Thermodynamics
Chapter 1 - Worldview
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Welcome to the Thermodynamics page
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A specific form of relationship is energy transfer. Thermodynamics studies the relationship between heat, work, temperature, and energy. The laws of thermodynamics describe how the energy in a system changes and whether the system can perform useful work on its surroundings.
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Core ideas
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Total energy in a system
A system can contain very different types of energy. Classical thermodynamics deals with the macroscopic properties of materials, such as temperature, pressure, and volume. Thermal energy from adding heat can be understood at the microscopic level as an increase in the kinetic energy of motion of the molecules making up a substance. Additionally, chemical energy is stored in the bonds holding the molecules together, and weaker long-range interactions between the molecules involve yet more energy. The total of all these forms of energy constitutes the total internal energy of the substance in a given thermodynamic state. The total energy of a system includes its internal energy plus any other forms of energy, such as kinetic energy due to the motion of the system as a whole (e.g., water flowing through a pipe) and gravitational potential energy due to its elevation.
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The Laws of thermodynamics
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The Zeroth Law of thermodynamics
When two systems are each in thermal equilibrium with a third system, the first two systems are in thermal equilibrium with each other. This property makes it meaningful to use thermometers as the “third system” and to define a temperature scale.
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The First Law of thermodynamics
The first law of thermodynamics deals with the total amount of energy in the universe, and in particular, it states that this total amount does not change. The First Law of Thermodynamics states that energy cannot be created or destroyed. It can only change form or be transferred from one object to another. For example:
- Light bulbs transform electrical energy into light energy (radiant energy)
- One pool ball hits another, transferring kinetic energy and moving the second ball
- Plants convert the energy of sunlight (radiant energy) into chemical energy stored in organic molecules
- You are transforming chemical energy from your last snack into kinetic energy as you walk, breathe, or even move your fingers
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The Second Law of thermodynamics
Energy can only change from more-useful forms into less-useful forms. As it turns out, in every real-world energy transfer or transformation, some amount of energy is converted to an unusable form (unavailable to do work).
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The Third Law of thermodynamics
The entropy of a perfect crystal of an element in its most stable form tends to zero as the temperature approaches absolute zero. This allows an absolute scale for entropy to be established that, from a statistical point of view, determines the degree of randomness or disorder in a system.
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Deep dive
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Flow
When two objects are brought into thermal contact, heat will flow between them until they reach equilibrium. When the flow of heat stops, they are said to be at the same temperature. The Zeroth law of thermodynamics formalizes this by asserting that if object A is in simultaneous thermal equilibrium with two other objects, B and C, then B and C will be in thermal equilibrium with each other if brought into thermal contact.
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Equilibrium
A fundamental concept is thermodynamic equilibrium, in which there is no tendency for the state of a system to change spontaneously.
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Process
The system can be changed to a new state only by an externally imposed change in one of the state functions, such as the temperature, by adding heat or changing the volume. A sequence of one or more such steps connecting different system states is called a process.
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Entropy
(The second building block of the worldview is about all aspects of entropy, but here is already a short preview.)
Since we know that every energy transfer results in the conversion of some energy to an unusable form, and since that energy does not do work but increases the randomness of the universe, we can state a biology-relevant version of the Second Law of Thermodynamics: every energy transfer that takes place will increase the entropy of the universe and reduce the amount of usable energy available to do work. In other words, any process, such as a chemical reaction or set of connected reactions, will proceed in a direction that increases the overall entropy of the universe.
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