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Energy and light

  1. Jun 24, 2012 #1
    Hi all.

    I've decided to take a physics course here and on youtube :-) (as uni it too much)
    Really enjoying this site, but dare not post to much as don't want to sound stupid.
    But anyway, first post and hope my question is not too stupid.

    Could it be said that Light is pure energy?
    And would this energy be the same energy that is in all matter and electrons, etc.
    Essentially is the energy all the the same and everything is made from it?
    from a photon of light to "what comes out of" split atoms/quarks/etc?

    Or are there different types of energy? (i.e there are no photons in electrons or mass, etc)
    (also is this a Special Relativity question, was not sure where to post it)

    I would love to write you all some nice formulas, but i'm not quiet there yet :-)

    Best Regards
  2. jcsd
  3. Jun 24, 2012 #2


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    Staff: Mentor

    Welcome to PF!
    If in doubt, ask! Never be afraid of looking stupid or silly! Once you realize that everyone here has to ask questions to learn anything then it's no big deal!

    Careful of falling into the trap of "popular terms" as I call it. What I mean is that many of the terms used in science are completely gutted and their true meaning is lost when they are used in public media such as most movies, books, etc. Energy is one of those.

    The definition of energy is generally "a conserved quantity that is a measure of the ability of one system to perform work on another system". Most people have this idea of energy being a "thing", much like an electron or a rock is. It turns out that when we do our calculations and such, we have this quantity that we can use to describe how much work a system can do. For example, if a falling rock would be able to do 10 joules of work we can say that it has 10 joules of energy without having to wait and measure what work is actually done. In this sense it doesn't really seem right to me to say things are "made of energy".

    A good example is light. Light is very commonly said to be made of energy, or it is "pure energy" as you put it. Light is an EM wave which consists of an electric and magnetic field that alternate from positive to negative and back as the light travels. It can transport energy from one place to another, as in the case of a light bulb, but to say that it is "pure energy" is, to me, similar to saying a water wave is made of pure energy.

    It is true that a system with more energy in it has more mass, and you can convert matter to energy, but like light saying that matter is made of energy is incorrect in my opinion. (How can something be made of "the ability to perform work?).

    It is correct that there are no photons inside an electron and other particles. As for types of energy, there are multiple types of energy that are commonly used, such as kinetic energy, chemical energy, mechanical energy, potential energy, electrical energy, etc. But all of these are still the same "energy" in that all of these merely describe the ability of one system to perform work on another system. The different names serve only to distinguish one type of system from another and the source of the energy.
  4. Jun 25, 2012 #3


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    Gold Member

    jonny512379, Welcome to PF :)

    Here is an excellent explanation of the different types of energy. Copy it into your computer memory and create a folder of “Basic Science References” and put this in there for your study and future reference. The answer to your above questions are all clearly on this webpage. If you have any more doubts or questions, come right back here and ask away. Members will be happy to help you on your journey to understanding of our natural world.

  5. Jun 28, 2012 #4

    Thanks for the replies, they are all great.
    I agree the nmsea.org descriptions of energy are great and easy to understand.
    (1 quick unrelated questions that it mentions but does not explain, is the formula for kinetic energy of "E kinetic = (1/2) m v2". my question is; why is it only half the mass, where is the rest used? and is this half an approximation?)

    I think i may be failing in to the trap of popular thinking, or have my own meaning of "energy" which is not of much use if no one else uses this understanding (and I'm probably wrong to do so of course). i was thinking along the lines of "matter can not exist without some form of "energy" " and in that was using energy as a more mythical unseen force than i probably should not be. maybe it would be better described as "the force" but that's getting a little to Jedi for real science (and me) i suspect.... :-)
    Light is the one that fascinates me and has caused me many sleepless night trying to work out a better understanding of it (for myself that is). I do have my own opinions, and from what i read, now i need to find out why i am wrong!

    Thank you again for the replies, ill keep reading and causing myself more sleepless nights
  6. Jun 28, 2012 #5


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    Staff: Mentor

    At non-relativistic velocities, aka at velocities much lower than the speed of light, the formula works fine. When you approach about 20% c the formula starts to become more and more inaccurate since it doesn't include relativity. It isn't that half the mass is being used, it's that you multiply mv2 by 1/2. You could just as easily do m1/2(v2) except it would look funny.
    Light. An electromagnetic wave! It's a wave! (Except when it's a pesky particle!)
  7. Jun 28, 2012 #6
    Thanks. i kind of understand about you :-).ill read the pages about energy again!

    Yes i understan light is a wave,maybe im overthinking it....but is that not just how we observe it? why would it travel at c?
    Sorry i dont mean to question as im sure ill answer my own questions the more i read.
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