Potential energy and kinetic energy

LESSON 19.
Potential energy and kinetic energy.
Although there are many kinds of energy in the world, they all fall into two broad categories: potential energy and kinetic energy. When energy is stored up and waiting to do things, we call it potential energy; "potential" simply means the energy has the ability to do something useful later on. When stored energy is being used to do something, we call it kinetic energy; "kinetic" means movement and, generally, when stored energy is being used up, it is making things move or happen.

Photo: You gain potential energy every time you walk up stairs. Your muscles pull your body against the force of gravity, doing work. In theory, the potential energy your body gains as you climb is exactly the same as the food energy it loses: one form of energy is simply converted into another. (In practice, you need to use more energy than you might think because your body wastes quite a lot of energy in the process.) At the top of a flight of stairs, you could turn your stored potential energy back into kinetic energy (movement) in various ways, such as sliding down the banisters or jumping down a fireman's pole!
It's easy to find examples of both potential energy and kinetic energy in the world around us. If you push a boulder up a hill, you'll find it's a real effort to get to the top. This is because the force of gravity is constantly trying to pull you (and the boulder) back down. In science, we say you have to do work against the force of gravity to push the boulder up the hill. Doing work means you have to use energy: the muscles in your body have to convert sugar and fat to make the energy you need to push the boulder. Where does this energy go? Although you use energy as you climb, your body and the boulder also gain energy—potential energy. When the boulder is at the top of the hill, you can let it go so it rolls back down again. It can roll down because it has stored potential energy. In other words, it has the potential to roll down the hill all by itself.
As the boulder starts to roll down the hill, the potential energy it had at the top is gradually converted into kinetic energy. When we talk about kinetic energy, we usually mean the energy something has because it is moving. Anything that has mass (contains some matter that takes up a volume) and moves along at a certain velocity (or speed) has kinetic energy. The more mass something has and the faster it goes (the higher the velocity), the more kinetic energy it has. If a truck and a car are driving parallel to one another down the freeway, at the same speed, the truck has more kinetic energy than the car because it has much more mass. (Read more about the science of motion.)
A lot of things we do each day involve converting energy between potential and kinetic. Pull yourself up a cliff on a rope and you have more potential energy the higher you go up. If you abseil down, your potential energy is converted into kinetic energy as you move. By the time you reach the bottom, the kinetic energy has turned to heat (your climbing equipment and the rope will get surprisingly hot) and sound (the rope will make a noise as you whiz down).
Other kinds of potential and kinetic energy
Things can have potential and kinetic energy for other reasons. Here are some more examples. A thundercloud passing overhead has "the potential" to release electrical energy as huge bolts of lightning. In other words, we say it has electrical potential energy. Suppose you want to fire an arrow from a bow. When you pull back the elastic bowstring, you have to stretch it well beyond its natural shape. As you do this, you give it what's known as elastic potential energy (it is sometimes also called mechanical potential energy). When you release the bowstring, it uses the stored potential energy to fire the arrow through the air.
Just as there are several kinds of potential energy, so there are different kinds of kinetic energy too. When a thundercloud releases its electrical potential energy as lightning, giant sparks fly from the sky to the ground. A bolt of lightning is a huge electric current (flow of electricity) moving through the air—in other words, it is what we might refer to as "electrical kinetic energy". We can also think of sound, heat, and light as examples of kinetic energy because they involve energy moving from one place to another.

Photo: Lightning is a huge release of electrical potential energy.
Lesson 20.
Heat energy.
Heat is one of the most familiar kinds of energy in our world—but is it potential energy or kinetic energy? Actually, it can be both. Suppose you heat an iron bar in a fire so it glows red hot. If you plunge it into a bucketful of cold water, you'll make a huge amount of steam. The energy from the hot bar goes into the water and heats that up too, losing some of its own energy in the process. This means that a hot bar—a bar with heat energy—has potential energy: it has the potential to heat something else up. But a hot bar also has kinetic energy. Inside an iron bar, there are billions of iron atoms held together in a rigid structure called a crystal lattice. It's a bit like a climbing frame with atoms at the joints. Although the atoms are pretty much fixed in the same place, they are constantly jiggling about. Each atom has a little bit of kinetic energy. The more you heat an iron bar and the hotter it becomes, the more the atoms jiggle about—and the more kinetic energy they have. In other words, heat is held inside the bar by the jiggling atoms and their kinetic energy. The idea that heat is caused by atoms and molecules moving around is known as the kinetic theory of matter.
Hot objects like to pass their heat energy to other things nearby. If you touch something hot, some of its heat energy flows into you—and you get burned. This is called heat conduction. But you don't have to touch something to feel its heat. If you sit some distance from a roaring fire, you'll be able to feel its heat energy on your cheeks even though the flames are not actually touching you. This happens because the fire passes its energy through empty space by a process called heat radiation. Radiation is the way the Sun passes its energy through about 150 million km (93 million miles) of empty space to earth in a journey that takes a little over 8 minutes.
Heat energy also moves in a third way, known as heat convection. If you put a pan of soup on top of the stove and heat it up, heat travels from the stove to the pan by conduction. The soup at the bottom of the pan quickly warms up. This makes it less dense ("thinner") than the soup above it, so it rises upward. As the warm soup rises, it pushes the colder soup at the top out of the way, and the cold soup falls back down to take its place. Pretty soon, there's a kind of invisible loop forming inside the soup, with heat energy constantly being carried up from the stove and circulating through the liquid up above. This process is also how heat travels through a hot air balloon, from the burner at the bottom, so it systematically heats up all the gas inside.