This post is a part of the series An Acre of Sunshine.
What is energy, really? With a focus on sustainability and land use, I will unsurprisingly spend a lot of time talking about sunlight streaming down on us from on high. We all have vague ideas about energy, thinking about the heat thrown off by a campfire, the electricity powering your lights or computer, but we don't often think about the threads that draw them all together. They seem like such totally different things, but they do have one thing in common; they are all forms of motion. All it means when one says that there is energy, or that something requires energy, is that it involves motion. While a physicist may quibble with this definition and want to talk about 'the ability to do work', at least for the sake of a human-scale discussion, energy simply is motion. For tiny objects, those smaller than can be seen with the human eye, it may be harder to think of them in terms of motion. Sunlight, electricity, and heat all include motion that is impossible to see and therefore hard to imagine. For the motion of large objects, such as a car, it is much easier to think about how energy really is just motion. The more massive the object and the faster the motion, the more energy is involved. Conceptually it is no different for microscopic motion. Electricity, and the running of all the devices powered by it, are all caused by the motion of small particles, usually electrons. The more electrons moving and the faster they move, the more electricity one has. Heat is also the motion of particles, and the faster the particles move the hotter that object is. For solid objects, say a metal stove, that motion is quite constrained, heat is the vibration of the particles that make up the stove; the particles of metal vibrate quickly, though they stay in close contact with each other, allowing the stove to keep it's shape and not melt away. And if you feel the heat coming off of a stove without touching it, you are actually feeling the high energy air particles, which picked up speed and vibration from coming into more direct contact with that stove.
Even chemical energy, such as what is used to power the human body or that found in gasoline, is about motion. In the case of chemical energy, it usually involves moving particles in relation to each other. Take the example of sugar. A molecule of sugar is simply a set of smaller pieces, atoms, arranged in a very particular way. That sugar molecule can be thought of as a tiny compressed spring; when the connections holding the parts together are broken, the pieces fly apart. Animals and plants have the ability to capture that motion, and use it in other places where it is needed. By the same token, making sugars, 'coiling the spring', requires an input of energy. Plants are able to capture some of the motion of tiny particles of sunlight, photons, and store it in molecules like sugar. It is no different for gasoline - the hydrocarbons in gasoline are just a different type of coiled spring that we release by burning, and then capture and use the resulting heat and pressure, which are again forms of motion.
Bringing up car engines, or cellular structures for that matter, illustrates the crucial point that there has to be some structure that can capture energy and use that motion to do something useful. Energy that is just 'out there' isn't useful at all. These energy capturing structures are all around us. Almost any object in our every-day environments can capture heat energy, be it rocks, engines, plants, or even people. Whether that energy is useful really depends on the situation. All living things are able to usefully use chemical energy of various sorts, and plants have the added abilities to capture and convert the sun's energy into chemical energy. For human technology, we have invented all sorts of other structures that can capture and use different forms of energy. The engines of cars, ships, planes, and trains usually capture the energy resulting from burning fossil fuels, and billions of devices are able to usefully use electricity. Windmills capture the motion of the air, hydropower is about capturing the motion of water, and finally photovoltaic solar panels, like plants, capture the motion of the photons coming from the sun.
To boil down all of the above: Energy = movement
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