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Why does tension increase with distance

  1. Oct 31, 2007 #1
    I don't get why the tension force should increase up to a point in a stretched metal wire as the wire is extended surely whatever attractive force exist within the wire (I am familiar with metallic bonding) should get less as you increase the average distance between the particles.

    I am very happy with compression the idea of reducing the distance between particles in a solid causes the overlap of electron shell hence repulsive forces between the like charges but tension is doing my head in please help.
  2. jcsd
  3. Oct 31, 2007 #2
    Can you explain this a little more? Can you give an example?
  4. Oct 31, 2007 #3
    You're assuming that the interactions between atoms are purely repulsive. They're not (it takes work to make a solid evaporate), perhaps don't think of the interference between electron clouds as always necessarily increasing with distance.

    If you actually plotted the potential energy as a function of separation, it would start very high, go down to a minimum, then rise again to a local maximum before flattening out. The force you must apply at a particular separation is given by the slope of this graph. Near a minimum, the graph gets steeper in both directions.
  5. Nov 1, 2007 #4
    Thanks for the help but I still don't get it. I have seen the plots of potential energy against distance and that the gradient of this generates a force extension plot.

    Now say you've got a 2m long metal wire and you add a weight at the end that causes it to stretch by 0.5mm, there is tension in the wire and this tension supports the weight now if you add more weight such that the extension is now 1.0mm the tension attractive force in the piece of wire has gone up what I don't understand is this if the average distance between the particles has now gone up (albeit very slightly) how does the attractive force between the particles go up? I just can't see how seperating particles more should increase attractive forces? If the source of the tension force is fundamentally electromagnetic why should seperating the particles a bit more increase the size of the attractive force and therefore generate the greater tension doesn't the electromagnetic force have an inverse square relationship with distance so as you increase the distance between particles you decrease the magnitude of the forces between them?
  6. Nov 1, 2007 #5
    You should consider the microscopic structure of a rubber band before and after stretching.
  7. Nov 1, 2007 #6
    Thanks but that doesn't help me.
  8. Nov 1, 2007 #7
    What I asked was so fundamental there must be an answer I've just not managed to find one yet.
  9. Nov 1, 2007 #8


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    Sketch the shape of the potential function, locate the minimum (and note the corresponding tension in the wire), then, stretch, or compress the wire in your mind, and note where you are on the potential function.
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