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Faynman talks about atoms repelling and attracting each other

  1. Feb 16, 2012 #1
    In this vid, Faynmen explains what a fire really is:

    just watch from start till 1:22


    He says that sometimes carbon repels air(O2) and sometimes they attract each other. If they attract, they start a fire. Whether they do repel or not is dependent by the distance between them or their kinetic energy. His analogy is a ball rolling up the side of a volcano, if the ball isn't pushed hard enough the ball goes back again. If its pushed hard enough the ball falls into the hole of the volcano.

    Is the force repelling and the force attracting the same force? (electromagnetic??) If so how does this work? What makes this one force do two opposite things?

    I know this might belong in the chemistry section...
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Feb 16, 2012 #2

    sophiecentaur

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    Each atom has a positive charge (protons) at its middle and a negative charge (electrons) on its outside. Individual molecules and solids are held together by the fact that the net energy of attraction between the positive and negative charges is greater than the energy of repulsion. The total Potential is said to be negative.
    When two atoms come close together, the negative charges of each are nearer to each other than they are to the other atom's positive charge. If they are just 'sitting there' the repulsion between each atom's electrons will keep them apart. To move them closer together would require some energy to be introduced (as in squashing a spring).
    If they come together fast enough (with enough Kinetic Energy - often called Activation Energy)) AND if the resulting molecule would happen to be stable, then the energy is sufficient to overcome the (positive) energy of repulsion and the electrons then come under the influence of the positive nucleus and are attracted. The electrical potential energy is then negative and you have a molecule. The equivalent to pushing a boulder up and over the lip of a deep volcano.
    In the case of an Exothermic Reaction, the resulting drop in the energy of the combination has to go somewhere and manifests itself as thermal energy because more energy comes out than you put in as KE.
    In the case of an Endothermic Reaction, the atoms still stay stuck together but the KE that you put in is greater than the overall change in Potential energy so thermal Energy is actually lost from the surroundings. (It takes heat out)
    In Thermal Decomposition, if you supply enough Kinetic energy (collisions from nearby fast molecules) the molecule can be knocked apart into its constituent atoms.

    It, of course, gets more complicated but that is the basics of it. Interestingly, there is a similar set of circumstances which determine when atomic nuclei come together with a bang, whether they will bounce off or fuse together.
     
  4. Feb 16, 2012 #3
    Thank you;) Im reading now, trying to understand everything you said. Perhaps Ill ask another question about it later when i know what to ask.
     
  5. Feb 16, 2012 #4
    What does this mean?
     
  6. Feb 16, 2012 #5
    You know the Internet cable that plugs into the back of your computer? It's called an RJ-45 connector.

    When you plug it in, it resists a little because the plastic spring tab needs to be pushed down as you push the connector in.

    Once you've pushed it in far enough the plastic spring clicks into place and now it would take even more force to pull them apart again.
     
  7. Feb 16, 2012 #6

    sophiecentaur

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    Negative potential means you get energy out as you get closer (i.e. like rolling down into a valley). Positive Potential needs energy put in to get closer - (like needing to be pushed up towards a peak).
    All Attractive systems (Earth's gravity etc) have negative potential. Repulsive systems (Two similar charges) have positive Potential. It has to be that way round to account for things gaining Kinetic Energy as they fall towards Earth (as height gets less).
     
  8. Feb 17, 2012 #7
    Im assuming this is an analogy, or isnt it? Confused becouse i know there is an electromagnetic force holding the plastic together..
     
  9. Feb 17, 2012 #8

    sophiecentaur

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    Of course it's an analogy. Every model you will ever hold in your head will be an analogy. If you can accept that than you may feel less bad when you don't feel you've understood something.
     
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