Why?

There is a lack of precision in the mentions of energy by the media and by politicians.  Even the “Greens” rarely present numbers.

Decision making in the field of energy should be based on calculations, not opinions.  To make calculations one needs to understand the units, quantities and interrelations.  That's not easy but it is not very difficult either.

Have a simple calculator ready, so you can check the numbers!

What and what not

Energy is present in the universe in many forms.  We will NOT be concerned with advanced concepts, rather only work with the simple stuff we need for everyday life.

However, you probably know one formula, though it is one we shall not need at all:

$E=m{c}^2$

Einstein's classic formula says energy and mass are equivalent, and the conversion factor is the square of the speed of light.  It is useful in high-energy physics, and in building nuclear reactors, but you will normally not need it.

We will only talk about:

• mechanical energy:  heavy stuff moving along, or rotating,
• chemical energy:  burning fuel,
• electrical energy:  lights, batteries,
• heat

Mechanical Energy

You have probably dropped something on your foot at one time or another.  This hurts:  while the object fell, it acquired motion energy and when it hit your foot, that energy was transferred to your foot, which did not quite know what to do with it other than causing pain.

Anything that moves has some motion energy.  To make yourself on your bicycle move along a flat road, you will have to pump the pedals for a while before you can coast along. That first effort to get going is the motion energy you have to supply.  After that, on a good bicycle, no wind, good flat road, the energy you need to keep putting in is just the small flow to overcome the friction with the road and the air.  That friction is the process that converts the motion energy into heat, and I will come back to it later.

The higher the speed of an object, the more motion energy it has.  Two cars crashing into each other have enough energy to make a real mess. but it does depend on the speed, and if it is low enough then the bumpers may just take it all up and nothing serious happens.

Mechanical energy can also be present in rotating objects:  this is why it can be quite dangerous to try to stop a spinning  item by touching it.

Lastly, mechanical energy is present by position in a graviataional field:  that sounds complicated, but all it says is that an object has more energy when it sits higher up than a similar object lower down.  Something falling off the top of a cabinet causes more damage than something falling off a low bench. Falling off the roof is not recommended, and we all know that the higher up we are, the worse we will suffer if we fall down.  This height energy can be used for storage:  a classic pendulum clock is driven by weights which are wound “up”, and release their energy to power the clock while they wind “down”.  There are proposals to store solar energy by lifting weights, see later.

Chemical Energy

Let's not try to go into the details, we all know that heat is released when we burn stuff.

There are three types of burning that we are familiar with, and they all involve combining fuel with oxygen from the atmosphere:

• flame-burning, whereby the rate of combingin fuel with oxygen is fairly fast, so that the release is violent enough to cause light coming from the process: a flame.
• explosive burning, whereby the rate is so fast that the result expands very quickly, causing havoc, noise, and damages.
• digestive burning, where the rate is very slow and very controlled, so it can be used to move our muscles, but note that's why we need to breathe to get the oxygen!

Electrical Energy

Electricity is the first “invisible” technology that we developed.  Unlike steam machines, electrical machines do not have any visible working parts.  The rotor of an electrical motor starts turning without any visible action on it.  Unlike the water current in plumbing, you cannot see electrical current flow.  Electrical energy is difficult to store:  apart from tiny amounts in capacitors, it is stored in batteries, all of which actually convert the electricity into chemical energy and release it in a reverse process. Therefore one can say that:

Units

We are going to need units of measurement.  If you are not familiar with the basic units and their derived units, first read the page on units before going on.  I will assume that you know the meaning of the units m, kg, s, A, J, K,… .

Energy

The unit of energy is the Joule (J).