# What is energy?

What is Energy?

by Bryan Elliot Sep. 7/2013

The simplest description of energy is the ability to do work.  It comes in a number of forms – heat, light, motion, gravitational, chemical, electrical – and these forms can be translated from one to another.  For example, you can take light energy from the sun and transform it into motion – kinetic energy – using a simple device as pictured here.

Generally speaking, energy, along with material resources, are the currency of a civilization.  You need energy to do things like till fields, clean water, mine for metals, manufacture goods, transport stuff from place to place, etc.  All of these things, in the physical sense, are work, and all work needs energy to happen.

The the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light.

## Power

To discuss energy, we also need to understand what power is – and why these terms often get confused.

Power is the rate at which work is performed.  To put it another way, it’s energy applied over time.  When in electricity, we talk about power output in Watts, and we talk about energy in Watt-hours.  Sometimes you’ll also see energy expressed in Joules – these are equivalent to Watt-seconds.

## Types of energy

### Potential energy

This is a general term for energy that is inherent in the state of a system. The classic example of this is a ball at the top of a hill. It doesn’t have energy in its motion, per se, but if something were to disturb it from its spot – starting its roll down – the object would acquire kinetic energy quickly. In this example, the potential energy is gravitational in nature – that is, gravitational attraction is temporarily thwarted by the bulk of the hill – but as the hill gives way, attraction becomes acceleration, and the ball is allowed to drift into a lower potential energy state as it gains kinetic energy.

### Kinetic energy

Speaking of kinetic energy, its simplest description is “mass in motion” – that is, if you have a thing that’s moving, it has energy. That energy, as any physics student will tell you, is $\frac{1}{2}mv^2$.

### Heat energy

Cooling towers are used to transfer heat from a turbine to a source of water, converting it to steam.

Heat energy is very, very similar to kinetic energy, except that it’s at the atomic scale. That is to say, it’s atoms in motion. As they bounce around, their kinetic energy gets distributed through the material – they make other atoms bounce around – thereby transferring heat. When all the atoms in a system have roughly the same heat energy, we say that the atoms are in thermodynamic equilibrium.

An interesting feature of this is that, without some place to transfer heat to – another system with lower energy – we can’t make use of it.

### Chemical energy

Chemical energy is more complex; it takes a certain amount of energy to pull atoms apart, and they release that energy when they snap back together. The difference between what it takes to break apart a fuel, and what it takes to break apart its combustion products, is the chemical energy embodied by that type of combustion.

Radiation is less a type of energy and more a class of how energy can be embodied. It comes from the latin “radiatio”, or to shine, and describes any travelling particle emanating from a source.

Most radiation you come in contact from day to day is harmless. That’s because most of that is electromagnetic radiation below the ultraviolet (visible light, infrared, radio waves, microwaves, wifi, and cell phones), which doesn’t carry enough energy per photon to break even the weakest of chemical bonds (which is what would be required to constitute tissue damage). If you dump enough of it into one place, like you do with a microwave oven, you can incur significant heating – but that’s a different story. On that subject, don’t stand next to radio towers.

An image of a solar storm. The sun is the largest source of radiation falling onto the earth.

… or, EM radiation, is the energy of photons. Photons don’t have a mass, but they do carry energy with them. Photons also have a fixed velocity – they travel, literally, at the speed of light. Because of this, as photons gain more and more energy, they get kind of bunched up – that is, their frequency increases. Translating from frequency to energy is direct: Energy = Planck’s constant * frequency.

EM radiation is the most common kind you’ll deal with, but other forms of radiation exist apart from electromagnetic. The most common three that are dealt with in nuclear physics are alpha, beta, and neutron. Gamma radiation, as we already noted, is electromagnetic – that is, it’s photons. The energy embodied in all of these is essentially kinetic – though, since they often occur at very high energies, the classic kinetic energy calculation noted above can break down, as they often suggest relativistic speeds.

When a nucleus that is too positively charged decays, it usually does so by releasing what’s called an “alpha particle” – so named before we knew what they really were: helium nuclei. An alpha decay results in a decrease in mass by 4 amu, and a decrease in atomic number (therefore transmuting the element) by 2. Any isotope that, in decaying, releases alpha particles is referred to as an “alpha emitter”.

Alpha particles are fairly heavy, and interact electromagnetically, so they can be stopped by a sheet of tissue paper, a few inches of air, or the outer layers of skin. That is not to say alpha particles can’t be dangerous – if an alpha emitter is ingested, the alphas can produce significant tissue damage, as well as genetic damage.

A heavy nucleus undergoing alpha decay.

A nucleus that is too heavy decays, it will release a “beta particle” – also named before we knew what they were: electrons. A beta decay results in no change in mass, but increases the atomic number (and therefore, transmutes the element) by one. Any isotope that, in decaying, releases a beta particle is referred to as a “beta emitter”.

Beta particles are more penetrating than alphas, but they still interact electromagnetically – so while a thin sheet of aluminum will stop them, you wouldn’t want to handle them without shielding.

That said, you deal with a particular beta emitter every day: tritium. It’s present in trace amounts in all water on earth, since it’s produced constantly by interactions between the atmosphere and cosmic rays.

Carbon 14 decays into Nitrogen 14, releasing a beta particle and an electron neutrino.