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Temperature vs Resistance Relationship.

  1. Jul 23, 2011 #1
    So, my intuition tells me that as a material heats up, the atoms start jiggling around more and more. That makes the atoms got all over the place, effectively causing more space between the atoms. Since there is more "free" space between the atoms that the electrons can travel through, the resistance must go down. However why is it not shown in real life? The resistance goes down as the temperature goes down. Look at super conductors. Since the cooler material's atoms are moving at a slower rate, I must assume that the material is also smaller, therefore less space to squeeze through and more resistance. But at the higher temperature, the atoms are going crazy, figuratively jumping from wall to wall, so there must be more collisions with the electrons. That would bring down the resistance.

    Can anyone point out where my intuition is flawed?
     
  2. jcsd
  3. Jul 23, 2011 #2
    I learned that the thermal expansion is insignificant at an atomic level.
     
  4. Jul 24, 2011 #3
    Who is to say that when a conductor temperature goes up there is more space in between atoms/molecules? There is a reason why materials expand when hotter....because every atom/molecule actually needs more space! It is not like they remain the same size and just move further apart for the heck of it...

    but that is besides the point, it is not a matter of space...it's a matter of energy levels...

    when you want to make an electron move across a conductor, you need to push it with some energy...in a cold conductor, a small amount of energy will make an electron jump easily...but in a hot conductor where an electron has already a lot of energy...the same amount will not do...you need more...this is perceived as higher resistance.
     
  5. Jul 25, 2011 #4
    Resistance isn't about the electrons having no space to move; it's really about them not moving in the direction of the applied field. As a metal heats up, the electrons gain more energy. Since electrons move in all different directions, this increases the amount of collisions and other scattering events that happen, which decreases the net motion of the electrons in the field direction. This causes the increase in resistivity.
     
  6. Jul 25, 2011 #5
    Electrical resistance in metals around room temperature is due mainly to scattering of electrons on the vibrations of the lattice (phonons).
    As you increase the temperature the vibrations became stronger and the probability of electrons being scattered increases.
    The increase in thermal energy of the conduction electrons is quite insignificant even when you heat the metal close to the melting point.

    The average distance between atoms in the lattice increases with temperature but this effect is quite irrelevant for conductivity. The wavelength of conduction electrons may be comparable or even larger than the distance between atoms. Think about electrons more like waves propagating through the lattice than particles trying to go through the holes.

    Of course, this is just part of the story. The electrical conduction depends also on the number of free carriers (electrons or other carriers). So the increase in resistance with temperature is observed for materials were the number of carriers is practically independent of temperature, like metals.
    In semiconductors the resistance may decrease with temperature.
    This is not because there is more room between atoms but because more electrons become free to move under the influence of the electric field.
     
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