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B What is energy?

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  1. Jan 11, 2019 at 3:29 PM #1
    While I was doing some practice questions it hit me "what is energy". I also don't understand how the formulas for potential and kinetic energy were derived. Was it due to their definitions or is there some reason behind their respective formulas.
     
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  3. Jan 11, 2019 at 4:09 PM #2

    BvU

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    Energy is a measure for work: if you do some work to move a book from the bottom shelf to the top shelf, the difference in potential energy of the book is the work you have done. Establishing a zero point for energy is awkward because in the real world all we have is energy differences.
     
  4. Jan 11, 2019 at 4:10 PM #3

    Merlin3189

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    An interesting question to which I do not know the answer! My recollection is that I first thought about energy when learning about Joule and work-heat equivalence. But maybe I'd met KE before and not thought much about it.

    If you do have a concept of KE, then PE seems obvious. When an object falls, it gains speed and hence KE due to the acceleration caused by gravity. So when an object is high up, it has the potential to gain KE by falling. Or if an object has speed, KE, then it has the potential to rise up against gravity.

    Quantitatively the change in KE with height when moving in a gravitational field works out at mgh, so that becomes the PE.
     
    Last edited: Jan 11, 2019 at 4:19 PM
  5. Jan 11, 2019 at 4:15 PM #4

    PeroK

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    There are many videos online on the subject "what is energy?". Perhaps what they all have in common is that it is not, in general. easy to define. One simple approach is to leave it essentially undefined, but list the different forms it can take: kinetic, potential, chemical, thermal, nuclear, radiation.
     
  6. Jan 11, 2019 at 5:09 PM #5

    Dale

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    There are several equally valid definitions used in different branches of physics. In Newtonian mechanics energy is defined as the capacity to do work. In thermodynamics an approach like @PeroK is used, energy is defined as kinetic energy (##\frac{1}{2}mv^2##) and anything that can be converted to kinetic energy. In Lagrangian mechanics it is defined as the conserved quantity associated with time translation symmetry of the Lagrangian.
     
  7. Jan 12, 2019 at 4:11 AM #6
    Do all forms of energy manifest as motion?
     
  8. Jan 12, 2019 at 4:17 AM #7

    PeroK

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    Can you think of any that don't? See the list in post #4.
     
  9. Jan 12, 2019 at 4:32 AM #8
    No
    kinetic = Motion
    potential = potential to move
    chemical = Motion caused by interactions of atoms / particles
    thermal = Motion of heat energy
    nuclear = motion of particles and subatomic
    radiation = Motion of photons
     
  10. Jan 12, 2019 at 4:37 AM #9

    PeroK

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    I might question whether potential, chemical and nuclear are directly "motion".
     
  11. Jan 12, 2019 at 6:42 AM #10

    Dale

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    No. KE and thermal energy of an ideal gas are motion. Thermal energy of other materials would have some of its energy in internal degrees of freedom corresponding to motion, but could also have non motion internal degrees of freedom if available. Most other forms of energy are not necessarily manifested as motion, although of course they can all be converted into KE. For example, electromagnetic energy can move (radiation) but you can also have static electromagnetic fields which hold energy without moving.
     
  12. Jan 12, 2019 at 6:52 AM #11

    jtbell

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    Does your textbook say anything about this stuff? It would help if we can use it as a starting point for discussion. Is this a grade-school or university textbook? The university level textbooks that I've used all make at least some attempt at this.
     
  13. Jan 14, 2019 at 2:16 AM #12

    A.T.

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    A quantity that is conserved in an isolated system over time.

    All the formulas for energy where defined such that the total energy is conserved in an isolated system over time.
     
  14. Jan 14, 2019 at 6:52 AM #13

    jbriggs444

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  15. Jan 15, 2019 at 2:21 AM #14
    That is when work is being done, something is converted to KE. What work doesn't involve moving something?
    No work is being done with the static field (PE), when work is done the field will move(KE) the charge or whatever is being moved/worked/energized?
     
  16. Jan 15, 2019 at 5:57 AM #15

    Dale

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    None. ##dW=F\cdot dx ##

    Sure, but it is still energy. The question was “do all forms of energy manifest as motion” not “do all forms of work manifest as motion”. Work and energy are closely related, but they are not the same thing.
     
  17. Jan 15, 2019 at 7:24 AM #16
    Yes in a stable configuration.
    So all forms of work manifest as motion? Judging by your "None. dW=F⋅dx" reply, then yes?
    And I need to look into my understanding of work and energy.
    Thanks
     
  18. Jan 15, 2019 at 11:45 AM #17

    Dale

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    Yes, the dx term is motion of the object on which work is done.
     
  19. Jan 16, 2019 at 9:45 PM #18
    Good question. I think you should hang on to it for a while. We can measure energy with incredible precision. We can describe its permutations with strict accounting. I don't know that we can say 'what' it is for the time being.
     
  20. Jan 16, 2019 at 10:02 PM #19

    Dale

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    Sure we can. I gave three definitions for what it is in post 5.
     
  21. Jan 16, 2019 at 10:30 PM #20
    truth -- the quality or state of being true.
    • that which is true or in accordance with fact or reality.
    • a fact or belief that is accepted as true.

    Certainly, you give a workable definition. Regards.
     
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