So I hope you'll permit me to go all explanatory for a bit. I promise it's really important and not very hard to understand. But it is too much for one post so I'm breaking it up into two. Today we'll look at what energy is and what it does. I'll follow that up with a post on how that's related to our modern world and the usual content of this site. So...
I'll start out with a really simple definition: Energy is what changes things.
Most of the obvious changes relate to how fast something is moving and in which direction: driving a car or hitting a tennis ball are intuitive examples of the way energy can be applied to change an object's motion. The energy that a moving object has is called kinetic energy.
But energy can take many forms (including the form of matter, as Einstein famously described in his "E equals m c squared" formula). Here are a few more:
Even the smallest objects, right down to atoms and molecules, move. As with large objects, the faster they're moving, the more kinetic energy they possess. But atoms have another kind of moving energy too. They can absorb energy in a form which makes them wriggle and bounce about in completely random ways. This atomic-level "Peter Garrett impersonation energy" is what we simply refer to as heat.
There's a very familiar and useful but extremely complex form of energy called electromagnetic radiation. This includes light, radio waves, x-rays, microwaves and so on.
And there are a number of kinds of fields in which energy can be stored and retrieved. One easy example is a gravitational field: you store energy in a gravitational field by raising an object to a higher position. That energy is released from the field as the object is allowed to fall. Other kinds of fields include electric and magnetic fields. Energy stored in fields is known as potential energy.
There are others but that'll do for now. Now that we've established that energy can take different forms, it mightn't surprise you to hear that energy can be converted from one form to another. Take the falling object as an example: at its highest point, before it starts to fall, it has a lot of gravitational potential energy but no kinetic energy (ie, it's not moving). When it's let go and allowed to start falling, the potential energy is converted to kinetic energy. As it falls lower and lower more and more energy comes out of the field and becomes embodied in the increasing downward speed of the object.
It's the conversion of energy from one form to another which enables most of the interesting changes in the universe - including life itself. But there are two rules to the energy game which you have to know before you can play...
1. Energy cannot be created or destroyed.
2. Whenever energy is used, some of it gets "lost" and can't be used again.
(In case you're wondering... yes, that does mean that eventually all the energy in the universe will be "lost" and nothing will ever change again. But don't sweat it - that's not going to happen for an incredibly long time.)
Right. So now you've got a grip on the rules, let me give you a quick update on the state of play in the great game of life on Earth.
For almost five billion years, our little rock has been spinning through the cold empty vacuum of space in orbit around an enormous nuclear fusion reactor. As a result of that reactor converting large quantities of matter into electromagnetic energy which travels quite efficiently through space, the Earth has had a steady stream of energy available to convert into different forms and enable all sorts of change.
In the early days most of the conversion was directly into heat, but a combination of heat and some tricky light-powered chemical reactions gradually snowballed to produce a soup of carbon-based compounds. At some point, and through a mechanism not clearly understood, life began.
Life and energy are inseparable. Life, by definition, must grow and sustain and replicate itself. Growth is change and energy is what changes things. And for all practical purposes, all of the energy for all of the things which have ever lived on this planet has come from the Sun.
The essential flow of energy goes like this:
- Plants convert energy from sunlight into stored chemical energy, with some "lost" as heat in the conversion process.
- Things which eat plants use some of that stored chemical energy to create their own tissues (muscle, blood etc), with some energy retained in the new chemical bonds and some "lost" as heat.
- Plant eaters also convert some of the plants' stored energy into energy of motion (walking, chewing etc) which is ultimately all "lost" as heat.
- Meat eaters rely on the chemical energy they obtain from the tissues of the plant eaters to power their own growth and movement - and of course most of that is "lost" as heat during the lifetime of the animal.
- Eventually all things die, but the energy in their tissues is fought over by lifeforms ranging from the smallest bacteria to the largest carnivorous scavenger. Some of the energy may get recycled (dead thing -> fungus -> plant eater -> meat eater) but at each step of the way some of the Sun's original input gets "lost" as heat, never to power the changes of life again.
Next time we'll look at why our modern abundance of energy is basically a once-off freak accident and why there is so much concern about what is going to happen to our civilisation as the Earth's energy flow returns to its four-billion-year-old business as usual.