Building an energy-converting device

In this activity, you get to build your own mini catapult out of simple materials. A catapult works by pulling back its throwing arm, which stores up energy. When you let go, that stored energy turns into motion and launches the object flying through the air. You will build a catapult, test how far and how accurately it launches things, and then look at what worked and what did not. Maybe your catapult launched too short, or veered off to one side. That is exactly what engineers do: build, test, and refine. You might make the arm longer, change the angle, or use a stronger material for pulling back, then test again to see if it improved. At the end, there is even a friendly competition to see whose design launches the farthest and most accurately.

When scientists talk about the law of conservation of energy, they do not mean saving energy like turning off lights. They mean something bigger: energy can never be created or destroyed. It can only change from one form into another. Think about a car. Its engine burns gasoline, and the chemical energy stored in that gasoline changes into mechanical energy that turns the wheels. Or think about a solar panel sitting in the sun. It takes radiant energy from sunlight and changes it into electrical energy you can use to power a lamp or charge a phone. In both cases, the total amount of energy stays exactly the same. It just shows up in a different form. This is the same idea at work whenever you design a device that changes one kind of energy into another, whether that is a toy that turns spring energy into motion, or a light that turns electrical energy into light and a little heat.

An electric generator is a device built to change one form of energy into electricity. There are many kinds of generators, but most of them work using an idea discovered by scientist Michael Faraday back in 1831. He found that moving a magnet inside a coil of wire makes an electric current flow through that wire. His very first generator was a simple spinning disk, called a Faraday disk, that turned the motion of spinning into an electric current. That is a real example of kinetic energy, the energy of movement, changing into electrical energy. Today's generators use this same basic idea, whether they are spun by flowing water, wind, or steam. Whenever you flip a light switch, there is a good chance a generator somewhere converted motion into the electricity that lit up your room.

Energy shows up all around us in different forms, and it can move from one form into another. A plant changes light energy from the sun into chemical energy stored in its leaves. A battery changes chemical energy into electrical energy that can light up a flashlight. A speaker changes electrical energy into sound energy you can hear. Scientists also sort energy sources into two big groups: renewable and nonrenewable. Renewable sources, like sunlight, wind, and moving water, can be used again and again without running out anytime soon. Nonrenewable sources, like coal, oil, and gas, took millions of years to form and will eventually run out if we keep using them.