What is Energy?

When you hear this word, you probably think of it as the opposite of when you are tired. If you feel wide awake and able to do a lot of physical or mental activities, we say that we “have energy.” In fact, the term energy in science is described as the ability to do work or cause a change. That makes sense, actually, because if you have a lot of energy, you can definitely get some work done!

The very first energy source in existence was the sun, which provides energy through heat and light. The sun provides an immense amount of energy, which plants harness to grow and humans use to power their homes through solar panels on their roofs. Humans have used fire and wood for warmth and cooking and then wind energy to sail ships. Now we use coal, oil, and natural gas as our primary sources of energy, though we still get some energy from water, wind, and the sun. Some parts of the world use dung as a source of energy, while others rely heavily on nuclear energy.


Energy Throughout History

The word energy stems from the Greek terms “en-” and “ergon” which mean “in” and “work”, respectively. The Greek word for energy was therefore “enérgeia”, which eventually was adopted in French as “énergie” and then became a widely used word in many languages starting in the 16th century.

Scientists have accepted the notion of energy since the 1800s, when Gustave-Gaspard Coriolis described kinetic energy in 1829 and William Rankine invented the term “potential energy” in 1853. The general acceptance and understanding of these concepts was greatly influenced by famous scientists and engineers such as Sadi Carnot, after whom the well-known Carnot cycle is named, and James Prescott Joule, after whom the unit of energy (the Joule) is named. In the mid-1800s, Rudolf Clausius and William Thomson declared the first and second laws of thermodynamics. After the 1920s, the science of energy transformations eventually came to be called “thermodynamics”, which is now a major area of study in both chemistry and physics.


Occurrence of Energy

In chemistry, energy is described as existing in three different forms: kinetic, thermal, and potential. According to the first law of thermodynamics, energy cannot be created or destroyed. It can only be changed or transformed from one form to another. This concept is known as the Law of Conservation of Energy. The universe already has all the energy it ever will have. Isn’t that fascinating?


Forms of energy

Kinetic Energy

Kinetic energy is the energy of movement. When you walk or bike or ride in a car you have kinetic energy. In chemistry, kinetic energy can be seen in the movement of molecules. On a windy day, the gas molecules in the air have lots of kinetic energy, but even molecules that don’t seem to move that much, like molecules in solids, still have some kinetic energy. The atoms in a solid have kinetic energy because, even though you can’t see it with the naked eye, those molecules are always vibrating in place. You can’t touch a solid object and feel the vibrations of the atoms it is composed of, but scientists can use special techniques to prove that these vibrations do occur and that they occur due to kinetic and thermal energy.

Thermal Energy

Kinetic energy and thermal energy are very closely related. Kinetic energy increases with temperature, and thermal energy is heat energy. The hotter an object is, the more thermal energy it has and vice versa. Heat refers to the transfer of thermal energy between two objects, and heat always moves from the hotter object to the colder object. When you provide heat to a system, like when you boil water to make pasta, you increase the thermal and kinetic energy of the water molecules. The average thermal energy of a collection of molecules is called the temperature. Once those water molecules gain enough kinetic energy, they are able to vibrate fast enough and powerfully enough to break away from the forces holding them together and leave the cooking pot as gas molecules. These gas molecules also have kinetic energy as they move around in the air. The warmer those gas molecules are, the faster they move and the higher the kinetic energy they have.

Potential Energy

Potential energy is stored energy. The food that you eat is a form of potential energy, and so is body fat. They are energy that is stored to be used to perform a particular function in the future, like keeping your body alive and healthy. Your body turns potential energy into kinetic energy when you eat food and then use the energy it provides to run, walk, and move your body in other ways. You can also probably imagine that a bowling ball held up in the air would also have a lot of potential energy or ability to do work. It has the ability to cause a lot of damage, that’s for sure! This example also illustrates the point that the heavier or more massive an object, the more potential energy it can have. Elastic energy is a specific form of potential energy that you might learn about in physics class. It is the mechanical potential energy stored within a temporarily compressed or stretched material or object. This is the type of energy stored in a compressed spring or in a sling shot when you pull back the pocket that holds a projectile (like a rock).


Forms of Energy in Chemistry

In chemistry, potential energy is often called chemical energy and is the energy contained within chemical bonds. This potential energy is related to the type of molecule and position and bonding of the atoms within that molecule. The engine in a car, for example, converts the potential (chemical) energy within the molecules that make up gasoline into kinetic energy, which moves the car forward. Some of that potential energy is also turned into thermal energy, which you’ll notice if you ever feel the heat coming from the tailpipe or under the hood of a running car.

Kinetic energy is turned into heat energy through friction. Try rubbing your hands together really quickly. What do you notice? The palms of your hands quickly start to become warm. This is caused by the friction, or drag, of rubbing your hands together, a motion that uses kinetic energy. Additionally, in moving your hands together, you have transformed potential energy from the food you have eaten into kinetic energy. In that one simple action you involve all three forms of energy!

Thermal energy is transformed into potential energy when heat is used to cause a chemical reaction to occur. However, there is always an energy barrier that reactants have to overcome in order for a reaction to occur. This is called the activation energy, which is the amount of energy that must be provided to reactants in order for a reaction to proceed. Indeed, thermal energy, or heating, is one way to provide sufficient energy to those reactants. The thermal energy is used to overcome the activation energy barrier so that new compounds that have chemical bonds, which contain chemical potential energy, can form.

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