1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The nature energy

  1. Nov 24, 2009 #1
    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?
     
  2. jcsd
  3. Nov 24, 2009 #2

    Dale

    Staff: Mentor

    Energy is definitely measurable. Otherwise the power companies would have a hard time knowing how much to charge :smile:. 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.
     
  4. Nov 24, 2009 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Yes. Energy is not measured directly, but derived from more basic quantities.
     
  5. Nov 24, 2009 #4
    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).
     
  6. Nov 25, 2009 #5
    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.

    all the kind of energy?
    what about electronic potential energy? what is it relative to? it seems absolute but i guess it not..
     
  7. Nov 25, 2009 #6

    russ_watters

    User Avatar

    Staff: Mentor

    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.
    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.
    It is relative to a ground, which is assumed to be at zero voltage, but doesn't have to be.
     
  8. Nov 25, 2009 #7

    Dale

    Staff: Mentor

    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.
    Yes, energy is relative.

    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.

    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.
     
  9. Nov 25, 2009 #8

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Yes they are all abstract. But some (like distance) can be observed directly by humans some (like energy) not.
     
  10. Nov 25, 2009 #9
    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
     
    Last edited: Nov 25, 2009
  11. Nov 25, 2009 #10

    Dale

    Staff: Mentor

    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.
     
    Last edited: Nov 25, 2009
  12. Nov 25, 2009 #11
    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??
     
    Last edited: Nov 25, 2009
  13. Nov 25, 2009 #12
    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...
     
  14. Nov 25, 2009 #13

    Dale

    Staff: Mentor

    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.
     
  15. Nov 25, 2009 #14

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    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.
     
    Last edited: Nov 25, 2009
  16. Nov 25, 2009 #15
    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.
     
  17. Nov 25, 2009 #16

    russ_watters

    User Avatar

    Staff: Mentor

    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. 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.
    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.
    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?
     
    Last edited: Nov 25, 2009
  18. Nov 25, 2009 #17

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    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
    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.
    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.
    In post #8 I already stated that all physical quantities are abstractions making them equally real / unreal.
     
  19. Nov 25, 2009 #18
    No. (In particular, different observers may disagree on whether energy is transferring from A to B or vice versa.)
    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.)
    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).
     
    Last edited: Nov 25, 2009
  20. Nov 25, 2009 #19
    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.
     
  21. Nov 26, 2009 #20
    It seems to me that ALL kinds of energy that we measure had to be 'derived' (obtained) from mass.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The nature energy
  1. Nature of vacuum (Replies: 7)

  2. Nature of energy (Replies: 1)

  3. The nature of time (Replies: 2)

Loading...