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What is electricity?

  1. Apr 8, 2009 #1
    What is electricity? Let me use the example of turning on a light in a room.

    I have been told that when I flip the switch, electrons flow in the wire at near the speed of light, and that is electricity.

    I have also been told that electricity propagates as a field between the two wires, and the energy is in that field.

    Further, I have heard of some phenomenon -- called drift current perhaps -- where there is a net accumulation or depletion of electrons in a wire over a period of a few seconds. Or, some propagation effect whereby electrons move only a few centimeters per second. Is there anything like this?

    Thank you for considering my question.
     
  2. jcsd
  3. Apr 8, 2009 #2

    MATLABdude

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    This is incorrect, but close enough to the truth that it's often repeated. In fact, it is the electric field that propagates at near light speed, as you say in the next point:

    This electric field actually does cause electrons to bounce around from atom to atom, but the net movement of any individual electron is actually rather slow (on the order of a few cm per second):

    The net current comes from the aggregate movement of a great many (10^19 ish) of these electrons. As an analogy, think of a wave in water: a buoy wouldn't move much in the direction of the wave (nor do the individual water molecules), but the wave itself moves forward at great speed. So it is with electricity.
     
  4. Apr 13, 2009 #3

    Redbelly98

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    Another water analogy that I like: consider a water hose, already filled with water though none is flowing.

    Somebody turns on the valve at one end of the hose. Almost instantly, water starts flowing out at the other end. Even though a given drop of water will take several seconds to travel the distance from the faucet to the open end.

    And I say "almost instantly" because there is a short delay, due to the speed of sound in water, before the high pressure from opening the faucet propagates the entire length of the hose.
     
  5. Apr 13, 2009 #4
    "Electron current" is the flow of conduction electrons in a conductor. "Conventional" curent, flow of positively charged current, is in the opposite direction. The direction of current was establiched long before electrons were discovered. The flow of current is based on the presence of a voltage, which creates an electric field along the conductor.
    Power is essentially the product of current and an in-phase voltage Power (watts) = I times V.

    The total circuit voltage is actually between the two conductors, one carrying the current, and the other carrying the return current. This voltage creates an electric field E between the two conductors. The currents create a magnetic field H around each current conductor. E and H are orthogonal to each other.
    The total power flow can be represented by the vector cross product of E and H. The Poynting** vector, the vector showing both the amplitude and direction of the power, is

    P = integral [E x H] dA, where A is the area of the surface between the two conductors where both E and H are non zero.

    The Poynting representation of electromagnetic power flow is especially useful when the currents are not confined to conductors, like for example high frequency power in waveguides, or in free space near antennas.

    ** Named after John Henry Poynting
     
  6. Apr 14, 2009 #5
    Ahem. Electricity is the effect obtained by turning the switch on, in the vernacular.

    A Dictionary: The phenomena of electromagnetism. (Doesn't help much, does it?)

    The stuff that moves in the wire isn't 'electricity'. It is current.
     
  7. Apr 24, 2009 #6
    Electrons in wires don't travel at the speed of light. They travel more like the
    speed of a minute hand on a clock! But lets back up a bit first.

    What is electricity? That's easy. Just use the scientific definition, and ignore
    the morass of incorrect and contradictory definitions used by the general public.
    That's the traditional technique for cutting through the Physics Fog found in
    popular culture. (Physics Phog?)

    "Quantity of electricity" means quantity of Electric Charge. That's the MKS definition
    of the word electricity. Amounts of "electricity" are measured in coulombs.
    A flow of electricity is called an "electric current."

    But if you adopt this definition, it creates certain problems when talking to
    non-experts. In that case a battery becomes an "electricity pump" which
    takes electricity in through one terminal and simultaneously spits it out the
    other. The half-cell reactions at the surfaces of electrodes are pumping the
    "electricity" through the electrolyte; through the battery, and back out again.
    Similarly, an electric generator can never generate any electricity (it's just a
    pump.) Also, electric companies don't sell electricity, and no electricity ever
    travels from the utility plants to your home. Electricity only wiggles slightly
    back and forth in any AC system. And if we use the scientific definition,
    then "electricity" is not a form of energy. Coulombs and Joules are two
    entirely separate things.

    If we accept that "Quantity of electricity" means Charge, then we're in line
    with Faraday, JC Maxwell, William Thompson, Einstein, and other heavy hitters.
    In particular, in 1839 Faraday published a set of experiments which proved
    that bioelectricity, triboelectricity, electrochemical electricity, and atmospheric
    electricity were all exactly the same entity.

    But later scientists stopped using this definition. They stopped using the word
    Electricity and instead replaced it with the word Charge. Our contemporary experts
    don't use any clear scientific definition of the word electricity, and they disagree
    among themselves. But the original definition is still in the MKS standards
    documents and in many physics refs (CRC handbook, for example.)

    More about this whole topic:

    Scientific definition of the word "Electricity"
    http://amasci.com/miscon/maxwell.html

    Is Electricity the charge, or is it the energy?
    http://amasci.com/elect/elefaq1.html#ae

    Electricity: list of definitions and their consequences
    http://amasci.com/miscon/whatdef.html

    Big list:
    http://amasci.com/ele-edu.html
     
  8. Apr 24, 2009 #7
    Try Beirce, Devil's Dictionary! Quite humerous. He actually figured it out.



    The stuff that moves in wires is called "charge." An electric current is a flow of
    charge, not a flow of "current." (And what is flowing in a river? is it the water, or
    is it the current?)
     
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