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!

Heat transfer and elastic collision

  1. Jul 11, 2012 #1
    I found the following:

    We have already seen that the flow of heat through conduction occurs when fast (hot) atoms collide with slow (cool) atoms, transferring some of their kinetic energy in the process. One might wonder why the fast atoms don't collide with the cool ones and subsequently speed up, thereby gaining kinetic energy as the cool ones lose kinetic energy - this would involve the spontaneous transfer of heat from a cool object to a hot one, in violation of the 2nd law. The answer lies in energy and momentum conservation in a collision - one can show, using these two principles, that in a collision between two objects which conserves energy (called an elastic collision) the faster object slows down and the slower object speeds up.

    I believe there are some configurations in which the hot atoms speed up and the colder atoms slow down.
    The energy flows from hot to cold, but it's possible that some energy also flow from cold to hot, naturally. But in the end, the flow is like 99% from hot to cold.

    Am I right or is the statement in bold true, and if so, how can you prove it?
  2. jcsd
  3. Jul 11, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    If you have an elastic collision between a fast object and a slow object of the same mass, the fast object will always lose energy and the slow object will gain energy. With different masses, I would assume that you can make the same statement with kinetic energy, but I did not check this.
    Note that individual atoms do not have a temperature - there are no "not atoms", just atoms with a high kinetic energy.

    Yes. This is related to the velocity distribution of the atoms - some atoms in a cold object are quite fast, too, and some atoms in a hot object are slow. If those hit each other, you transfer some energy from the cold object to the hot one. However, this is rare, the common process is energy transfer hot -> cold.

    In this post, I neglected other energy types which can be present, but the concept is similar for them.
  4. Jul 11, 2012 #3
    Thanks a lot for the answer.

    But look at this example (in the attachment):

    Atom A is slower and atom B is faster. The configuration in the left side is when the atoms start to really feel the repelling forces from each other. If this is the real configuration, then atom B will gain |Vx| and |Vy| so it will get even faster than atom A.

    So, according to this example, hot atoms can speed up and colder atoms slow down. What do you think now?

    I really need someone to support this idea, or explain me why I'm possibly wrong !! :)

    Attached Files:

    Last edited: Jul 11, 2012
  5. Jul 11, 2012 #4
    Anyone? :eek:

    Look at the picture. It seems there are collisions in which the faster object gets even faster than the slower object.
  6. Jul 11, 2012 #5


    User Avatar
    2017 Award

    Staff: Mentor

    Let's see, all units are m/s:
    The center of mass system moves with 5 to the right, 0.5 down. In this, both objects have a velocity of sqrt(5^2+0.5^2). This velocity is the same after the impact, but the direction can be different. In the ideal case, the fast object moves in the same direction as the center of mass system in our lab system. Transformed back, it gets a velocity of 2sqrt(5^2+0.5^2)=~10.05. Which means you are right. Even more: One object can stop completely (maybe a feature of the orthongonal velocity vectors). Unless I made a mistake here.

    However, this is quite unlikely - on average, there is an transfer energy to the slower particle.
  7. Jul 11, 2012 #6


    User Avatar
    Gold Member

  8. Jul 11, 2012 #7
    I didn't quite understand how did you make the calculations but I could see that the faster atom most speed up! If the slower atom comes perpendicularly or from behind, the faster object will speed up.
    I think this could be likely in a system with few atoms. In the real situation, atoms are surrounded in many directions, and so the entropy goes up. Energy flows from hot to cold.

    Another point: You said atom B gained 0.05 m/s, but we could come up with another situation in which the faster atom (B) would gain even more velocity, right?
    Last edited: Jul 11, 2012
  9. Jul 11, 2012 #8
    Thanks. The problem is: I'm talking about collisions in at least 2 dimensions. Calculations become harder. :\
  10. Jul 12, 2012 #9
    I'm looking at your picture, and wondering how two "billiard balls" can collide in such a way. Specifically, you're requiring that the slow moving ball strikes the rear of the fast moving ball. If they are to collide with those precise velocities, would they not collide in such a way that the contact point is on the forward-moving face of the faster ball?

    At the point when they are perfectly aligned vertically, if they have not yet collided, then I suggest that they never will, because the receding ball is travelling faster than the approaching ball, so the separation between the centres will increase, and since they are both circular, this means the minimum separation between the edges will also increase.
  11. Jul 12, 2012 #10


    User Avatar
    2017 Award

    Staff: Mentor

    The whole kinetic energy is transferred, there is no way to make it faster.

    The contact point is the uppermost point of the lower ball. The horizontal velocity remains unchanged, and the vertical velocity is as in a 1-dimensional collision.

    That is not right, as the movement of the slower ball is directly in the direction of the ball separation, while the other ball moves perpendicular to that. Those collisions are possible, too, but the energy of the faster ball is a bit lower there.
  12. Jul 12, 2012 #11
    Right! One last question

    The equation of conservation of energy is:

    Ma*sqrt(Viax^2 + Viay^2) + Mb*sqrt(Vibx^2 + Viby^2) = Ma*sqrt(Vfax^2 + Vfay^2) + Mb*sqrt(Vfbx^2 + Vfby^2)

    and not separated like this:

    Ma*Viax^2 + Mb*Vibx^2 = Ma*Vfax^2 + Mb*Vfbx^2 , for direction x

    Ma*Viay^2 + Mb*Viby^2 = Ma*Vfay^2 + Mb*Vfby^2 , for direction y

    Right? It's a stupid question, I'm just not sure x_x

    Note: Via= initial velocity of body A; Vfb= final velocity of body B...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook