# The nature energy

hi..
i wondered...
is energy is something tangible?
i mean.. theoretically.. is it possible, with some (theoretical maybe) test machine to test how much energy there is in a body? or is is a theoretical concept (that obviously helps solving problems, but is unmeasurable)

my guess was that no the second option.. but i want to be sure..

and a second quest.
there are kinds of energy, such as kinetic, which are relative to something else. is all kinds are? if so, what is electronic energy, or spring energy (kx2) are relative to?

Dale
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Energy is definitely measurable. Otherwise the power companies would have a hard time knowing how much to charge . There are many ways of measuring the energy in a system, depending largely on what kind of energy you want to measure and how destructive you want to be.

A.T.
or is is a theoretical concept (that obviously helps solving problems, but is unmeasurable)
Yes. Energy is not measured directly, but derived from more basic quantities.

Conservation of Energy in Thermodynamics postulates the equivalence of work, heat, and energy. The measurement of work is something equivalent to raising a weight against the standard gravity. This means measuring force (weight) times displacement (height change) in a fixed Earth reference frame. Also measuring temperature changes relative to absolute zero (thermodynamic temperature scale).

Heat, work, and energy are equivalent based on experimental measurements within the error of measurement, and these are theoretical concepts derived from the basic measurements. There is no way to find absolute energy because measurements require a reference frame (absolute zero temperature does imply zero heat energy, in theory).

Energy is definitely measurable. Otherwise the power companies would have a hard time knowing how much to charge . There are many ways of measuring the energy in a system, depending largely on what kind of energy you want to measure and how destructive you want to be.

Yes. Energy is not measured directly, but derived from more basic quantities.

first of all, see how energy is a complicated concept, you don't even agree!

"but derived from more basic quantities"
but still, mv^2/2, that depends on the reference frame, so you can say some body has 40J kinetic energy or 90J kinetic energy, and both will be true. (for example, 20kg body in a train that go north in a constant velocity of 1m/s. the body is also going north in constant velocity of 2m/s relative to the train. if your reference frame is the train, the energy of the body is 40J, but if you reference is earth, the energy of the body is 90J).

my point is, energy is not some property of a body.. it's a theortical concept.

Conservation of Energy in Thermodynamics postulates the equivalence of work, heat, and energy. The measurement of work is something equivalent to raising a weight against the standard gravity. This means measuring force (weight) times displacement (height change) in a fixed Earth reference frame. Also measuring temperature changes relative to absolute zero (thermodynamic temperature scale).

Heat, work, and energy are equivalent based on experimental measurements within the error of measurement, and these are theoretical concepts derived from the basic measurements. There is no way to find absolute energy because measurements require a reference frame (absolute zero temperature does imply zero heat energy, in theory).
all the kind of energy?
what about electronic potential energy? what is it relative to? it seems absolute but i guess it not..

russ_watters
Mentor
first of all, see how energy is a complicated concept, you don't even agree!

"but derived from more basic quantities"
AT's point while true is really a minor qualification. Measuring energy requires measuring two quantities and combining them to get energy. In the case of electrical energy, that's voltage and amperage.
but still, mv^2/2, that depends on the reference frame, so you can say some body has 40J kinetic energy or 90J kinetic energy, and both will be true.

my point is, energy is not some property of a body.. it's a theortical concept.
That means energy is relative. Don't make the mistake of thinking that that makes energy not real. For an example, when a car collides with another car while moving, the energy of the collision is different from if the car collides with a wall. The energy of the collision is relative to the speed difference between the objects.
all the kind of energy?
what about electronic potential energy? what is it relative to? it seems absolute but i guess it not..
It is relative to a ground, which is assumed to be at zero voltage, but doesn't have to be.

Dale
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first of all, see how energy is a complicated concept, you don't even agree!
I do agree with A.T. In fact, I would go further than that and say that most things are "not measured directly". For example, distance is not measured directly (in the SI system) but instead the time for a light beam to travel the distance is measured and then the distance is obtained by multiplying by the speed of light.
but still, mv^2/2, that depends on the reference frame, so you can say some body has 40J kinetic energy or 90J kinetic energy, and both will be true. (for example, 20kg body in a train that go north in a constant velocity of 1m/s. the body is also going north in constant velocity of 2m/s relative to the train. if your reference frame is the train, the energy of the body is 40J, but if you reference is earth, the energy of the body is 90J).
Yes, energy is relative.

my point is, energy is not some property of a body.. it's a theortical concept.
Typically you would say that energy is a property of a system. But I don't understand your point about energy being a theoretical concept. All physics concepts are theoretical concepts, but the theory matches well with experiment so it accurately describes and predicts the behavior of nature.

all the kind of energy?
what about electronic potential energy? what is it relative to? it seems absolute but i guess it not..
The scalar potential in one reference frame transforms into a vector potential in another reference frame, and a charge in one frame transforms into a current in another. So yes, it is relative also.

A.T.
But I don't understand your point about energy being a theoretical concept. All physics concepts are theoretical concepts,
Yes they are all abstract. But some (like distance) can be observed directly by humans some (like energy) not.

Any object has "energy" until entropy is maximized. There are two basic types: external and internal. External includes all types of kinetic and potential, electrical (e.g., capacitive (1/2CV2 and msgnrtic, inductive (1/2LI2) and mechanical (mgh and 1/2 mv2), pressure (in a gas)). Internal includes chemical (inter and intra molecular)(energy of oxidation (external oxygen)), Phase change, Heat of fusion (water->ice), thermal (temperature relative to surroundings,, etc.) and nuclear (fission/fusion).

Bob S

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Dale
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Yes they are all abstract. But some (like distance) can be observed directly by humans some (like energy) not.
I disagree completely with this. Auditory receptors are sensitive to acoustic energy at least as "directly" as proprioceptors are to distance. Similarly with the retina being sensitive to electromagnetic energy in the visible range and skin receptors being sensitive to thermal energy.

Personally I think your original point was better. Almost all measurements are a synthesis of multiple measurements. This is certainly true with the human perception of distance which is a synthesis of various muscle and joint position receptors and prior knowledge of the body geometry.

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Dalespam's post #8 is right on: it's silly to define "abstract" as anything we can't sense directly. That might mean light is "tangible", X-rays are "abstract". Or something at 98.6degrees is "abstract" because we might not feel it while all other temps are "tangible".

In any case, energy is as "tangible" or as "abstract" as space, time, mass, etc, etc....we just think we intuitively know what some are....who would think "mass" like a rock or wood or lead is 99.9.....% empty space??

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But when you see light, you don't measure the energy, you are measuring the electric field. Same with sound energy, you don't measure the energy in a sound wave but the pressure.

A higher frequency wave, and a more intense wave are two different experiences to a human eye, but the energy of these may be the same...

Dale
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But when you see light, you don't measure the energy, you are measuring the electric field. Same with sound energy, you don't measure the energy in a sound wave but the pressure.
I am not sure that either of those are correct nor that they are a distinction with any meaning. In both cases the energy is proportional to the square of the amplitude, and the neural response is a non-linear function of either.

A.T.
Yes they are all abstract. But some (like distance) can be observed directly by humans some (like energy) not.

I disagree completely with this. Auditory receptors are sensitive to acoustic energy at least as "directly" as proprioceptors are to distance. Similarly with the retina being sensitive to electromagnetic energy in the visible range and skin receptors being sensitive to thermal energy.

You are describing technicalities and making the trivial point, that you can compute the energy of any process. By "directly observing by humans" I meant something completely different, something that happens in the brain, not somewhere in the retina: The conscious recognition of abstract quantifiable properties.

For example, length: You see two completely different objects, and you know which one is longer.

For example, time: You hear two different sounds, and you know which one lasted longer.

These senses might be inaccurate measurement devices with a limited range of measurement, but they are built in into humans. Yet we don't have such a sense for energy, that would tell us how the potential energy of that apple on the tree compares to the light energy it receives per day.

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Pilots and race car drivers with greater skill also have a greater sense of energy. Bringing this up to the conscious level is powerful, yet birds of prey also use energy much like a pilot, and glider pilots are now learning much about effective energy strategies from birds.

The shape and skin of a dolphin reduces drag in water far more efficiently than anything we can build ... a natural energy optimization adaptation.

The shape of a drop of water in space is spherical to minimize the energy of surface tension.

It would appear that energy optimization is a property of physical and biological adaptation, and it is very likely that we have both an intuitive biological and a conscious grasp of energy as a biological species.

I'm not sure the debate over what is tangible versus intangible/abstract is not a mere matter of personal judgment and interpretation of terms (semantics). But it is an interesting thread.

russ_watters
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Yes they are all abstract. But some (like distance) can be observed directly by humans some (like energy) not.
Drop a paperclip and an anvil on your foot and then see if you still feel that way...
For example, length: You see two completely different objects, and you know which one is longer.

For example, time: You hear two different sounds, and you know which one lasted longer.
You drop two different objects on your foot and you know which had more energy. You put your hand in two different glasses of water, feel the difference in temperature and know which has more energy.

Note also that temperature is a measure of average kinetic energy of a substance.
These senses might be inaccurate measurement devices with a limited range of measurement, but they are built in into humans. Yet we don't have such a sense for energy, that would tell us how the potential energy of that apple on the tree compares to the light energy it receives per day.
Using the same terms you used, it can be said that our eyes, ears, and touch receptors all sense energy as directly as our eyes sense length.

And the fact that we aren't able to convert between different forms of energy doesn't mean any more than the fact that we aren't able to compare certain distances, such as the distance between you and the apple and the distance between the apple and the sun.
You are describing technicalities and making the trivial point...
IMO, it is you who is splitting a hair and adding a technicality to the issue that is confusing the OP.

The OP wants to know if energy is real. How have you helped convey the answer to that question?

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A.T.
Drop a paperclip and an anvil on your foot and then see if you still feel that way... You drop two different objects on your foot and you know which had more energy.
Yes, yes, we also taste energy with our tongue and smell it with our nose because the chemical reactions involve energy. You could just as well argue that we smell time, because the chemical reactions have a duration
Using the same terms you used, it can be said that our eyes, ears, and touch receptors all sense energy as directly as our eyes sense length.
Eyes do not sense length. The concept of length is created in our brains. And unlike with energy we don't need to be taught the definition of it. That was my point in the prev. post which you completely ignored, by talking about receptors again.
And the fact that we aren't able to convert between different forms of energy doesn't mean any more than the fact that we aren't able to compare certain distances, such as the distance between you and the apple and the distance between the apple and the sun.
Different forms of energy assigned to completely different phenomena is a different level of abstraction, than length which has only one form and can always be measured in the same way. And the distance between the apple and the sun is just a matter of the range of measurement, which isn't infinite in most devices.
The OP wants to know if energy is real. How have you helped convey the answer to that question?
In post #8 I already stated that all physical quantities are abstractions making them equally real / unreal.

is energy is something tangible?
No. (In particular, different observers may disagree on whether energy is transferring from A to B or vice versa.)
is it possible, with some (theoretical maybe) test machine to test how much energy there is in a body?
Yes, use a bathroom scale. (Energy is proportional to inertia, E=mc2, though it might be more useful to know how much can be thermodynamically harnessed.)
there are kinds of energy, such as kinetic, which are relative to something else. is all kinds are?
Kinetic energy of (the centre of mass of) an isolated system (as a whole) isn't physically important, and subtracting that, even (total) kinetic energy of the individual moving parts of a system is frame invariant (not relative).

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different observers may disagree on whether energy is transferring from A to B or vice versa.

Is this based on the arbitrary choice of sign convention for work done on or by a system? If not, could you provide an example of this type of observation?

Do you regard E = mc2 as indicating a relative or absolute amount of energy? My intuition says it is relative if only due to the fact that mass and the speed of light require a unit scheme which may be arbitrarily imposed (by conventional custom).

Again, if the kinetic energy of an isolated system is computed by an observer, say, moving with the center of mass, is this not a relative measure anyway when calculated because mass and velocity are specified in a conventional unit system, which is relative?

In other words, it is my understanding that the definitions of physical quantities and laws of physics do not change, yet there is no way to determine the absolute energy of a system because the reference frame and the unit system are both arbitrarily imposed. I would find a compelling explaination to the contrary to be of interest.

It seems to me that ALL kinds of energy that we measure had to be 'derived' (obtained) from mass.

Dale
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You are describing technicalities and making the trivial point, that you can compute the energy of any process. By "directly observing by humans" I meant something completely different, something that happens in the brain, not somewhere in the retina: The conscious recognition of abstract quantifiable properties.

For example, length: You see two completely different objects, and you know which one is longer.

For example, time: You hear two different sounds, and you know which one lasted longer.

These senses might be inaccurate measurement devices with a limited range of measurement, but they are built in into humans. Yet we don't have such a sense for energy, that would tell us how the potential energy of that apple on the tree compares to the light energy it receives per day.
To me your reasoning remains unconvincing, everything you say about distance seems to me to apply to energy also. However, I am not interested in pursuing it further. Regardless of the (unimportant) biological/cognitive disagreement we may have I think that we both agree on the (important) scientific facts:

1) Energy is part of all successful theories of physics
2) Those theories have been experimentally verified through an enormous amount of experimental data

A.T.
I think that we both agree on the (important) scientific facts:
1) Energy is part of all successful theories of physics
2) Those theories have been experimentally verified through an enormous amount of experimental data

Sure, but I think more relevant to the OPs questionis this:

3) Energy, like all other physical quantities, is an abstract concept, and merely a property assigned to different observed phenomena, not a phenomenon itself.

If you state it like this, you never have to wonder why completelly unrelated phenomena can all be quantified by energy. I think the confusion arises from sentences like this:

"Light is a form of energy"

To me it is like saying:

"A ruler is a form of length"

It is just semantics, but energy / length are just properties of light / ruler, not the generalisation of them. The above statments confuse observation and abstract concepts used to quantify it.

Dale
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Sure, but I think more relevant to the OPs questionis this:

3) Energy, like all other physical quantities, is an abstract concept, and merely a property assigned to different observed phenomena, not a phenomenon itself.

If you state it like this, you never have to wonder why completelly unrelated phenomena can all be quantified by energy. I think the confusion arises from sentences like this:

"Light is a form of energy"

To me it is like saying:

"A ruler is a form of length"

It is just semantics, but energy / length are just properties of light / ruler, not the generalisation of them. The above statments confuse observation and abstract concepts used to quantify it.
I agree with this (except the word "merely" in your point 3). The more correct way of saying it is "light has energy" and "a ruler has length".

I do not say 'abstract concept, and merely a property... ' as DaleSpam.

In this Universe I see a distinction between four kind of 'top level entities'

-- entities with Mass (usually some form of 'rigid bodies')
-- entities MassLess (with Energy and rets mass=0), photons ...
-- entities Field (gravito/electomagnetic..)
-- entity Space, (vacuum, field, aether,... 'the stage'...)

All of them have Energy and this property is the common denominator to all entities.
We can not have an entity without energy.

The Universe evolves because entities exchange energy.
The all universe is about energy.

Any TOE needs to derive all entities from just one. (not in my time)

3) Energy, like all other physical quantities, is an abstract concept, and merely a property assigned to different observed phenomena, not a phenomenon itself.

If energy is an "abstract concept", if you buy batteries for your flashlight, but when you get home you find that the batteries are dead, you don't have much of a case to demand your money back. It's no one else's responsibility that you have decided to form a particular concept. If your own mind decided to form the concept "dead batteries" instead of thinking about, say, Hegel's theory of the Absolute, or any other conceivable concept, that's your own choice.

This reminds me of the famous debate between George Berkeley and Samuel Johnson in the 1700s. Berkeley talked for a long time about the absense of any evidence for the existence of a physical world, since all we ever perceive are sense impressions. When it was Johnson's turn, he got up, kicked a stone with his toe, said, "I refute it thus," and sat down.