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Why are two different conductors needed for the Seebeck-Peltier effect

  1. Jun 2, 2013 #1
    I've been doing some reading about the thermo-electric effect but I don't get why two different conductors are needed for the electrons to flow.

    I mean if I take a piece of copper wire and heat one end and cool the other, the electrons at the hotter end will get excited and move towards the cooler end thus producing the desired effect then why are different conductors required
     
  2. jcsd
  3. Jun 2, 2013 #2
    You need two conductors because the effect is very hard to measure otherwise. Lets say you have a copper bar with a thermal voltage across it. It is easier to understand if we use an Ampere meter to measure this voltage. If you connect the Ampere meter to the piece of copper using copper wire, the thermal voltage will be the same on the copper wire as it is on the bar because the endpoints are at the same temperatures, thus there will be the same thermal voltage across the wire as on the bar, if you go around the loop made of the bar and the round trip voltage is zero therefore no current will flow.

    Due to Kirchhof's laws all voltages in a loop add to zero, normally some voltages come from voltage sources and some from the voltage over the resistors due to currents. Because the voltage sources all already add up to zero there can be no voltage in the loop anywhere where you might put a voltmeter.
     
  4. Jun 2, 2013 #3

    sophiecentaur

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    But you can put a solenoid in series and the thermoelectric current through it can be enough to activate a massive gas valve. That involves a finite emf across the ends of the coil (finite resistance) and your solenoid is, thus, a form of voltmeter. Not an ideal voltmeter, of course, because voltmeters have infinite resistance.
     
  5. Jun 3, 2013 #4
    The point is that you need two different materials, so that the voltages are not balanced. Otherwise there is no measurable current or voltage. You cannot operate a solenoid valve on a thermocouple made only of copper.
     
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