal gas.

Expansion of a Gas at Constant Pressure Surface area, A Pressure, p cylinder l

However If the distance l is small l, then the pressure can be considered constant. If the pressure is constant then the force F will be constant.

And The work done by the gas is W = Fl but since pressure F = pA W = pA l but since volume V = A l  W = pV

That is (work done / J) = (pressure / Nm-2) x (volume change / m3) So that,  W = p V = p(V2-Vl) The sign of the work done by the gas depends on whether volume change is positive or negative.

Explanation When a gas expands, then work is done by the gas, and the volume increases. As V is positive, then W is positive.

More This equation is also valid if the gas is compressed. In the compression, work is done on the gas and the volume is decreased. Therefore, V is negative which means that W will be negative. From the first law of thermodynamics this means that positive work is done on the gas.

Internal Energy Internal energy U was defined as the sum total of the potential energy and kinetic energy of the particles making up the system. From a microscopic viewpoint, the internal energy of an ideal gas is due to the kinetic energy of the thermal motion of its molecules.

Internal Energy of an Ideal Gas There are no intermolecular forces and thus there cannot be any increase in potential energy. Therefore a change in the temperature of the gas will change the internal energy of the gas.

More From the macroscopic point of view of thermodynamics, one would expect that the internal energy of the system would be changed if: work is done on the system work is done by the system thermal energy is added to the system thermal energy is removed from the system

Internal Energy is a property of the system that depends on the "state" of the system. (In thermodynamics, a system is said to have changed "state" if some macroscopic property of the system has changed e.g. phase, temperature, pressure, volume, mass, internal energy).

What can change a system Heat and work can change the state of the system but they are not a property of the system. They are not characteristic of the state itself but rather they are involved in the thermodynamic process that can change the system from one state to another.

The Absolute Value The absolute value for internal energy is not known. This does not cause a problem as one is mainly concerned with changes in internal energy Denoted by U, in thermodynamic processes