Electron kinetic energy, movement

  1. I read that based on temperature , electrons have kinetic energy in metals so they randomly move around the ion lattice.
    when we apply current to a room temperature conductor, the electrons being charge carriers align and form current?
    but what happens in the plates of a capacitor , when the capacitor is charged the electrons on one plate and the positive charge on the other, do electrons have kinetic energy in such situation or are they locked due to the attraction force?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    Not really. Their movements are still random, they just have a small net movement along the direction of current flow. It's kind of like air. When you blow out of your mouth the volume of air is moving fairly slowly, but the molecules themselves are pinging about randomly at around 1100 mph. They just have a small net motion in the direction away from your mouth, leading to airflow.

    Both. They still have random motions within the plates, but they cannot get off of the plates until you discharge the capacitor.
     
  4. current decreases in a wire due to the resistance mainly caused by electron collisions with ions, then if the same electrons collide wth the same ions while on the plates of a capacitor , then why doesn't the capacitor loose it's charge as heat in the metal plates ?
     
  5. For the same reason the current doesn't decrease in a wire over time, because there is a battery supplying a constant voltage. If you remove the battery the accumulated charge will slowly dissipate (as the current will also go to zero in a connected circuit).
     
  6. so a capacitor looses charge the same way a wire looses current , by electron ion interactions?
     
  7. The "current decrease in a wire" is a vague statement. If you are thinking about electrical resistance, this is not due to collisions between electrons and ions. You are trying to apply a model which is not appropriate. It is true that this electron-ion collision is presented often in introductory texts.

    Your observation may be a hint to the fact that the electrons are not slowed down by electron-ion collisions. The electrons in an atom are not slowed down by collisions with the nucleus, are they?
     
  8. Drakkith

    Staff: Mentor

    No. All capacitors have a small "leakage" current which drains the plates slowly over time. This is because it is impossible to have infinite electrical resistance so there is always a small current flowing.
     
  9. Drakkith

    Staff: Mentor

    You are talking about two different things.

    A plate that is electrically charged simply has an excess of one charge over another. The plate remains charged until those excess chargers are drained away. Remember that the charges are actual particles. You have to physically remove or add them to the plate in order to charge or discharge it. A current carrying wire is usually not electrically charged, and even if it is it has little effect on current flow.
     
  10. Yes I was thinking about a current in a wire when the power source is disconnected. It stops , due to resistance and the resistance is caused by what then?
     
  11. Drakkith

    Staff: Mentor

  12. sophiecentaur

    sophiecentaur 13,583
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    It stops because the PD changes - charges would build up at the ends of the wire, producing a force to stop the flow (as with a charged capacitor). Resistance is not necessary - except to account for the fact that an oscillation does not occur.
     
  13. Yes , I see , when there is no PD , there is no force that would move the charge, and PD could be described as a seperation of opposite charges.Correct?

    when current flows due to a PD it encounters resistance in an ordinary conductor.We were talking about ions and electrons interacting wtih them and each other previously, what would happen if we would make a glass tube with electron gas in it as a conductor , would current still have resistance there?
     
  14. Drakkith

    Staff: Mentor

    The electron gas would behave like a plasma, not a normal conductor, so the normal models of electrical conductivity don't apply there any more. That's not to say it doesn't have resistance, it's just that this is beyond the original scope of the thread.
     
  15. Yes, but if the conductor consists of only electrons and they travel through vacuum , much like in a CRT then were and from what resistance arises?
     
  16. UltrafastPED

    UltrafastPED 1,919
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    Gold Member

    Ohm's law is not applicable to electron beams; resistance is a property of _metals_.
     
  17. sophiecentaur

    sophiecentaur 13,583
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    'Resistance' is just the ratio of V/I and applies to anything; that's why you can include diodes and other elements in circuit analysis. 'Ohm's Law' is not actually just R = V/I. Ohm's Law applies to metals and means that V/I is constant at a constant temperature. It even applies to Lamp Filaments - or it would, if you could keep the temperature constant.
    For an electron beam, there will be a measurable PD and a measurable current. But of course, an electron beam needs to start with a source of electrons (thermionic cathode or lots of volts) the V/I characteristic will be very non-linear
     
  18. UltrafastPED

    UltrafastPED 1,919
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    When I design a photo-electron gun the voltage at the photocathode may be 30 kV, and the anode is grounded. During transit each electron will start with almost zero energy, but by the end each one will have 30 keV.

    Depending upon the laser fluence the average current may be anywhere in the range 10^-14 up to 10^-3 amps. By your prescription this vacuum has a resistance varying from:

    30,000/10^-14 = 3*10^18 ohms for the low fluence, to
    30,000/10^-3 = 3*10^7 ohms for the high fluence.

    Funny how the vacuum has a variable resistance!

    I believe you are thinking of "transimpedance"; we used this term when studying solid state devices. "Transfer resistance", "transfer conductance", "transfer impedance" ... these are all for non-ohmic devices. See http://en.wikipedia.org/wiki/Transimpedance

    When teaching an introductory circuits course I would limit the use of the term "resistance" to when Ohm's law is applicable. I even made some notes about Ohm's law last year - it's available in this thread on resistors: https://www.physicsforums.com/showthread.php?t=740567#15
     
  19. sophiecentaur

    sophiecentaur 13,583
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    Yes - I take your point about the 'vacuum' but there will still be a V and an I, measured somewhere and they would, for instance, tell you the power dissipated and how the volts are shared with any series (actual) resistances. And it's not so much the 'vacuum' you are describing as a tube with available electrons in it. They shouldn't be dissipating much power until they hit the end but they are still gaining VI power as they go through the tube.

    I though that 'transfer***' described a device with three or more terminals.
    What would do about a carbon filament? I wouldn't think you would want to treat that differently from a tungsten filament in a circuit. Also, the resistance of a diode in an RF circuit could be very relevant for small signals.

    I really think that "Ohm's Law should be a term reserved for situations where it applies fully. Why not just state Resistance (defined by the ratio), which says nothing about the temperature dependence? You can then include a device in your circuit and solve whatever equations are needed, to get an answer. Amplifiers all have an output and input impedance associated with them and they are definitely not metallic ('ohmic') resistors. (And what about a load resistance, as seen via a transformer?)
    We are just coming to this from different directions of experience, I think. I don't think we are disagreeing about anything other than use of terms.
     
  20. UltrafastPED

    UltrafastPED 1,919
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    Except ... what you are saying is not what is taught in physics or electrical engineering. Ohm's law is a material dependent statement, as pointed out in the notes I referenced.

    You are using resistance when the technical word is transresistance:
    http://www.answers.com/topic/transresistance
     
  21. sophiecentaur

    sophiecentaur 13,583
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    I can only find the term 'transresistance' **used to describe an amplification or mutual effect, along with transconductance. Your link only took me to a page full of adverts. Google the term and there are many pages of hits with references to amplifiers. Where is it taught 'your way'?

    Sometimes the resistance of a non-ohmic component is referred to as dynamic resistance (dV/DI)

    Of course Ohm's Law is material dependent but Ohm does not actually mention the word Resistance. It is just states that metals have a constant ratio of V/I at constant temperature. You can measure the 'resistance' of anything and get an answer - that doesn't say anything about its Ohmic behaviour. Furthermore, Ohm's Law is not a 'Law'; it just describes a behaviour, as does Hooke's 'Law'.

    ** Which is where the term 'Transistor' came from, I believe.
     
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