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What is Thermal Energy & How Does it Work?

Posted on by Everything Energy 5 minutes, 4 seconds

What is thermal energy?

Thermal energy is the energy that comes from the temperature of a heated substance.

Thermal energy is one of the six basic forms of energy, which are: chemical energy, electrical energy, mechanical energy, nuclear energy, radiant energy and thermal energy.

What causes thermal energy?

As matter is heated up, its atoms and molecules move faster and collide with each other. Thermal energy is the energy that comes from this heated-up matter. The hotter that matter gets, the more the particles move — and the higher its thermal energy.

What are the 4 laws of thermodynamics?

Thermodynamics is the study of the relationship between heat, work, temperature and energy. There are four laws of thermodynamics that are important to know.

Zeroth Law of Thermodynamics

This law states that when two bodies are each in thermal equilibrium (the same temperature) with a third body, they must also be in thermal equilibrium with each other.

This “law” actually came after the other three laws of thermodynamics. However, scientists noted that even though this concept is very basic and understood to be true, it needed to be formally stated. Since the other three laws had already been numbered, and this law is the foundation for the other three, the name “zeroth law” was settled on in the 1930s.

First Law of Thermodynamics

This law, also known as the law of energy conservation, states that energy cannot be created or destroyed; it can only change forms or be transferred from one place to another.

The transfer of potential energy into kinetic energy, demonstrates the First Law of Thermodynamics.

Second Law of Thermodynamics

This law states that heat doesn’t flow spontaneously from a cold area to a hotter area. So, the entropy (the measure of the disorder of the material) of a closed system increases over time toward a maximum value.

If this sounds confusing, here’s an example of the second law of thermodynamics in action: An ice cube sits on a table. As time passes, it starts to melt. This spontaneous process increases the entropy of the universe.

Third Law of Thermodynamics

This law states that entropy of an isolated system approaches a constant value as the temperature of the system approaches absolute zero (the bottom point on the Kelvin temperature scale, equivalent to -459.67° Fahrenheit).

This is another tricky one. It basically says that absolute zero can never be reached. As the Second Law of Thermodynamics states, heat can never spontaneously move from a colder body to a hotter body. As a system approaches absolute zero, it will eventually have to draw energy from nearby systems. And drawing that energy prevents it from ever reaching absolute zero.

Did you know…? James Prescott Joule produced groundbreaking work in thermodynamics. Joule’s First Law provides a formula that shows the relationship between heat produced by flowing electric current through a conductor. The “joule,” a unit of energy, was named after him.

How is thermal energy transferred?

Though energy can’t be created or destroyed, it can be transferred from one object to another.

There are three main ways thermal energy transfer occurs: conduction, convection and radiation.


Conduction is when thermal energy moves between molecules that are touching each other. Think of dipping a metal spoon halfway into a pot of boiling water. Even the part of the spoon you’re holding on to will get hot, thanks to conduction.

Some materials, like wood and plastic, do not conduct heat very well. These materials are called insulators. That’s why appliance cords are coated in protective, non-conductive plastic.


Convection happens in fluids, such as liquids and gases. When a liquid is heated, the molecules closest to the heat source gain thermal energy first. They move faster and spread out, decreasing the density of the molecules closest to the heat source. These molecules move further from the heat source, pushing cooler, denser molecules toward the heat.

The process repeats, with molecules sinking, heating up, rising, cooling down and sinking again. An easy way to envision this is a pot of water boiling on a stove.


Radiation occurs when a heat source travels through space, without needing to directly touch the object it’s transferring energy to. Most thermal energy on Earth comes from the sun and radiates to the planet’s surface in the form of visible light. Materials then absorb the waves to be used for energy or reflect them back into space.

How is thermal energy converted into electricity?

There is a lot of heat that gets “wasted,” so to speak — from the sun, furnaces, and engines, etc. Converting that heat directly into electricity has proven difficult so far.

Thermophotovoltaic devices that convert a temperature difference into electricity without any moving parts could recycle some of this wasted energy. They could also allow compact systems for grid energy storage that are less expensive than batteries. Although research is being done to improve their efficiency, thermophotovoltaic devices of that level are not economically viable currently.

What is thermal energy used for? 5 thermal energy examples

So where do we see examples of thermal energy in everyday life? Here are some thermal energy examples that demonstrate the concept in action:


Now that you know the answer to the question What is thermal energy? you will probably see many cases of it as you go about your day. Via convection, conduction or radiation, the transfer of thermal energy happens all around us.

If all this talk about energy has you thinking about energy in your own home, you’re in luck! Everything Energy has easy-to-use search tools that make shopping for home electricity a breeze. Try them out for free here.

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