Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Unit Joule is the unit of work and of kynetic energy

  1. Jul 23, 2011 #1
    1. The problem statement, all variables and given/known data
    That is what I learnt, and that it is expressed in newton*meter.
    Then I learnt that
    G is equal to 6.47 x 10^ -11 joules and that

    h is equal to 6.6262 x 10^ -34 joules x sec (N*m*s)and that
    one photon of light (say red) is equal to 4x 10^ 14 Hz(frequency) and its energy is

    E (light) = h * f, so it has energy= 4x 10^ 14 h (N*m*s)

    If these data are correct, can you explain to me

    1) what kind of energy is the energy of light[/I] , what is it called,
    is it just Kynetic Energy (* sec)? or has it got its own definition?
    edit "should I call it just energy of light"?
    in EMR ( R stands for , not energy)
    "is it electromagnetic energy?
    (hope it is clear now what I am asking for)

    2) how do you relate h to KE?
    A photon has E= Joules* sec
    an electron ( in motion has KE = 4x 10^14) Joules .
    why its energy is not Joules*sec,
    hasn't it got the same energy of a red-ray photon?
    the same energy every sec? they both run every second and indefinitely!
    what is the difference between the two energies?

    (when a ray meets an electron in Compton scattering its energy is transformed in the sane amount of KE, so what is the difference?

    Likewise an electron dropping from an orbit changes its KE into light
    how do you account for the chance of unit?)

    3)and how do you relate h to G?
    is h (Nms)= G(Nm)* sec?

    3. The attempt at a solution
    Last edited: Jul 23, 2011
  2. jcsd
  3. Jul 23, 2011 #2


    User Avatar

    Staff: Mentor

    A single photon in that beam of red light carries an energy = h.f

    If you were to shine a beam of light onto a black rock, the rock would heat up as it absorbs the light energy. That's energy being transferred from the photon to the solid. The SI unit of energy is the Joule.

    It carries that energy every second, every minute, and for perpetuity, if it loses none. If it bumps into something, and transfers all its energy, it does 4 x 1014 Joules of work, no more, and no less.

    A ray of light carries energy, and it only loses some when it transfers it to another body, such as when it's absorbed by a dark body. The dark body can absorb visible light, become heated, and radiate infrared light. A beam of light is a stream of photons, and each photon is like a parcel of energy.

    Difference in what?
  4. Jul 23, 2011 #3
    hallo NascentO.
    Thanks for you post.Probably my thread was not clear.I have thoroughly edited it.I hope you or some other helper can give me the three answers?
    I'll give an example of the shortest answer

    1) energy of a photon is **** energy

    2) an electron and a photon moving alongside with (samevalue) energy
    are different in that ********
    a photon hitting an electron at rest (***divides its energy/ sec)
    an electron dropping to lower orbit changes its KE (multiplying by sec)

    3) G relates to h in the ( ***sameway)

    I do hope someone can give me more detailed info
    thank you
    Last edited: Jul 23, 2011
  5. Jul 24, 2011 #4


    User Avatar

    Staff: Mentor

    The newton and the joule are 'compound units', names for specific groupings of fundamental units that are assigned because they represent particularly useful quantities in physics. The newton is a unit for what we call force, and the joule is a unit for what we call energy, in whatever form it may take. Energy is the ability to do work.

    Energy can reside in systems in a number of ways (gravitational potential, chemical potential, kinetic, heat, light, etc.). No matter what form it takes its quantity is expressed in units of joules.

    The joule, being a compound unit, can be formed from other units in a variety of ways. Often these ways of combining units can tell you what sort of forms of conversions are going on between 'types' of energy (by 'type' of energy we mean how it is stored in the system -- we give different ways that energy is stored different names, like 'potential' or 'kinetic', but it is all just energy with the same units and is inter-convertible from one form to another). So, for examples, N*m yields joules, and so does kg*m2/s2. The first example is typical of work done by a force acting through some distance, and the second example is typical of the kinetic energy attributed to a moving mass.

    No, that's not correct. G has units of m3*kg-1*s-2. If you wanted to squeeze joules out of that you'd get something like J*m*kg-2. I can't off-hand think of a useful interpretation for that grouping of units.
    Don't forget that frequency has units s-1. So the energy of a photon, being given by h*f, has units (J*s)*(s-1) = J. That's just Joules, as it should be for energy!

    The bottom line is that all energy is measured in units of joules (in the unit system we're discussing here). All energy 'types' are really the same thing: the ability to do work -- and are inter-convertible via various physical processes or interactions within a physical system. Labeling energy as 'potential' or 'kinetic' doesn't change this; it's just a handy way to remind ourselves how the energy is stored in the system and what sort of process might be required to 'get at it'. For example, to get at potential energy stored in a field (such as gravitational or electric fields) you might have to move something from one location to another in that field. To get at the energy in a photon, you have to have the photon interact with an electric charge such as the charge on an electron (which may be in an atom, and thus cause the electron to change its energy level there,... ).

    Regarding what to call the energy held by a photon, since the photon has no rest mass you can't really call the energy of a photon kinetic energy. It's more like a 'portable potential energy'. It's sort of like energy being bundled into storage at one end (emission) and then unpacked and converted back to kinetic energy at the other (reception).
  6. Jul 24, 2011 #5
    Don't forget: volt-amp-second, volt-ohm squared-seconds. XD
  7. Jul 24, 2011 #6
    That is an excellent explanation,there are some dark points.Let's start from these,
    maybe you'll clarify everything

    1) I read everywhere that Joule is unity of WORK, and I've been corrected for saying energy
    and that confused me.Now,
    Why can't we say that h is the unity of energy, then?
    as you say....in....
    6) that energy is packed in a photon once and for all,
    why should h be energy(Joule)*sec. ,

    also a ball gets its energy(kynetic=joule) only once
    the difference is only that a photon has N herz every second
    and a ball has N meter every second
    why must frequency affect dimension ??

    a photon has/carries the same energy of a mass! .That really beats me

    I'll tell other points after your reply
    Thanks a lot!
    Last edited: Jul 24, 2011
  8. Jul 24, 2011 #7


    User Avatar

    Staff: Mentor

    Since energy is the ability to do work, work is measured in energy units. So many joules of energy = so much work that it can do. Typically, when we do "work", we're converting energy from one form to another: such as kinetic energy to potential energy, or to heat energy."
    Technically, the units of h, being J*s, form a combined unit called "action". It's not joules (energy) alone. Similarly, we don't call momentum velocity, although p = m*v for objects with rest mass m.
    In the case of photons, yes. Frequency matters. The higher the energy content of a photon, the higher its frequency. That's just how the 'packaging' works.
    You're going to love Relativity, where it will be shown that there is an equivalence relationship between mass and energy :smile:

    As previously stated, what we call "energy" is the ability to do work. As such, it's more of a concept than something you can hold in your hand; you can't put kinetic energy or potential energy under a microscope and and say "Aha! That's what energy looks like".
  9. Jul 25, 2011 #8
    Hi Gneill, you must have read my mind !, I have just read the thread "momentum is (quantity of motion) =...." and I was shocked,
    at the question : 5 children have each 10 sweets, multiplying we get 50 what ?, .... everybody answered ....50 children*sweet!
    Do you subscribe to that?, it is incredible to me!..much worse than GR!

    I was going to send a post, but please (take a look there and) tell me if I'd make a fool of myself if I replied:

    "sir, I do not know if you were(/ are) a schoolteacher if you would teach/ expect that from your students. If you mean that there are two maths, one for schoolboys and one for others, you destroy my belief in maths.

    I have been taught that multiplication is just repeated addition , and it is under the domain of set theory.
    So we have n (5) sets (children) with m (10) elements (sweets) each, multiplying m*n we get the total number of elements 50 (sweets).

    If we change dimensions (child => mass , sweet => v) we must get the same result 50 (vectors)

    Of course one is free, for a particular reason, to create a new, derivative dimension, but if it is not a basic, intuitive one (like lenght, time, speed...) he should define it.

    Is my argument sound or stupid? could I send such post?
    Thanks, tou are a great Helper!
    Last edited: Jul 25, 2011
  10. Jul 25, 2011 #9


    User Avatar

    Staff: Mentor

    Physics is all about models of the world and the details and keeping track of relevant properties and their associations in those models. We do this with units assigned to things. Mathematics happens to be a handy way to keep score. Fortunately, mathematics is not just about counting.

    The question is, is there a relevant difference between children with sweets and children without, and do the particular number of sweets per child matter as well? I suppose it depends upon what it is you're trying to model. If it's just a sweet-counting exercise and only the sweets matter, then the children are irrelevant: they could be replaced by chairs or bottles without affecting the outcome. If you're modelling a particular disease associated with blood sugar levels, or anxiety levels in a population of children, then perhaps the units child*sweet or sweet/child could be useful.
  11. Jul 25, 2011 #10
    You are quite right, Gneill.
    I do not know if my analisys is correct, but
    I view Mass exactly like a child or a jar: a vessel , a container

    When you say E = M v^2/ 2 you are stating that each mass carries, contains E= v^2/2
    and if you have 5 masses you multiply E by 5
    I apply the same argument to momentum:
    1 Mass has v=10 m/s , => has ten vectors
    if the body has 5 masses it has, carries 50 vectors
    I can't see any other implication or diagnosis
    Am I wrong?

    II) the point is that in that thread I read an ├Činteresting reference.
    in fact:
    a) If I state that light HAS NO mass
    b) and I state that momentum is M*V .., I have made a sillogism
    and I put myself in a blind alley, it takes more than a century of arguing and mirror-climbing to try to disprove it

    2)this example is completely different so I'll leave it out, if you are interested, we can discuss it.
    You didn't tell me if I may send the post!
    Last edited: Jul 25, 2011
  12. Jul 25, 2011 #11


    User Avatar

    Staff: Mentor

    In physics it's not always a practical thing to do (or even valid!) to treat mass simply as a holder for something like velocity. After all, thanks to the equivalence principle we know that mass is in fact equivalent to energy (E = M*C2) So now you have energy containing energy? And when you change reference frames, the relative velocity changes, so the kinetic energy of the given mass changes; it's different for every different frame of reference all at the same time! So it's not just a matter of counting or a container holding a certain amount.

    And as for momentum, while momentum is always a conserved quantity in physics, neither mass nor velocity separately need be conserved when you allow for E = M*C2. And momentum does not even require mass when it comes to massless particles (photons).

    With that said, there are concepts that deal with things like the energy per unit mass. That's called "specific energy". It can be handy in certain circumstances where the actual mass isn't important but the relative energies are. A practical example is in astrodynamics where orbit geometry depends upon the specific energy of the orbit, not the actual mass of the object in orbit.
  13. Jul 26, 2011 #12
    physical point of view
    1) There is no problem there. Mass is a form of energy, trapped energy.Surely it has different
    We do not know if the exact word is it contains energy or is towed by pure energy , or ......

    Now, no problem, a glass (matter) contains water (matter),( a book does not,. it depends on properties)
    a breakdown truck is not contained by a car, but nevertheless it can tow it, it depends on properties

    2) I don't know why you are ignoring the sillogism.
    if you say photons have no mass
    and add momentum is m*v,
    you are implying that photon has not momentum
    if you add that momentum is sum-total of vectors of a body of mass M , you are not in trouble any more.
    there are all advantages and no disadvantage

    Maths point of view (sorry I can't put the arrows)
    v is a vector and m(***) is a scalar a..... (***was put by the system,foul word)
    multiplying dot you get p, a vector p= a v = 50 vectors .....ergo
    momentum is "quantitas motum" translated in modern English as...

    momentum is the sum-total of the vectors of a body ....ergo....
    .....a photon has momentum

    Is there anything wrong with this?
    Last edited: Jul 26, 2011
  14. Jul 26, 2011 #13

    sorry, Gneill, I forgot those points

    1) if this is a problem it is so independently of the definition of momentum.
    I think it is a false problem.
    if train A and B are running alongside on two tracks,
    A(velocity,Ek...) are different from
    B( v,Ek...) but A()= B() + Δ AB

    2)you are right ! it is a conserved quantity in collision between masses,
    but if you are transforming mass into light you are allowing for a new set of properties.
    as in mass you have variable v,
    in photon you have constant v
    The question remains independently if you allow momentum be m*v or v

    (if you set m=1 you'll see all problems differently
    what is the moment of unitary mass?)
    my question is : why create GMO like children*seets, when this creates only problems?
    3)this point too is covered by the sillogism
  15. Jul 26, 2011 #14


    User Avatar

    Staff: Mentor

    So the discussion is becoming one of semantics or philosophy rather than physics...
    More philosophy. Or perhaps stamp collecting... collecting lists of properties and building up analogies as physics. Very Aristotelian. Besides, anyone who has ever read in the bath knows that a wet book holds plenty of water. :smile:
    And yet photons most assuredly DO have momentum. It's an easily measured property. Since all photons travel at the same speed, c, momentum is NOT velocity. Nor is it mass. It is apparently a separately identifiable quality or property in nature (mathematically it is an "integral of the motion").
    Except that "vector" is a mathematical construct for a quantity that has both magnitude and direction. As such it is not wedded to any particular quantity, quality, or property. There are all sorts of vectors for different quantities: Velocity, momentum, kinetic energy, forces, field lines, and so on. If you examine a moving body very closely, you're not going to see a bundle of vectors associated with it.
    A rather poor translation. Better: "Quantity of motion". Sums it up nicely I think, since at the time the word "momentum" was coined for this property of motion it was recognized that it was not velocity, nor was it kinetic energy, yet it clearly described an "amount" of the motion attributed to a body.

    Claiming that multiplying a vector by a scalar produces multiple vectors rather than simply magnifying the original vector magnitude would seem to be a pointless mathematical exercise. What good are 50 vectors to deal with when the notation and rules say that one will suffice? So you have to write out every one of them and treat them all separately as you solve every mathematical problem? A simple geometry exercise could take reams of paper to write out. Is there a basic quantum of "vector"? Do we have to deal with billions of vectors all the time because the smallest unit vector is minute?
    It would appear to me to be an exercise in semantics with no practical advantages. The mathematical rules of vector manipulation are defined to be consistent and practical. Remember Occam's Razor.

    Playing mathematical games within the rules can be an interesting diversion, but mathematics does not impose its form upon the physics of the world (the map is not the territory). Rather we choose appropriate mathematical tools to help us describe what we measure in the world.
  16. Jul 26, 2011 #15
    All right, Gneill,
    you have shown great patience, with me, I do not wish to try it further
    just a quick note:

    1) when you (one in general, of course) talk of dimensional analysis (DA), of properties
    it is physics, when I do that it is philosophy or semantics.:cry:
    Remember that DA is philosophy, ontology. In Newton's words it is
    "the Great Principle of Similitude"

    What you call (improperly because substance, quality is not quantity)
    quantities,dimensions, units (that is) : TIME,SPACE LENGTH,MASS,SUBSTANCE,...
    you forget that these are nothing but what for XXV centuries cultured people all over the world have called and still call

    categories, according to good, old, dated ,Aristotle. Latin: Summa genera.
    categories,predicaments, ontological predicates, according to Kant

    when you (one) use physical quantity (indicating by that word Space,Length,mass etc..)
    you (one) delude yourself you are talking physics, but you are talking philosophy,ontology because you are handling dangerous weapons such as abstrac ideas, what Kant defines Pure Concepts
    The fundamental equation of DA Q= {Q}[Q] is not at all an equation and needs a lot of philosophy

    2)Again (you and) I are talking semantics. So I'll refrain, abstain, unless you are interested.
    Just a quick (semantics) note 'motion' is more a synonym of speed and not of v
    in the thread I mentioned "momentum is..." there is another good reference. BruceW states
    that Einstein affirmed (ipse dixit !) momentum be velocity.

    3) watch out (for owngoals) Gneill ! :tongue:
    that razor cuts both ways.You are backing my point of view! remember:
    "...entia non sunt multiplicanda praeter necessitatem!"
    That is exactly what I am trying to assert :smile:

    I'll wait for your remarks, and then if you are not interested or intrigued by this issue,
    we'll conclude our primitive discourse about Joule

    Thank you, Gneill you are really a Great Helper! :cool:
    Last edited: Jul 26, 2011
  17. Jul 26, 2011 #16


    User Avatar

    Staff: Mentor

    The thread has meandered away from being a Homework and Coursework topic, and would probably be more at home in a general discussion forum, perhaps General Physics. You might find interested parties to discuss the semantics, etymology, and philosophy of science there. :smile:
  18. Jul 27, 2011 #17
    I'll try to wrap up what I have learnt in this thread:
    correct my mistake, whe you say OK, I'll move on

    1) energy of photon is called just energy and is expressed by unit Joule, as all kinds of energies
    Photon/ light is energy, mass is a form of energy , => all is energy

    if I may, for taxonomic balancing, (in order to have similar coordinate hyponims)
    I'll call light: pure energy or Ea,
    a) so that E => a, k, (m)

    b) the unit of measure of E is J(oule) equal to 1,5 x 10^ 33 h/s
    Energy is a derived or derivative (?)quantity (not a basic concept,category)
    it is not a basic physical quantities like space-length L, time T, mass-substance M

    c) Joule is expressed by basic dimensions of : M*L^2 / T^2

    (isn't it odd, I mean diminishing, that the most important thing in the world is not a basic unit?)

    d) an electron has Em mass-energy of 1.2356 x 10^ 20 h/s.
    when it meets a positron they release (2 x) a photon of the same Em= Ea and Hz= f = 1.2...

    (:uhh:) e) if the particles have Ek it adds up to Hz of photons(?)
    Last edited: Jul 27, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook