Introduction
Thermal physics is the study of heat and temperature and their relation to energy and work. It is a branch of physics that deals with the macroscopic behavior of matter and energy, and it is based on the principles of thermodynamics and statistical mechanics.
The Fundamental Concepts
Matter is made up of building blocks called atoms. They make up molecules, which in turn make up solids, liquids, and gases. The behavior of matter is governed by the laws of thermodynamics, which describe how energy is transferred between systems and how it affects their properties.
In thermal physics, we study matter from both a macroscopic and microscopic perspective. The macroscopic perspective focuses on the bulk properties of matter, such as temperature, pressure, volume, and density. Conversely, the microscopic perspective focuses on the behavior of individual atoms and molecules, such as their motion, interactions, and energy levels. The two perspectives are related through the principles of statistical mechanics, which provide a framework for understanding how the macroscopic properties of matter arise from the microscopic behavior of its constituent particles.
Mass and Density
Mass is more difficult to define than it seems. There are a few interpretations of mass in the context of classical physics:
- Inertial mass: This is the mass of an object as measured by its resistance to acceleration when a force is applied.
Newton's second law states that
, where is the inertial mass of the object. - Gravitational mass: This is the mass of an object as measured by the gravitational force it exerts on other objects.
The gravitational force between two objects is given by Newton's law of gravitation, which states
, where and are the gravitational masses of the two objects. All experiments have shown that inertial mass and gravitational mass are equivalent. Hence, we can use the term "mass" to refer to either one. This is known as the weak equivalence principle. - Sometimes, mass is defined as the amount of matter in an object. This is a more intuitive definition, but it is not as precise as the previous two definitions. In fact, this definition is not even well-defined, since it does not specify what an "amount" of matter is.
The volume of an object is (roughly) the amount of space it occupies, measured in cubic units. Once again, this is not a precise definition, but it is good enough for our purposes.
The density of an object is defined as its mass per unit volume:
Temperature
Temperature is a measure of the average kinetic energy of the particles in a system. It is intuitively defined as the "hotness" or "coldness" of an object. In other words, the hotter an object is, the higher its temperature.
Temperature is a scalar quantity, and it is measured in degrees Celsius (°C), Kelvin (K), or Fahrenheit (°F). Fahrenheit is not commonly used in scientific contexts, so we will focus on Celsius and Kelvin. The reason we use Kelvin in scientific contexts is that it is an absolute temperature scale, meaning that it has a true zero point. This has a few advantages—for example, it makes the temperature proportional to the average kinetic energy of the particles in a system. If you graph the average kinetic energy of a system as a function of temperature, you will get a straight line that passes through the origin.
The relationship between Celsius and Kelvin is given by:
The Kelvin scale is based on the absolute zero point, which is the temperature at which all molecular motion ceases.
This is the lowest possible temperature, and it is defined as
It is worth noting that although Kelvin and Celsius are different scales, they are related by a simple addition/subtraction.
This means that a change in temperature of
Internal Energy and Thermal Energy
The internal energy of a system is the total energy of all the particles in the system, in the system's rest frame. This means that even if the system is moving, the kinetic energy from that overall motion is not included in the internal energy. The internal energy consists of:
- The kinetic energy of the particles due to their motion (translational, rotational, and vibrational).
- The potential energy of the particles due to their interactions with each other (e.g., intramolecular forces and intermolecular forces).
- The energy associated with the internal structure of the particles (e.g., electronic energy levels).
- The rest mass energy of the particles (i.e., the energy associated with their mass,
).
The thermal energy of a system is the part of the internal energy that is associated with the temperature of the system. It includes the types of energies that are transferred between systems due to a temperature difference.
Pressure
As previously stated, molecules are in constant motion. When they collide with the walls of a container, they exert a force on the walls. This is because they bounce off the walls, and this bouncing causes a change in momentum. The pressure of a system is defined as the force per unit area exerted by the particles in the system on the walls of the container:
where
Intensive and Extensive Properties
Intensive properties are properties that do not depend on the amount of matter in a system. On the other hand, extensive properties are properties that do depend on the amount of matter in a system.
For example, mass is an extensive property because more matter means more mass. However, density is an intensive property because it does not depend on the amount of matter in a system.
Summary of Macroscopic Properties
| Property | Type | Definition |
|---|---|---|
| Mass ( | Extensive | Amount of matter in a system |
| Volume ( | Extensive | Amount of space occupied by a system |
| Density ( | Intensive | Mass per unit volume |
| Temperature ( | Intensive | Measure of the average kinetic energy of the particles in a system |
| Internal Energy ( | Extensive | Total energy of all the particles in a system |
| Thermal Energy ( | Extensive | Part of the internal energy transferred due to a temperature difference |
| Pressure ( | Intensive | Force per unit area exerted by the particles in a system on the walls |