Voltage/Current

Discussion in 'Homework Help' started by Boka, Jul 22, 2004.

  1. Boka

    Thread Starter Active Member

    Apr 21, 2004
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    Hi

    How faat does Electricity travel? And what affects it's speed(im thinkin resistance right?) Does the 20,000volts travel faster than say 10V?.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Hi,

    All electromagnetic phenomena propagate at the speed of light. Due to their mass, electrons travel more slowly. Their speed in a conductor is dependant on the propelling voltage. The resistivity of the conductor may have some bearing on that, too.

    Since one coulomb of charge has to pass a point in one second to make one ampere of current, passing 100 amps past that point would argue for a higher velocity of the electrons. Their mutual repulsion would prevent much of an increase in electron density in the conductor.
     
  3. Boka

    Thread Starter Active Member

    Apr 21, 2004
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    hello beenthere,

    Thanks for the reply i was playing with a switch and i thought to myself that if i pressed the switch fast enough i might not get electricuted (<---stupid thought) well thanx for the info:)
     
  4. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
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    hi,

    what made you think that way, was it just plain curiousity? that's one curoiusity that can bring you six feet under the ground or disable you for the rest of your life. always be on the safe side when you start being curious ;) :p :D :lol:
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
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    If you look at the speed at which free electrons randomly move between atoms in a conductor, it is around 10^6 m/s dependant on certain factors such as temperature. Now if we apply an electric field to the conductor these free electrons will experience a force which accelerates the electrons in the opposite direction to the field. They collide with the atom nuclei and and are deflected from their flowpath, loosing energy with each collision (this energy loss is the heat we feel when we hold an insulated wire which is carrying a current). What we actually now have on a macro scale is a steady drift velocity superimposed over the random movement of the electrons on the micro scale. The current in the conductor is defined by:

    I = ne x VdA

    Where:

    n is the number of atoms per cubic metre
    e is the electron charge
    Vd is the drift velocity
    A is the cross-sectional area

    Although the above equation requires you to deal with a vector product, it is simple to reduce the equation to the form:

    Vd = J / ne

    Where:

    J is the current density

    Using typical values, you will see that the drift velocity of the current in the conductor is only around 0.22 mm/s - this is incredably slow!! If you don't belive me try putting typical values into the above equation.

    From a strict Physics and Electromagnetics sense, current is a vast quantity of rapidly moving electrons upon which an incredably slow drift velocity is imposed.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi Dave,

    Thanks for quantifying the figure. I always use my image of electrons going from cathode to the screen on a CRT. Somehow, it seems that 6 to the eighteenth electrons need lots of room.
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
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    The speed of current is one of those very confusing and somewhat difficult to imagine concepts. At first I didn't believe the figure of 0.22 mm/s, it just seemed completely incomprehensible that such a large quantity of charge could move so slow. What is an interesting point to note about the figures and method I quoted above is the quanitity of charge in copper, which is the material I assumed for this explaination, is around 13 C/mm - which a huge quanitity of charge! If we were to look at the conductor on a small scale we would find that the positive charge on the ionised atoms is the same as negative free charge, even though it is carrying a current. The peculiarities of the Quantum world! :rolleyes:

    Just to look at the original question posted by the author of this thread:

    Does the 20,000volts travel faster than say 10V?

    Assuming all else is equal (which is very unlikely!) 20000volts will be just as quick as 10volts or even 10millivolts - in all fairness voltage has no speed because voltage is electrical pressure, which can even have no speed at all. Try to imagine pressure of water in a vessel, the water can be completely static but under any degree of pressure. At the same time it can be dynamic at any speed irrespective of the pressure applied.
     
  8. haditya

    Senior Member

    Jan 19, 2004
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    considering bernaulli's equation for stream lined flow of a fluid.. i think pressue on water would effect its forward velocity... no doubt
    changes in pressure can effect its velocity.
     
  9. Dave

    Retired Moderator

    Nov 17, 2003
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    Bernoulli's equation refers to dynamic physics, where as water can easily be under pressure in a static model - think of water in a cup, that water is exerting presurre on the cup and conversly is under pressure from the atmosphere, however the water is static in the cup. Also consider water held at a height, it has potential energy which is exerting pressure on the vessel - this is probably a better way of considering voltage as a pressure - releasing the potential energy causes the water to flow. In a way this is analogous to have a voltage in a circuit and you closing the circuit to allow a current to flow.

    Certainly changes in pressure will affect the velocity of the fluid, i.e. dP/dt ≠ 0, assuming no changes in any other variable. When I said "it can be dynamic at any speed irrespective of the pressure applied" I was refering to a dynamic model (the fluid was dynamic) where there is no change in pressure, i.e. dP/dt = 0.
     
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