ENERGY COMES IN TWO BASIC FORMS: POTENTIAL AND KINETIC
Potential Energy is any type of stored energy. It can be chemical, nuclear, gravitational, or mechanical.
Kinetic Energy is found in movement. An airplane flying or a meteor plummeting both have kinetic energy. Even the tiniest things have kinetic energy, like atoms vibrating when they are hot or when they transmit sound waves. Light from a burning match is kinetic energy — electromagnetic energy packed in the photons that radiate from a source of light, like a match. Electricity is the kinetic energy of flowing electrons between atoms.
Energy can shift between forms, but it is never destroyed or created.
A car transforms the potential energy trapped in gasoline into various types of energy that help the wheels turn and get the car to move. Most of the energy is actually lost to heat and other forms of energy used to keep the engine working.
Power plants transform one form of energy into another form, electricity. Coal and natural gas plants use the chemical potential energy trapped in fossil fuels. Nuclear power plants change the nuclear potential energy of uranium or plutonium into electricity too. Wind turbines change the kinetic energy of air molecules in wind into electricity. Hydroelectric power plants take advantage of the gravitational potential energy of water as it falls from the top of a dam to the bottom. These transformations are hardly perfect. An efficient fossil fuel power plant loses more than half of energy to other forms than electricity, such as heat, light, and sound.
Forms of Potential Energy
Systems can increase gravitational energy as mass moves away from the center of Earth or other objects that are large enough to generate significant gravity (our sun, the planets and stars).
For example, the farther you lift an anvil away from the ground, the more potential energy it gains. The energy used to lift the anvil is called work, and the more work performed, the more potential energy the anvil gains. If the anvil is dropped, that potential energy transforms to kinetic energy as the anvil moves faster and faster toward Earth.
Chemical energy is stored in the bonds between the atoms in compounds. This stored energy is transformed when bonds are broken or formed: chemical reactions. Like letters of the alphabet that can be rearranged to form new words with very different meanings, atoms get moved around during chemical reactions, and they form new compounds with vastly different personalities.
When we burn sugar (a compound made of the elements hydrogen, oxygen, and carbon) in our bodies, the elements are reorganized into water and carbon dioxide. These reactions both absorb and release energy, but the overall result is that we get energy from the sugar, and our bodies use that energy.
Chemical reactions that produce net energy are exothermic. When wood is burned, the chemical reactions taking place are exothermic. Electromagnetic and thermal energy are released. Only some chemical reactions release energy. Endothermic reactions need energy to start, such as by adding heat or light.
Today’s nuclear power plants are fueled by fission. Uranium or plutonium atoms are broken apart, freeing lots of energy. Hydrogen atoms in the sun experience nuclear fusion, combining to form helium and subsequently releasing large amounts of kinetic energy in the form of electromagnetic radiation and heat.
Nuclear energy is the stored potential of the nucleus of an atom. Most atoms are stable on Earth; they keep their identities as particular elements, like hydrogen, helium, iron, and carbon, as identified in the Periodic Table of Elements. The number of protons in the nucleus tells you which element it is. Nuclear reactions change the fundamental identity of elements by splitting up an atom’s nucleus or fusing together more than one nucleus. These changes are called fission and fusion, respectively.
Elastic energy can be stored mechanically in a compressed gas or liquid, a coiled spring, or a stretched elastic band. On an atomic scale, the stored energy is a temporary strain placed on the bonds between atoms, meaning there’s no permanent change to the material. These bonds absorb energy as they are stressed, and release that energy as they relax.
Forms of Kinetic Energy
A moving object has kinetic energy. A basketball passed between players shows translational energy. That kinetic energy is proportional to the ball’s mass and the square of its velocity. To throw the same ball twice as fast, a player uses four times the energy.
If a player shoots a basketball with backspin or topspin, the basketball will also have rotational energy as it spins. Rotational energy is proportional to how many times it spins per second, as well as the ball’s mass, and the size and shape of the ball.
In shooting a basketball, players often try to add rotational energy as backspin, because it results in the greatest slowdown in speed when the basketball hits the rim or the backboard, increasing the chance that the ball stays near the basket. The opposite direction of spin, a topspin, can be used in games like tennis, because it will help speed up a ball after impact and lowers the angle it travels after the bounce.
THERMAL ENERGY AND TEMPERATURE
Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating, but we can feel their kinetic energies as temperature. When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat.
A hot cup of tea loses some of its thermal energy as heat flows from the tea to the room. Over time, the tea cools to the same temperature as the room. At the same time, the room gains the lost thermal energy from the tea, but because the room is much larger than the tea, the temperature of the room increases by so little, and a person in the room wouldn’t notice it.
Objects next to each other that are different temperatures will spontaneously transfer heat to try to come to the same temperature. However, how much energy it takes to change the temperature of an object is based on what it’s made of, a principle called heat capacity or thermal capacity. Water has a higher heat capacity than steel, for example. An empty steel pot on the stove takes almost no time to get to 212 degrees Fahrenheit (the boiling temperature of water). A pot half-full of water will take much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to get as hot as metal.
Sound waves are made through the transmitted vibration of stuff, instrument strings or gas molecules in the air. Sound waves can travel by the motion of atoms regardless of whether they are in liquid, solid, or gaseous states. Sound cannot travel in a vacuum because a vacuum has no atoms to transmit the vibration.
Solids, liquids, and gases transmit sounds as waves, but the atoms that pass along the sound don’t travel (unlike the photons in light). The sound wave travels between atoms, like people passing along a “wave” in a sports stadium. Sounds have different frequencies and wavelengths (related to pitch) and different magnitudes (related to how loud).
Even though radio waves can transmit information about sound, they are a completely different kind of energy, called electromagnetic.
Electromagnetic energy is the same as radiation or light energy. This type of kinetic energy can take the form of visible light waves, like the light from a candle or a light bulb, or invisible waves, like radio waves, microwaves, x-rays and gamma rays. Radiation — whether it’s coming from a candle or an x-ray tube — can travel in a vacuum. Sometimes physicists describe electromagnetic radiation as divided into tiny energy packets called photons. Each photon has a characteristic frequency, wavelength, and energy, but all photons travel at the same speed, the speed of light, or nearly 1 billion feet per second.
Electromagnetic energy can be converted to stored chemical energy by plants during photosynthesis, the process by which plants and algae use the sun’s electromagnetic radiation to turn carbon dioxide gas into sugar and carbohydrates.
Electric energy is to the kinetic energy of moving electrons, the negatively-charged particles in atoms. For more information about electricity, see Basics of Electricity.