The Concept and Forms of Energy and Energy Conversion


By Robert M. Hazen, Ph.D., George Mason University

Energy is at once an extremely useful and an extremely subtle concept. It took scientists centuries of studying heat and light and various mechanical processes before the basic definition of energy—the ability to do work—emerged. The concept of energy can also be understood through analyzing different forms of energy and how they are converted from one form to another.

A 3D illustration of Newton's Cradle on a white background.
In Newton’s Cradle, potential energy is converted into heat and kinetic energy. (Image: CD_works27/Shutterstock)

What Is Energy?

To understand energy, the formal concept of work, which is a force acting over a distance, has to be understood. As Newton says, anytime an object accelerates, there’s a force involved, and a force has to be acting over a distance.

So, according to Newton’s first law of motion, energy is what allows acceleration and forces to operate in the first place. Energy is a very fundamental concept in this regard. Energy is the characteristic of a physical system that gives it the ability to exert that force over a distance.

Six Ways Energy Reveals Itself

A moving object has kinetic energy. The potential energy could be stored up, like water behind a dam or a coiled spring. Wave is another form of energy, for example, a wave of sound, a wave in water, or an earthquake. 

Heat is a form of energy that is associated with the kinetic energy of individual atoms. Light is a form of energy. It comes from the Sun and travels 300,000 kilometers per second. And finally, under special circumstances, mass itself is a form of energy that can do work. 

This is a transcript from the video series The Joy of Science. Watch it now, on The Great Courses Plus.

The Concept of Closed and Open Systems

An image of a dam and a power plant.
The energy of water is converted into electricity. (Image: BORJA PD/Shutterstock)

When a quantity like mass doesn’t change, physicists refer to it as conservation law or a closed system. A closed system is like a box that contains matter and energy. And if it’s a closed system, that box is closed. No energy flows in or out of the box.

An open system is one in which matter and energy can flow in and out. That would be true of most of the systems in the human body, and also of ecosystems. In the ecosystem, sunlight can come in and matter flows in and out. In general, these systems are sometimes partially open and sometimes closed, depending on how they are defined. But, the first law of thermodynamics relates to closed systems.

Learn more about ecosystems and the law of unintended consequences.

The Transferring of Energy in Everyday Life

There are many different ways of transforming energy in everyday life that occur all the time. One of the more dramatic ones is riding a roller coaster. This is a great way to think about transferring energy from one form to another. A lot of gravitational potential energy, as well as kinetic energy, is transferred from one form to another when the roller coaster starts climbing until it gets back to its original position. 

A bouncing ball is another good example of energy conversion from one form to another. The more elastic the ball, the more efficient is the energy conversion because there will be more gravitational potential energy and kinetic energy as the ball hits the floor and goes up higher and higher.

Another example is provided by the famous Newton’s Cradle, in which there is constant switching of motion of one sort into another. The energy seems to be constantly being transferred from one place to another, and gradually as these balls hit each other, they’re also heating up slightly. There is also friction in the various parts of the machine, but that’s another example of the transfer of energy from one form to another.

A car is a great example of the transfer of energy from one form to another. The car is filled with gasoline, a form of chemical potential energy. That energy is converted to heat in the engine, helping drive the engine. The heat is converted into kinetic energy. The kinetic energy then drives the car. It can also be converted to electrical energy in the generator, powering the radio. That can be turned into light energy when the car headlights are turned on, or sound energy if the radio is turned on. There are all different kinds of energy transfer going on constantly. 

Learn more about the fact of evolution—the fossil record.

Modern Technology and Energy Conversion

An image of wind turbines in a mountainous area.
Power plants convert wind energy into kinetic and finally electrical energy. (Image: Virrage Images/Shutterstock)

One of the greatest challenges of modern technology is to find cheap and efficient ways of converting one kind of energy to another. Power plants convert coal into chemical energy. They convert water into gravitational potential energy, wind into kinetic energy, and even mass into nuclear energy. Actually, the aim of converting these energies is to produce electricity which is expensive energy. That’s why converting energy should be conducted as efficiently as possible.

Modern appliances are designed to produce kinetic energy in sound, light, and heat from electricity. So that’s another thing, we are constantly looking for ways of converting one form of energy into another.

Learn more about the chemical evolution of life.

Is Qualifying Energy Enough?

Energy can be converted from one form into another. There are lots of ways to do that, but is energy conserved? Is the total amount of heat and potential energy and kinetic energy constant through all these different transformations?

To answer these questions, the concept of energy should be quantified. For example, how much heat energy is contained in a pot of boiling water? How much gravitational potential energy is there in suspended weight? How much energy in joules? How can that be calculated? These calculations require a much more quantitative way of thinking and understanding energy.

Common Questions about the Concept and Forms of Energy and Energy Conversion

Q: What is the definition of energy according to Newton’s Law of Motion?

According to the first law of motion, energy is what allows acceleration and forces to operate. Energy is the characteristic of a physical system that gives it the ability to exert force over a distance.

Q: What are closed and open systems?

When a matter’s quantity doesn’t change, this is called a closed system or conservative law. On the other hand, when forms of energy can easily flow in and out of a system, this is called an open system.

Q: What are the various forms of energy?

There are different forms of energy that can be transferred into each other. Kinetic, potential, light, heat, sound, nuclear, and gravitational are some forms of energy that impact everyday lives.

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