Basic Concepts of Thermodynamics

1 Introduction to State, Work, Heat, Energy, Temperature

Thermodynamics is the science relating heat and work and transfer energy in the working substance. The working substance is isolated from its surrounding in order to determine its properties.

  • System is a collection of matter within prescribed and identified boundary. A system may be open, close or isolated referring to mass transfer takes place or does not takes place across the boundary.
  • Surrounding is usually restricted to those particle of matter external to the system which may be affected by changes in the system and surrounding themselves may form another system.
  • Boundary is a physical or imaginary surface enveloping the system and separating it from the surrounding.
  • Internal energy is the energy of the system covering all the energies arising from the internal structure of the system.
  • Enthalpy is the property of a system conveniently defined as h=u+PV where u is the internal energy.
  • Entropy is the microscopic disorder of the system and depends upon temperature change. As the temperature increase it increases and with the decrease of temperature it decreases.

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

 

The zeroth law of thermodynamics states that if two separatethermodynamic systems are each in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.

If a body A, be in thermal equilibrium with two other bodies, B and C, then B and C are in thermal equilibrium with one another.

The 1st Law of Thermodyamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing.

Examples of  energy conversion processes.

Automobile Engine

Chemical � Kinetic

Heater/Furnace

Chemical � Heat

System and Surroundings

The 1st Law of Thermodynamics tells us that energy is neither created nor destroyed, thus the energy of the universe is a constant. However, energy can certainly be transferred from one part of the universe to another. To work out thermodynamic problems we will need to isolate a certain portion of the universe (the system) from the remainder of the universe (the surroundings).

DE = q + w (1st Law of Thermodynamics)

  • DE = The change internal energy of the system,
  • q =The heat transferred into/out of the system,
  • w = The work done by/on the system.

Internal Energy

We have already discussed work and heat extensively, but a few comments are in order regarding internal energy. The internal energy encompasses many different things, including:

  • The kinetic energy associated with the motions of the atoms,
  • The potential energy stored in the chemical bonds of the molecules,
  • The gravitational energy of the system.

It is nearly impossible to sum all of these contributions up to determine the absolute energy of the system. That is why we only worry about DE, the change in the energy of the system. This saves all of us a lot of work, for example:

  • if the temperature doesn’t change we can ignore the kinetic energy of the atoms,
  • if no bonds are broken or destroyed we can ignore the chemical energy of the system,
  • if the height of the system doesn’t change then we can ignore gravitational potential energy of the system.

Our convention for DE is to subtract the initial energy of the system from the final energy of the system.

DE = E(final) – E(initial) = q + w

In a chemical reaction the energy of the reactants is E(initial) and the heat of the products is E(final).

 

Sign Convention

When working numerical problems we will quickly become confused if we don’t adopt a universal convention for when we use a positive sign or a negative sign.

Sign Convention for heat, q

  • Heat is transferred into the system  q > 0
  • Heat is transferred out of the system  q < 0

Sign Convention for work, w

  • Work is done upon the system by the surroundings  w > 0
  • Work is done by the system on the surroundings  w < 0

Lets look at some processes to get a better feel for defining a thermodynamic system and using the proper sign convention.

Example

Hold a piece of ice in your hand until it melts

Solution A

  • System � You
  • Surroundings � Ice + the rest of the universe
  • q < 0 � Heat flows out of the system (you) into the ice.

Solution B

  • System � Ice
  • Surroundings � You + the rest of the universe
  • q > 0 � Heat flows into the system (ice) from you.

You can see that the answer changes depending upon how you define the system, but the physical reality is exactly the same, but both solutions A and B are correct. It doesn’t matter how you define the system as long as you are consistent.