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How voltage affects electrons

  1. May 11, 2017 #1
    Can someone please tell/explain to me how voltage affects electrons. Does it determine the speed of the electrons in a conductor? Is so why a power supply rated 12v 3A and another rated 9v 3A have the same amount of electrons traveling at the same speed through the conductor. My big question is: What is the purpose of the deference in voltage?
     
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  3. May 11, 2017 #2

    anorlunda

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    :welcome:

    I hate the water analogy, but it may help you. Think of a water hose. Voltage difference one end of the wire to the other, is like the pressure difference one end of the hose to the other. Current in amps is like the flow of water in the hose in liters per second. The power is voltage times current. You need both voltage and current to get power.

    It is best to forget about electrons. They just confuse people. Electrons are not like little capsules of energy nor is a cup full of electrons like a cup of water.
     
  4. May 11, 2017 #3

    jim hardy

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    The water analogy leads one to the mistaken idea that like water, electricity always falls to ground.
    But that's gravity not electrical attraction..

    It takes a long time to get one's thinking aligned so that such apparent conflicts resolve.

    I suggest that at hobbyist level Bill Beatty's articles are very useful. That's because he explores one by one the logical conflicts that arise from less-than-perfect methods of explanation.

    Sample:
    from https://www.electrical-contractor.net/ESF/Why Electricity is Impossible to Understand.htm
    but
    his website is http://amasci.com/ele-edu.html
    old jim
     
  5. May 12, 2017 #4
    How do you calculate the power output of a generator. I know that P = I x E and that you can know the output voltage but how do you calculate the current in order to determine the power output?
     
  6. May 13, 2017 #5

    anorlunda

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    Actual power out depends on the actual load connected to the generator. If the load is a resistor R, then I=E/R

    Maximum power out is limited by heating of the wires. Look at the name plate for max power of max current.
     
  7. May 13, 2017 #6

    sophiecentaur

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    Firstly, the speed of electrons (the mean square drift velocity) is extremely low (around 1mm / s.
    This link tells you the whole story about charges flowing through a uniform wire. You can see the drift velocity is proportional to the PD (Voltage drop) across a length of wire but it's so low that the Kinetic Energy is just not relevant as a mechanism to transfer power.
     
  8. May 15, 2017 #7

    sophiecentaur

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    You have to get your thinking in the right order if you want to suss this out properly. It's all a matter of cause and effect and carts coming AFTER horses. The energy source in a circuit is the Battery (etc. etc.) and it is the Potential Difference (Voltage) that counts. It tells you the Energy available for each unit of charge. You put a load across the battery (Resistor, say) and that will give you the current that flows. If you want to determine what Resistance is needed for a particular current to flow, that's quite possible of course but what you end up with is not a simple cause - effect thing but a "what do I have to do to get that cause to have this effect?"
    You can only know the output power that a generator is actually supplying by knowing the V and the I that happens to be flowing - but that's determined by the load and not the generator.
    On a higher level, you can calculate the power that the Generator could supply by knowing details of the design like the magnetic field, the resistance of the coils and the power of the engine (etc.) that's turning the generator. (That's another cause and effect thing). The resistance of the coils can give you an indication of the possible current available before they overheat but that's a practical thing.
    So you need to be very precise in your question(above) to get an answer.
     
  9. May 15, 2017 #8

    jim hardy

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    If Load = R then current I = E/R .
     
  10. May 16, 2017 #9

    analogdesign

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    I've never understood why people hate the water analogy so much. Like any analogy, it is incorrect in a holistic sense, and it has its limitations, but for a lot of purposes it helps develop a lot of intuition about electric circuits.

    Also, I disagree with Mr. Beatty's point about cancelled charge resulting in matter. That simply isn't true. The matter is always there. In a semiconductor, a minus charge is an electron and a postiive charge is a hole (a kind of lack-of-electron) in a lattice site. So, when a - meets a +, they really do cancel in an electrical sense and make no more matter because the electron was already there (it just wasn't in the thrall of a particular atom). Calling it "frozen electricity" makes no sense, as there are still electrical interactions between the electrons in the various shells and the protons in each nucleus (and surrounding nuclei since the each outer shell electron has an excursion radius of several lattice constants so it interacts with a handful of nuclei). Indeed, I think this analogy is more misleading that the hydraulic analogy.

    I guess this goes to show that all "analogies" are flawed. The only way to really understand electric circuits is through Maxwell's Equations and the only way to really understand transistors is through Quantum Mechanics. However, you can do useful things without "really understanding".
     
  11. May 16, 2017 #10

    jim hardy

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    Yes , one can provide valuable service to industry with just enough understanding of how things work to keep them in good repair and running smoothly..

    It's a different skillset.
     
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