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Unit 3•Lesson 1 2 min read

Thermodynamics: Laws and Processes

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Learning Objectives

State the First and Second Laws of Thermodynamics.
Define entropy as a measure of disorder.
Describe the four main thermodynamic processes: isobaric, isochoric, isothermal, and adiabatic.

The Laws of Energy and Disorder

Thermodynamics is the branch of physics that deals with heat, work, and temperature, and their relation to energy.

The Laws of Thermodynamics

1.First Law (Conservation of Energy): Energy cannot be created or destroyed, only transferred or transformed. The change in a system's internal energy (ΔU) is equal to the heat added to the system (Q) minus the work done by the system (W).
ΔU = Q - W
2.Second Law (The Entropy Law): The total entropy of an isolated system can only increase over time.
Entropy (S) is a measure of the disorder or randomness of a system.
This law dictates the 'arrow of time' and sets fundamental limits on the efficiency of heat engines, as some energy must always be lost as waste heat to increase the overall entropy of the universe.

Thermodynamic Processes

These are idealized processes that describe how a system changes state.

1.Isobaric Process: Occurs at constant pressure (ΔP = 0).
Example: Water boiling in a pot with a loose lid. The pressure is constant (atmospheric pressure), but the volume changes dramatically.
2.Isochoric (or Isometric) Process: Occurs at constant volume (ΔV = 0).
Since Work = PΔV, no work is done in an isochoric process. All heat added goes into increasing the internal energy.
Example: Heating a gas in a rigid, sealed container.
3.Isothermal Process: Occurs at constant temperature (ΔT = 0).
Since internal energy is proportional to temperature for an ideal gas, ΔU = 0.
Therefore, from the First Law, all heat added to the system must be converted into work done by the system (Q = W). This requires the process to be very slow to allow for heat exchange.
4.Adiabatic Process: Occurs with no heat transfer between the system and its surroundings (Q = 0).
From the First Law, ΔU = -W. Any work done by the system comes from its own internal energy, causing its temperature to drop.
Example: The rapid expansion of a gas from a compressed air can. The can gets cold because the gas does work on the surroundings, and there is no time for heat to flow in.

Key Terms

Thermodynamics
The branch of physical science that deals with the relations between heat and other forms of energy.
First Law of Thermodynamics
A version of the law of conservation of energy, stating that the change in the internal energy of a system is equal to the heat added to the system minus the work done by the system.
Second Law of Thermodynamics
The law stating that the total entropy of an isolated system always increases over time.
Entropy
A thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work, often interpreted as the degree of disorder.
Adiabatic Process
A thermodynamic process in which there is no heat transfer into or out of the system (Q=0).

Check Your Understanding

1

What are the four main types of thermodynamic processes defined by a constant property?

2

If a gas in a rigid container is heated, what type of thermodynamic process is this, and how much work is done by the gas?

3

According to the First Law of Thermodynamics, what happens to the internal energy of a gas that undergoes an adiabatic expansion?

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Special Relativity