Why is thermal energy classified as kinetic energy rather than potential energy?

Broadly speaking, all energy in the universe can be categorized as either potential energy or kinetic energy. Potential energy is the energy associated with position, like a ball held up in the air. When you let go of that ball and let it fall, the potential energy converts into kinetic energy, or the energy associated with motion.

There are five types of kinetic energy: radiant, thermal, sound, electrical and mechanical. Let's explore several kinetic energy examples to better illustrate these various forms.

Radiant energy is a type of kinetic energy, referring to energy that travels by waves or particles. The energy is created through electromagnetic waves and is most commonly experienced by humans in the form of heat. Some examples include:

• Incandescent light bulb: When you turn on a light with a traditional incandescent light bulb, it gives off two forms of energy. There is the visible light that you see, as well as the warmth that it generates. Both of those are forms of radiant energy.
• X-rays: Just as visible light travels in waves, so do electromagnetic waves beyond the visible spectrum. X-rays are one such example.
• Electric toaster: As the heating elements inside the toaster get warmer, they emit radiant energy that heats and toasts the bread.
• Radio signals: Similar to x-rays, radio waves also travel in the form of waves. That's how radio stations are able to transmit their programming across vast distances.
• Sunshine: Have you ever stood outside on a warm, sunny day? Did you notice that it is hotter out in the sun than it is in the shade? That's because you're experiencing radiant energy from the sun's rays!

Thermal energy is similar to radiant energy in that both can be experienced in the form of heat or warmth. The difference is that while radiant energy refers to waves or particles, thermal energy describes the level of activity among the atoms and molecules in an object.

As they move more quickly, they collide more frequently with one another. That motion is why thermal energy is considered an example of kinetic energy, even if you may not be able to see that motion with the naked eye. Some examples include:

• Geothermal energy: A great example of a renewable resource, geothermal energy comes from the decay of natural minerals and the volcanic action of the earth. This warmth can then be harnessed to heat and cool homes, as well as generate electricity.
• Baking in an oven: When you place a frozen pizza in the oven, you are heating it up and raising its temperature. The molecules that make up the pizza are moving more quickly when the pizza is piping hot.
• Boiling water: Visually, boiling water is perhaps one of the better examples of thermal energy. If you heat water to a rolling boil on the stove, like when cooking pasta, you can see the active, kinetic energy of the very hot water. On a microscopic level, the individual water molecules are equally active.
• Geysers: You know Old Faithful in Yellowstone National Park? The geyser is a terrific example of thermal energy at work. The geothermal energy of the earth heats up groundwater and turns it into steam. The individual water molecules are moving more quickly, building up pressure until the water bursts through in the form of a geyser.
• A running car engine: Have you ever been near the engine of a car while the motor is running? The warmth that you feel emanating from the engine is an example of thermal energy.

The human experience of sound is caused by vibrations. The object creating the sound creates waves of movement through a medium, like air, until it reaches our eardrums, which then vibrate and our brain interprets that as sound. Here are some examples of sound energy:

• Your voice: Speak at a normal volume while placing your fingers on your throat. You'll feel the vibrations (movement) of your vocal cords.
• Stereo speakers: If you place your hand on a speaker, particularly one that is playing loudly, you'll also feel it vibrate. The mechanisms in the speaker are generating what you experience as sound.
• Stomping your feet: When you stomp your feet, you cause the surface beneath your feet (the floor) to vibrate. That movement passes through the air, which is then perceived as sound.
• Drums: When a drummer strikes a drum, the surface vibrates and causes sound.
• A buzzing bee: When you hear a bee buzzing, what you are actually hearing is the bee flapping its wings at a very rapid pace. The flapping is so fast that it creates perceivable sound waves in the air.

Electrical energy, which we more commonly refer to as electricity, is caused by the flow of negatively-charged electrons around a circuit. It is the very movement of these electrons that powers our everyday devices.

Here are some examples of electrical energy:

• Lightning: When you see a lightning strike, what you are actually seeing is a very fast discharge of electrons, caused by static electricity in the clouds.
• Batteries in use: If you set up a simple circuit connecting a lightbulb with the positive and negative ends of a battery, and the lightbulb illuminates, you are witnessing the power of electrical energy.
• Desk lamp: A light fixture that is plugged into a wall outlet and is turned on is another example of electrical energy. The flow of electricity goes from the power outlet to the lamp, and back out to the wall outlet again.
• Electric eels: These fascinating creatures have specialized organs for producing electricity, including two that function like tiny batteries.
• AC/DC devices: Everyday electrical outlets, devices, and appliances are classified as using alternating current (AC) or direct current (DC), depending on how they are configured to work.

Visually, the most obvious kinetic energy examples are examples of mechanical energy. It is literally the energy associated with the mechanical movement of an object. The more and faster the object moves, the more mechanical energy it has and the more ability it has to do work.

• Wind energy: Harnessed by windmills, wind energy captures that natural movement of air. This can spin a turbine, which in turn can generate electricity.
• A flowing river: The movement of the water down a river is a great example of mechanical energy. The underlying principle is how hydroelectric dams can be used to generate electricity.
• A bullet fired from a gun: The bullet flying through the air is literally moving. This energy is then transferred to the object the bullet hits in the form of damage.
• A bowling ball heading down the lane: The movement of the ball rolling down the lane is an example of mechanical energy. The "work" is then apparent when the ball strikes the bowling pins at the other end.

• Playing a piano: The person playing the piano uses his or her fingers to strike the keys. The movement of the fingers is itself an example of kinetic energy, which is then transferred through the piano until the hammer strikes a string (more kinetic energy), resulting in sound energy.

  Kinetic Energy Moves You

  It is important to note that these different types of kinetic energy are not mutually exclusive. You may observe multiple kinetic energy examples in a single instance, like how a toaster is an example of both radiant and thermal energy. In all cases, though, you're witnessing energy in the form of motion.

  To learn more about kinetic energy's counterpart, read up on these Examples of Potential Energy. It'll really set your learning into motion!