How Do We Get Thermal Energy?

Thermal energy refers to the internal energy present in substances in the form of kinetic energy associated with the random motion of atoms and molecules. It is often referred to as heat energy and is responsible for the sensation of warmth. Thermal energy is essential in our everyday lives and has many important applications and uses.

Thermal energy is crucial for regulating body temperature, enabling chemical reactions, powering engines and machinery, generating electricity, enabling phase changes, and transferring energy. It also plays a vital role in Earth’s climate system. Without thermal energy, the world as we know it would not be able to function.

The Sun

The sun is the predominant natural source of thermal energy on Earth. As a star, the sun produces enormous amounts of energy through nuclear fusion reactions in its core. This process converts hydrogen into helium and releases tremendous heat and light. The inner core of the sun has temperatures over 15 million degrees Celsius.

The sun radiates this heat and light energy outward in all directions into space. Although the sun is over 149 million kilometers away, a tiny fraction of its radiation reaches Earth. Approximately 1,360 watts per square meter of solar energy hits the outer atmosphere. As it passes through the atmosphere, some of this radiation is absorbed or reflected by air molecules, clouds, dust and greenhouse gases. But much of it continues through and reaches the Earth’s surface.

When solar radiation hits objects on Earth, it is absorbed and converted to thermal energy. The amount of sunlight reaching a given spot depends on factors like time of day, seasons, latitude and cloud cover. But the Sun provides the original source for much of Earth’s heat energy that drives our weather, climate and ecosystems.


Friction is a force that occurs when two surfaces move against each other. The friction between the surfaces converts the mechanical energy of motion into thermal energy in the form of heat. Here’s how it works:

When two surfaces rub against each other, interactions between microscopic ridges and valleys on the surfaces resist the motion. The resistance to motion requires a force to overcome it, which is called friction. The energy required to overcome friction does not disappear, but rather is converted into heat.

The amount of heat generated depends on the force pushing the surfaces together and the roughness of the surfaces. Rougher surfaces cause more friction and generate more heat. This is why rubbing your hands together warms them up – the friction between your hands converts your motion into thermal energy.

Friction is responsible for heat generation in many mechanical processes. For example, friction in car brakes, engines, and tires generates heat. Friction also heats up bearings in machines and even heats up fluid flows inside pipes due to viscosity. Anywhere mechanical motion is impeded, friction can convert that motion into thermal energy in the form of heat.

Chemical Reactions

Chemical reactions involve the breaking and formation of chemical bonds between atoms and molecules. When a chemical reaction occurs, the energy stored in the bonds is either absorbed or released. Reactions that release energy in the form of heat are called exothermic reactions. The thermal energy produced comes from the energy stored in the chemical bonds of the reactants.

During an exothermic reaction, the chemical bonds in the reactants contain more stored energy than the bonds that form in the products. When the bonds break and reform, this excess energy must be released from the system, usually in the form of heat. The amount of thermal energy released depends on how much stronger the bonds are in the reactants versus the products.

Some common examples of exothermic chemical reactions are combustion reactions and oxidation reactions. For instance, when wood burns the chemical bonds in the reactants (wood and oxygen) contain more energy than the bonds that form between carbon dioxide and water. This releases a large amount of heat and light energy, which provides thermal energy. Similar exothermic reactions occur in our bodies when we metabolize food to produce energy.

Exothermic chemical reactions are very useful for generating thermal energy for heating and electricity production. The burning of fossil fuels like coal, oil, and natural gas rely on exothermic combustion reactions to produce heat that can be used to generate steam and spin turbines. Chemical heating packs work because when certain chemicals mix together they trigger an exothermic reaction that releases heat. Overall, chemical reactions are an important everyday source of thermal energy.

Nuclear Fission/Fusion

Nuclear fission and fusion are nuclear reactions that produce enormous amounts of thermal energy. Nuclear fission occurs when a heavy atomic nucleus, like uranium or plutonium, splits into two or more lighter nuclei. This split releases neutrons, photons, and a huge amount of energy in the form of heat. Nuclear power plants use the thermal energy from fission to boil water into steam that spins turbines to generate electricity.

Nuclear fusion is the opposite reaction, where two light atomic nuclei fuse together to form a heavier nucleus. The sun produces energy through nuclear fusion of hydrogen atoms into helium. This fusion releases energy in the form of thermal and electromagnetic radiation. Scientists are working to replicate fusion here on Earth as a clean, renewable energy source. The extremely high temperatures required to overcome the repulsion of positively charged nuclei makes fusion very difficult to achieve in a controlled environment. But the promise of harnessing the power source of the stars continues to drive fusion energy research.

Geothermal Energy

Geothermal energy refers to the heat contained within the Earth that generates geological phenomena on a planetary scale. This vast amount of thermal energy originates from the original formation of the planet, the radioactive decay of minerals, and the residual heat from ongoing tectonic processes. Humans are now tapping into this vast source of internal heat as an alternative energy resource.

The inner core of the Earth is as hot as the Sun’s surface, while the outer core is over 4,000°C and the mantle is around 700-900°C. This heat gradually flows outward, warming rock, water, and other fluids. Where these heated subsurface reservoirs are concentrated near the surface, they can be accessed to generate geothermal power. Hot water or steam from deep underground can be pumped to the surface and used to drive turbines and generate electricity. For direct heating applications, hot water can be piped straight to facilities or residences.

Geothermal energy is considered a renewable power source because the Earth constantly produces heat, replenishing the thermal energy supply. Once geothermal reservoirs are tapped, the heat extraction is miniscule compared to the tremendous amounts of heat stored in the Earth’s core. Geothermal energy is also a clean, emission-free power source. While geothermal sites are location-specific, this technology taps into the virtually limitless inner heat of the planet.

Fossil Fuels

Fossil fuels like coal, oil, and natural gas are also important sources of thermal energy for us. These fuels are made from the remains of plants and animals that lived hundreds of millions of years ago. Over long periods of time, the remains were buried under layers of rock and dirt. The heat and pressure from these layers turned the remains into fossil fuels.

When we burn fossil fuels like coal and natural gas, a chemical reaction called combustion releases the energy that had been stored inside them. This energy is in the form of heat which we can use as thermal energy. For example, we can burn coal or natural gas to heat water into steam and spin turbines to generate electricity.

The problem with fossil fuels is that burning them also releases carbon dioxide gas into the atmosphere. This gas traps heat and causes global warming. So while fossil fuels give us lots of useful thermal energy, they also damage the environment. We need to be careful and try to get more of our energy from cleaner renewable sources.


Animals, including humans, produce thermal energy through their metabolic processes to maintain a stable internal body temperature. Metabolism refers to all the chemical reactions that take place inside the body to sustain life. These reactions require energy, which is obtained from food through the process of cellular respiration.

Cellular respiration breaks down the sugars, fats and proteins from food into simpler compounds. This process releases energy in the form of ATP molecules. As ATP delivers energy to power metabolic reactions, heat is generated as a byproduct. The amount of heat released depends on how rapidly these reactions occur, which is influenced by the animal’s activity level, size and surrounding temperature.

If an animal starts to gain or lose too much heat, sensors in the body detect these changes in core temperature. Signals are sent to adjust metabolism to produce more or less heat accordingly. For example, shivering generates heat through increased muscular activity when the body is cold. Sweating cools the body through evaporation when the body is too warm. These mechanisms help maintain a steady internal body temperature despite changing external temperatures.

Mechanical Work

One way we get thermal energy is through mechanical work. Mechanical work is when a force causes an object to move. This movement and force causes the atoms and molecules in that object to move faster and collide, which generates heat. So any time we exert a force to move something, some of that energy is converted into thermal energy in the form of heat.

For example, when you bend a piece of metal back and forth repeatedly, the force you exert causes the metal to move and bend. This bending and flexing of the metal makes the atoms and molecules vibrate faster, which heats up the metal. The friction from these fast-moving particles crashing into each other generates thermal energy. This is why after repeatedly bending a piece of metal, it will feel warm to the touch – some of your mechanical work has been converted into heat.

Other examples are rubbing your hands together, stirring a spoon in a cup of coffee, or driving a car. The force exerted leads to movement and friction at molecular levels, converting some of that mechanical energy into thermal energy in the form of heat. So we can generate heat through many kinds of mechanical work in our daily lives.


Thermal energy, also known as heat energy, is essential to life on Earth and is a byproduct of many natural processes and human activities. As we have seen, there are a variety of sources that provide thermal energy, both natural and man-made.

Some of the main natural sources are the Sun, friction, chemical reactions, nuclear reactions, geothermal energy, and the metabolic processes of living organisms. The Sun delivers a constant supply of thermal energy to the Earth through radiation. Friction converts mechanical energy into thermal energy through resistance. Exothermic chemical reactions like combustion also generate heat.

Nuclear fission and fusion in stars, reactors, and weapons produce enormous amounts of thermal energy. The internal heat of the Earth itself powers geothermal energy sources. All living things, including humans, generate heat as a byproduct of metabolic processes that sustain life.

On the human side, the burning of fossil fuels like coal, oil, and natural gas in power plants, vehicles, and other machines releases thermal energy for conversion into electricity, motion, and more. Humans also utilize other mechanical and electrical processes that give off heat, like welding, electric heating, friction from brakes, and more.

This thermal energy powers innumerable essential tasks that form the foundation of modern civilization – everything from electricity generation to transportation, manufacturing, heating and cooling, cooking, and more. Understanding the diverse sources of thermal energy helps us harness it more efficiently and effectively for human use.

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