# Eli5: what exactly is energy?

268 views

Like, i know it’s the ability to do work and all that, but this isn’t what energy is, it’s what energy does. What IS energy tho?

In: Physics

While it seems unrelated, I think this video is very relevant:

It’s part of an interview with Richard Feynman, in which he was asked about what magnetism is. His answer discusses how the problem with this sort of question is that the answers can keep being broken down with more questions until you eventually get to the real answer: that’s the way things are.

The most fundamental definition of energy is probably based on [Noether’s theorem](https://en.wikipedia.org/wiki/Noether%27s_theorem), which tells us that any symmetry has an associated conservation law. In this case, it is the symmetry of time translation which has the associated property we call energy which is conserved. By time translation symmetry I basically mean that you get the same result if you do the same thing at different times.

This doesn’t really say what energy is, though, it just says that it’s a property which has a certain property…

[removed]

In short — energy is a property of a physical system, which is conserved*. There is no substance to the energy that we know of. We can, however, calculate how much energy a system has and we expect this amount to be constant.

According to the Noether Theorem the conservation of energy is really just a consequence of the fact, that the laws of physics don’t change with time. I.e. you could make the same physical experiment the day before or the day after and it would give the same results. To show how, however, one has to understand Lagrange’s physics, which is outside the ELI5

*) in fact the expansion of the Universe doesn’t conserve energy in the large scale (which is consistent with Noether Theorem as the Universe in large scale does change with time)

Energy isn’t really a tangible “thing” at all. It’s essentially a measurement of a thing’s ability to do work. You can think of it as “measurable capacity to do work”.

>What IS energy tho?

In some cases, the only way to see, in the physical world, how much energy something has, is to measure how far away it is from other objects.

For example, we talk about [gravitational potential energy](https://en.wikipedia.org/wiki/Gravitational_energy); two bodies have gravitational potential energy if they are far apart.

* A mass of water has gravitational potential energy because of how the force of gravity is trying to pull it towards the center of the earth’s mass.
* The farther it is from the center of mass, the more energy it has; and we can use this energy to do work; that’s how a [water wheel](https://en.wikipedia.org/wiki/Water_wheel), or a hydroelectric plant.

But gravity isn’t the only force of attraction. Electromagnetic attraction is possible too. In fact, electromagnetic attraction is the force that governs chemical reactions, and all the chemical bonds that hold objects together. So what does an electromagnetic version of potential energy look like?

A rubber band has what we call [elastic energy](https://en.wikipedia.org/wiki/Elastic_energy). We can think of elastic energy as a form of distance too: it exists when we stretch the ends of the rubber band a farther distance from one another, than they would ordinarily be.

Ultimately, elastic energy exists because of elasticity, a property where the particles that make up an object try to return themselves into a certain shape. When we stretch those particles out of shape, the energy is embedded in the distances of how far away they each are from their ideal position.

So we could say that energy is the potential for one of the fundamental forces of nature, to move an object across a distance. This fits with how, mathematically, energy is force times distance.

Energy is more of a measurable property than a physical thing. Like speed. You can’t put speed in a cup, but you can measure how much speed something has.

Energy is the potential to do something useful.

Cars and bullets use kinetic energy.

Dropping something = potential energy.

Most engines and power plants use chemical energy. Etc.

Depends what terms you’re thinking. For example in electrical terms, power (watts) x time (hours) = energy (watt-hours or kWh)

Simply put, energy is the ability to effect change. There are a lot of ways that manifests, and a lot of complicated descriptions for how it works, but it isn’t a thing, any more than knowledge is a thing.

When an object moves, that movement is energy. When that object hits another object, it will transfer some of that energy to the other object, and change how it moves. There may also be sound (other movement), heat (molecules within things moving faster), and light (which deserves its own ELI, as things moving as fast as light does start to have weird interactions with time).

When you put clothes in the dryer, and they come out stuck together, and they crackle a bit when you pull them off, it’s because some clothes have transferred electrons to other clothes, resulting in electricity, which is energy related to the movement of electrons.

Just as motion isn’t a thing you can touch, so is energy not a thing you can touch. Rather, it’s a measurement of how much one thing can change other things.