Thermal Energy and Entropy
Student Expectation
The student is expected to analyze and explain everyday examples that illustrate the laws of thermodynamics, including the Law of Conservation of Energy and the Law of Entropy.
Key Concepts
The zeroth law of thermodynamics states that if systems A and B are in thermodynamic equilibrium with each other and system A is in thermodynamic equilibrium with system C, then systems B and C are also in thermodynamic equilibrium with each other.
Within a closed system the thermal energy of the system is the sum of all the kinetic and potential energy within the system and does not change as energy is transformed. This is the Law of Conservation of Energy.
Entropy is a measure of how much energy is dispersed or spread out. Barriers may exist such that an increase in entropy is prevented.
The entropy of a closed system always stays the same or increases. In the real world, entropy of isolated systems may increase or decrease, but the entropy of the universe as a whole is increasing.
The entropy of a closed system approaches a constant value as the temperature approaches absolute zero.
Thermal Energy and Entropy
Thermal Energy
The laws of thermodynamics helps us understand the concepts of heat and thermal energy. Thermal energy is the energy in a substance due to the random motion of particles inside it. Thermal energy can be thought of as the average kinetic energy of particles in a substance. The measure of thermal energy is temperature. However, thermal energy does not describe the total energy in a system. The total energy in a system is called internal energy. The internal energy of the system is the total kinetic and potential energy in the system. It includes the translational kinetic energy, or thermal energy, of particles. It also includes the vibrational energy and rotational kinetic energy of all the particles in the system. Furthermore, it includes the gravitational potential energy and electric potential energy between particles.
The Zeroth Law of Thermodynamics
The “zeroth” law of thermodynamics relates temperature to the concept of thermal equilibrium. A system in thermal equilibrium does not gain or lose thermal energy. The zeroth law of thermodynamics states that if systems A and B are in thermodynamic equilibrium with each other, and system A is in thermodynamic equilibrium with system C, then systems B and C are also in thermodynamic equilibrium with each other. In other words, if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. It can be illustrated in the following graph.
If we know that for systems A, B, and C,
Diagram of the zeroth law of thermodynamics.
Then we can conclude,
Diagram of the zeroth law of thermodynamics.
Thermal Equilibrium and Themometers
Application of the Zeroth Law of Thermodynamics allows us to create a thermometer. We can calibrate the change in a thermal property, such as the length of a column of mercury, by putting the thermometer in thermal equilibrium with a known physical system at several reference points. Celsius thermometers have the reference points fixed at the freezing and boiling point of pure water. If we then bring the thermometer into thermal equilibrium with any other system, such as the bottom of a person’s tongue, we can determine the temperature of the other system by noting the change in the thermal property. Objects in thermodynamic equilibrium have the same temperature.
1st Law of Thermodynamics - Conservation of Energy
Energy can neither be created nor destroyed. It can only be transferred from one type to another. Within a closed system, the thermal energy of the system is the sum of all the kinetic and potential energy within the system and does not change as energy is transformed. This is the Law of Conservation of Energy. The change in internal energy of a system is equal to the heat added to (or lost from) the system minus the work that the system does on the environment:
Change in internal energy = Change in heat – Work done on environment
In another example, a tennis ball is dropped from a second floor and the velocity increases until it hits the ground. Before it is dropped, the ball contains potential energy. After it is released, the gravity starts to accelerate its speed and potential energy begins to transfer into kinetic energy. However, the total amount of energy does not change. The cost of the faster speed of the ball is the potential energy reduces.
The Law of Thermodynamics and Entropy
Rudolf Clausius is the famous scientist who originally gave the concept of entropy. Entropy is a measure of the disorder in a system. Systems in which the energy is dispersed or spread out have greater disorder and hence greater entropy. It has units in J/K (joules per Kelvin). In a thermodynamic system, pressure, density, and temperature tend to become uniform over time because this equilibrium state has higher probability (more possible combinations of microstates) than any other. The entropy of the thermodynamic system is a measure of how far the equalization has progressed. The second law of thermodynamics states that, in a closed system, the entropy does not decrease. Cars rust, dead trees decay, buildings collapse; all these things are examples of entropy in action, the spontaneous movement from order to disorder
Also, in statistical mechanics, the entropy tells the degree of uncertainty. Because states of a system with greater disorder are more probable, the entropy of a closed system always stays the same or increases. In the real world, entropy of isolated systems may increase or decrease, but the entropy of the universe as a whole is increasing. This is the Law of Entropy. Furthermore, barriers may exist such that an increase in entropy is prevented. The entropy of a closed system approaches a constant value as the temperature approaches absolute zero. In another point of view, a Markov process may also be named as entropy.