Work, Heat, Energy. and the First Law
|Introduction||What is this thing called Thermodynamics??? | Definitions | Thermal Equilibrium and Zeroth Law | Limitations|
|First Law||Work, Heat, Energy, and the First Law | Work, Heat, Energy, and the First Law (simplied) | Derivatives | Derivatives Exercise | Reversibility, Enthalpy, and Heat Capacity|
|Second Law||Things to Think About | Observations and Second Law of Thermodynamics | Alternative Approach - the Clausis Inequality | Consequences of the Second Law | Consequences of the Second Law (simplified) | Carnot Principle - motivation and examples | Equivalence of Second Law Statements*|
|Third Law||Third Law of Thermodynamics | Consequences of Third Law*|
|Development of Thermodynamics||The Thermodynamic Network | Network Exercise | Equations of State | Thermochemistry|
* Optional Section
- Force acting through a distance
Therefore, Work, W is
where F is force and dl is the distance through which the force acts The reason for the minus sign is explained below.
In most applications of thermodynamics we are mainly interested in mechanical work due to pressure of a fluid. Since pressure is force per unit area, force is simply pressure times area:
If we consider a volume, V, then the distance l is
If we then assume a constant area then we can take the A inside the differential and:
- Capacity to do work
- Energy transferred due to a temperature difference
- No heat transfer between a system and its surroundings
- Exothermic process
- A process which releases heat
- Endothermic process
- A process which adsorbs heat
Heat is denoted by the symbol, Q
Heat and work are not properties
It is important to note that heat and work are not intrinsic properties of a system. They refer only to energy which is transferred. We cannot say, for example, that a brick has 15 J of heat. It may however, have 15 J of energy.
Sign convention and notation
Δ is used to indicate finite change (for example, ΔU)
d is used to indicate differential change (for example, dU)
However, we do not use ΔQ or ΔW for finite changes in heat or work, since Q and W only refer to change. We but simply use just Q or W. We still do use dQ and dW for differential change.
The laws of thermodynamics are based on observations of the natural world. The first law is based on two observations concerning energy:
- Energy can be transferred between a system and its surroundings by only two ways: work and heat
- The total energy of a system and its surroundings is always constant (The conservation of energy)
These two observations can be combined into the First Law of Thermodynamics:
The internal energy of a system is constant unless changed by doing work or by heating
Mathematically, the change in internal energy is the sum of the work and heat entering or leaving the system:
- Note that some references say the internal energy is the energy due to the internal vibrations, etc. In other words that other than kinetic or potential energy. However, since we are interested only in energy differences the definition used here is equivalent and is easier to understand.
- It is important to note that previously engineering used a different convention: Heat was the same, but work was considered positive if it was transferred from the system to the surroundings.