Energy Definition

I have found a definition of energy that I like…
But is there any other definition that is better…
You all out there can help…

“Energy is the ability to do work on matter”

Energy is the violin and matter is the song. Told you I was free flying.

Matter is energy as well…
Energy works on itself.

I would assume that for physicalists (or materialists) this definition is redundant, since it implies there could be work on something other than matter. I think the word ability is problematic also. It sounds like something held in potential OR a skill. If a potential, what is it now and how is it different from active energy.

Off the top of my head it seems like ‘change’ might be better than ‘work’, given the latters human centered meanings.

Energy is the inherent possibility of or actual unfolding change in stuff.

Regardless of the matter angle, It’s a circular definition, because work just is the transfer of energy. See wikipedia, and read this bit: In physics, mechanical work is the amount of energy transferred by a force acting through a distance in the direction of the force. Like energy, it is a scalar quantity, with SI units of joules.

A better definition is energy is a volume of stressed space. That might sound a bit surprising, but when you plumb the fundamental physics, that’s where you end up. See Energy Explained for details.

thanks again farsight…you are helpful…

A pleasure turtle. The thing to bear in mind is stress-energy, like Einstein talked about about. Energy typically travels through space in the form of electromagnetic waves. Electromagnetic waves are waves in space, and when you ask yourself what’s waving? and then take a look at displacement current, the answer has to be space itself. Displacement current is “a time-varying electric field”, but you should take it at face value - it’s space being displaced. And how do you displace space? You insert a bit more space into it!

Gravitational waves are a bit different to electromagnetic waves, see LIGO. There’s a subtle difference between waves in space and ripples in space-time, but we can cover that another day.

Energy is a physical quantity whose definition is context-related. Kinetic energy, for example, is 0.5mv^2 (in classical physics).

There’s an interesting little snippet at the back of Beyond space-time: Welcome to phase space, a New Scientist article:

“It has been obvious for a long time that the separation between space-time and energy-momentum is misleading when dealing with quantum gravity,” says physicist João Magueijo of Imperial College London.

As it happens I don’t think gravity can be quantized, at least not the way it’s usually presented, so I’d say focus on It has been obvious for a long time that the separation between space-time and energy-momentum is misleading.

Indeed, the stress-energy tensor is a very important element of physics when one uses it as Einstein and other physicists use it. Unfortunately, this is not how Farsight uses it.

A stress in physics is the sum total of any interaction between a set of objects. It can be broken down into certain kinds of interactions as we need to do it. For example, think about pushing a cup across a table. We can think of the stress of pushing a cup and the pressure back on one’s fingers that the cup provides. We can think of the friction: the pressure of the table on the cup and the pressure of the cup on the table. Even with the cup sitting in place we can think of the weight of the cup on the table and the pressure of the table upwards on the cup.

The stress tensor was developed to describe stress, that is any kind of physical interaction, in all directions. Einstein uses the stress-energy tensor to include the energy of a system as described in all directions. (The good thing about a tensor is that it can more easily be converted from different systems of coordinates than some mathematical objects.) Einstein relates this tensor to a tensor describing the geometry of spacetime.

You still don’t get this, PhysBang. Think about pushing at that cup. It’s made up of atoms, atoms are 99% empty space, and they’re held together by electromagnetic bonds. The stress is in the bonds, in the electromagnetic field, which is in the space. So that stress really is in the space.

Now think about compressing a steel spring, adding energy, then ask yourself where the energy is stored. Steel is typically an alloy of iron and carbon, and the energy stored in a compressed steel spring isn’t in the iron atoms or the carbon atoms. It’s in the bonds between them, in the space between them. The energy is stored in what we call a field, but that’s in the space. It’s the same for chemical energy. My favourite example of this is cubane, where Philip Eaton included the quote below in “Octanitrocubane: A New Nitrocarbon. Propellants, Explosives, Pyrotechnics, 27, 1-6 (2002)” :

”Cubane, the hydrocarbon (CH)[size=85]8[/size], is named appropriately; its skeleton is in the shape of a cube. At each corner of this cube there is a carbon atom (carrying a hydrogen) bound to three identical neighboring carbons… The large bond angle deformations in cubane make it a powerhouse of stored energy. Each strained bond is like the spring in a set mousetrap…”

[size=200]ENERGY NEEDS NO DEFINITION[/size]

Why is that?

That the stress has a location in space and time does not mean that you aren’t using the relevant terms incorrectly. As many have suggested to you, I suggest that you actually take the time to learn physics rather than simply imagine it into being.

I’m not the one imagining things into being, PhysBang. In The Foundation of the General Theory of Relativity on page 185 of Doc 30, 3.6 Mbytes, Einstein says “the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy”. And where is that energy? In the space around a planet.

The ability to cherry pick quotations without ever learning any of the actual content of the relevant theory is one of the hallmarks of a certain kind of crank. You have admitted that you have not learned the relevant mathematics of general relativity. The theory cannot be expressed without that mathematics.

Regardless, the use of “stress” (and indeed the use of the stress tensor) goes back well beyond the creation of relativity theory.

Funny how people try to play the abuse card when they’ve been caught bang to rights. Be it a steel spring or a gravitational field, the energy is in the space, end of story. Wittering on about mathematics and cherry picking in an attempt to dismiss that and what Einstein said won’t change things one bit.

Yes, stress and tensors predate relativity. See for example the history of tensors.

Stop reading wikipedia and pick up a college enrolment guide.

And you put down your 50-year-old textbook and start thinking for yourself.

Let’s be genteel here. :slight_smile: