What is thermal energy?

The flow of thermal energy from one system to another is the basis for a branch of physics known as thermodynamics. Scientists have made innovative leaps and bounds across the physical sciences thanks to discoveries in the field of thermodynamics. Today, those findings are helping fuel a new era of energy alternatives.

The origin of the term “thermal energy” dates back to antiquity (around 500 B.C). However, its discovery is often attributed to James Prescott Joule, a nineteenth century English physicist, mathematician and brewer.

Joule experimented with mechanical energy conversion and noticed that the more he manipulated the speed of a substance, the hotter it became. By observing temperature changes through friction and chemical reactions, Joule discovered that energy can manifest in different forms, such as heat, and that there was a direct correlation between heat and mechanical work (energy transferred to or from an object by applying force).

Joule and his findings were met with skepticism throughout his career. And yet, we now measure the amount of work produced by a system in joules, a unit of energy within the International System of Units (SI unit). His findings paved the way for the law of conservation of energy, which states that the total energy of an isolated system remains constant. This discovery led to the creation of the first law of thermodynamics

Among the four physical sciences, thermodynamics is a branch of physics that focuses on heat, work and temperature and explores their relationship with energy, entropy and physical properties like matter and radiation. The behaviors observed between these elements are governed by four laws:

Initially, the zeroth law wasn’t seen as a separate law of thermodynamics since it’s implied in the other three laws. It focuses on thermal equilibrium, which is when two objects in close proximity reach the same temperature and no longer exchange thermal energy (think hot water and a cool mug both reaching room temperature). The law states that if two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other. In many ways, the zeroth law operates as a transitive property.

Represented as a formula, the first law of thermodynamics is an expression of the law of conservation of energy. It states that energy can neither be created nor destroyed, only transformed from one form of energy to another. Therefore, the heat within a system will be equal to the heat from a source.

In its simplest form, the second law of thermodynamics states that heat flows spontaneously from hotter regions to colder regions. However, it forbids the inverse: heat will not spontaneously flow from cold regions to hot regions. This distinction is key as it establishes the concept of entropy (the degree of disorder or uncertainty in a system) as a physical property. Entropy will increase until it reaches its height at thermal equilibrium.

Though considered impossible, the third law of thermodynamics states that as the temperature of a system approaches absolute zero, the entropy of the system will approach a minimum value. The concept of absolute zero, in which all activity within a system comes to a halt, is considered unachievable since molecules can never become entirely motionless. However, it’s theorized that the zero point, or lowest possible temperature, is -273.15 degrees Celsius (or -459.67 degrees Fahrenheit) on the Kelvin temperature scale.

Energy can be categorized as either kinetic or potential energy. Kinetic energy is measured by the movement of an object and accounts for mass and speed. Potential energy is the potential for an object to move based on several factors such as its position (is the object suspended in the air or on the floor?), properties (what’s the object made of?), and its relation to other objects (could another object cause it to move?).

Consider a ball hanging from a string. As the ball hangs, it’s storing potential energy. It’s not in motion, but it could be since gravity is acting upon it as a potential force. If the string were cut and the ball were to fall, it would then have kinetic energy because it’s a moving object. Some prominent examples of potential and kinetic energy include:

Thermal energy is the total kinetic energy within a system, observed as either vibrational, rotational or translational kinetic energy. However, there is also a “hidden” (or rather, microscopic) energy that exists in the form of internal energy which considers all the particles in a system, and accounts for both kinetic and potential energy.

Thermal energy can be transferred through three methods: conduction, convection and radiation. To best understand how each works, consider a pot of boiling water on a stove.

• Conduction is the flow of energy through solid materials. As the flame heats the pot, energy travels throughout the object and increases its surface temperature.
• Convection is the flow of energy through fluid movement. As the pot gets hotter, the water adjusts to the temperature difference. Hot water rises to the top as cold water is pulled down, creating a circular current known as mass motion of fluid.
• Radiation is the flow of energy through waves. The kinetic energy from the flame travels through electromagnetic waves and is converted into heat once it touches an object, in this case, the pot.

The heat being distributed in this example moves through three different states: solid, liquid and gas. Thermal energy can alter objects across each state and can even initiate a phase change depending on the amount of heat applied. This depends on latent and sensible heat.

Latent heat refers to the amount of heat or energy needed to trigger a phase change (turning boiling water into steam). Sensible heat refers to the energy needed to raise the temperature of a substance (the flame making the pot hotter). Each object has its own specific heat capacity, which is the amount of heat needed to raise the temperature by one degree Celsius. Water has a high specific heat meaning it takes a lot of energy to raise its temperature, whereas air has a low specific heat since gasses typically have a lower specific heat capacity.

Thermal energy is often used interchangeably with heat, though there are slight nuances. Thermal energy refers to the movement of molecules and atoms within a system. Heat, on the other hand, is the transfer or flow of thermal energy from one system to another. Both thermal energy and heat are measured in joules.

Temperature refers to the average kinetic energy generated within a system, and is measured in Celsius, Fahrenheit, Kelvin or Rankine. It’s important to note that temperature records an object’s “hotness” or “coldness” at a specific time, but not its energy. For instance, temperature cannot tell you the quantity of heat leaving a system.

Another way of thinking about the relationship between the three is that thermal energy is the total amount of energy in a system, heat is the flow of energy from that system to another, and temperature is the average kinetic energy of molecules.

At a time when concerns around climate change are mounting, the push for businesses to move towards net zero operations is growing. Thermal energy offers organizations an opportunity to embrace renewable energy sources and move away from fossil fuels.

Examples include: 

Harnessing thermal energy as a renewable energy source can be an effective way for companies to diversify their energy management strategy. What’s more, it can help businesses mitigate any further damage to the planet by reducing consumption and improving energy conservation.