The origin of the thermal energy resides, on the one hand, in the constant movement of the particles at the molecular level, which gives them kinetic energy, which is the energy associated with the movement.
On the other hand, particles have a property called electric charge, according to which they interact according to their relative positions. This contribution to the body’s thermal energy is potential energy.
It must be emphasized that thermal energy is not a new form of energy, but the way of referring to the sum of the kinetic and potential energies of a very large system of particles. The measure of this energy is the temperature, therefore, the higher the temperature of something, the more thermal or heat energy it has.
Characteristics of thermal energy
The thermal energy of a system is characterized by:
-Have the same units as work and any other form of energy.
-Easily transferring from one material to another through certain fundamental mechanisms that are described later.
-Variable in two ways: the first by exchanging energy with the environment, which in this case is talking about transferring heat, and the other is by doing some work on the system that adds or subtracts energy.
Units and formulas
The unit of thermal energy in the International System is the joule , abbreviated J, in honor of the English physicist James Prescott Joule. However, when it comes to thermal energy, a commonly used unit is the calorie .
In terms of the joule, a thermochemical calorie equals 4.1840 J and a kilocalorie represents 1000 calories.
Thermal energy is proportional to body temperature. If E c is the kinetic energy and T is the temperature, the proportionality constant is k B or Boltzmann’s constant, the mean kinetic energy of the particle for each degree of freedom is given by the following equation:
E c = ½ k B ∙ T
For example, a monatomic gas molecule, such as helium or argon, can move anywhere in a room, so it has 3 degrees of freedom and its translational kinetic energy is equal to 3 times the above equation:
E c = 3/2 ∙ k B ∙ T
In units of the International System, Boltzmann’s constant is equal to 1.380649 × 10 −23 J / K.
Assuming that gas molecules interact very little with each other (ideal gas) and that they only have translational motion, the internal energy U is entirely equal to the kinetic energy E c .
When other contributions are taken into account, such as rotational movement for example, E = ½ k ∙ T is added for each possibility of movement.
Where is the thermal energy obtained from?
When two bodies with different temperatures are brought into contact, energy spontaneously flows from warmer to colder, until thermal equilibrium is achieved and the temperatures equalize.
Once in thermal equilibrium with its surroundings, a body absorbs as much thermal energy as it emits.
Often these changes produce transformations. For example, when heated, most substances expand and when cooled they contract. Changes of state can also take place, such as going from solid to liquid or undergoing chemical transformations.
Obtaining thermal energy is possible through various ways. For the Earth, the primordial source is the Sun , but the Earth itself generates heat on its own through the radioactive decay of some unstable elements.
Chemical reactions and electricity also generate thermal energy that can be harnessed.
At the core of most stars, hydrogen, the simplest and most abundant element in the universe, fuses to produce helium, the next most complex element after hydrogen. This process of nuclear fusion, which occurs continuously inside the Sun, releases large amounts of energy that reach the Earth in the form of light and heat.
Combustion is a chemical reaction that quickly releases heat. It is always produced in the presence of oxygen and requires a combustible material, such as wood, coal or gasoline. In them there is an exchange of electrons in which oxygen takes them from the fuel, releasing light and heat in the process.
In the example at the beginning, rubbing your hands in cold weather gives you a comforting sensation of warmth. In doing so, kinetic friction increases the energy of the particles on the skin’s surface and thereby increases the thermal energy.
The same happens when pushing a book on a table and in general whenever there is relative movement of surfaces in contact. At the microscopic level, the particles on the two surfaces experience an increase in their kinetic energy, which translates into a rise in temperature, which can be perceived simply by touching the surfaces.
Through the passage of electric current
The materials are heated by the passage of electric current, therefore, the cables of electrical appliances, when they are connected to the socket, feel hot when they touch the plastic coating. This heating is called the Joule effect .
By radioactive decay
Inside the Earth there are unstable elements that decay naturally, that is, they expel particles from their nuclei to transform into other more stable elements. This process is accompanied by the emission of thermal energy, which heats the interior of the planet.
Thermal energy transfer
There are three fundamental mechanisms for transferring thermal energy, that is, transferring heat from one body to another: conduction, convection, and radiation.
It occurs preferably in solid materials, whose particles collide with each other, without them moving appreciably within the material. Metals are good conductors of heat thanks to the free electrons they have.
Through this process the heat is transported together with portions of the dough, which is generally a fluid, for example a liquid. When the water is boiled in a pot, the mass that is at the bottom, close to the flame, heats up and expands, so its density decreases and the fluid rises. Thus the colder portions sink to warm in turn.
Unlike conduction and convection, radiation does not need the material medium to propagate, since it does so through electromagnetic waves. In this way, the thermal energy from the Sun reaches the Earth through empty space.