Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The truth about electric current

  1. Jul 12, 2010 #1
    A professor of mine was talking about the electric current. He said electric current is not exactly the flow of electrons through a conductor. He said the flow of electrons is low but energy is propagated through the conductor somehow, when a electric current is present.

    I always thought the electric current as the flow of electrons like a water pipe.

    What do REALLY happens? Is the electric current EXACTLY equal to the water pipe (flow of electrons) or is the professor right?

    I would appreciate if someone explain me this clearly...

    Thank you,
    Rafael Andreatta
  2. jcsd
  3. Jul 12, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    Current is the flow of charge, which is usually electrons in most cases. However, the flow of power is not done by the charges. The drift velocity of charges is very very small, but the power and changes in the power delivered transmits on speeds on the order of the speed of light.

    In non-DC situations, the power is transmitted by electromagnetic waves. At the input to the power lines, called a transmission line, we excite the waves using a voltage or localized currents. As the waves travel down the transmission line, they in turn induce currents on the line. This is how the currents can propagate down a wire at speeds much faster than the those of the charges themselves. The waves also induce voltage differences across the structure of the line. So this is how you can see a voltage across the wires at a component for example. This voltage can be used to induce currents inside devices where we do not use the electromagnetic wave model.

    In a DC circuit, well it's not really DC since there is a starting and stopping time. But in the DC sense we induce an applied electric field along the wires of the ciruit. Same idea.
  4. Jul 12, 2010 #3
    The water pipe analogy works very well. Though the water moves slowly in the pipe, the pressure change due to flow is much faster. What really happens is that the electric current cannot be disassociated with the electric field (which looks like pressure in the analogy) and the magnetic field (which, in the water pipe analogy would correspond to inertia of the fluid).
  5. Jul 12, 2010 #4
    Analogies work well, but are not what really happens.

    It's hard to find answers to this seemingly simple question

    There are electric and magnetic fields and electrons moving according F=qE but in the crystal. However, I haven't found how to make a consistent picture out of it.
  6. Jul 12, 2010 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Without knowing the exact wording that your professor gave, it is hard to judge if he really meant what you THINK he meant.

    Electric current is the flow of charge. Period. However, it isn't simply like a "river" of charge. This is because, as has been mentioned, the actual drift velocity of the charge is actually quite small. If we simplify the model of the electrons in a conductor as a free electron gas, they are actually moving in a random fashion. It is only when we look at the overall statistics do we see a movement to one direction.

    So the flow of charge in a conductor that results in a current isn't quite the same as, say, the flow of charges in a particle accelerator beam pipe. The latter is closer to what we view as a "fluid flow".

  7. Jul 12, 2010 #6
    Even that is a simplification,

    electrons are not 'hard little round things' as you picture in your mind. They are vague, fuzzy 'quantum' objects that spread out to fill a conductor completely, existing more like a wave than a particle.

    Our minds are simply not equipped to understand this completely because it's something outside our common experience of the world.

    The trick in getting to understand it is not to try to grasp it immediately as a whole but simply go along with the various models your professors give, which work in some situations, not in others - eventually you begin to get a small inkling of what the truth must be like.
  8. Jul 12, 2010 #7
    It doesn't have to made too complicating. Say you take a quasi-classical model with bouncing balls. In this simplified picture, can you write down a consistent equation for electron flow, electric field and magnetic field?
  9. Jul 12, 2010 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I did say "simplify", didn't I?

    Still, as simple as the model is, it is the basis for the Drude model that has been quite successful in describing the properties of a "standard conductor", including the derivation of Ohm's law. It certainly has its limitations, but such simplifications have been shown to be quite useful in many instances.

    You'd be surprised that in many particle beam dynamic codes (example: the PAMELA code) that model the beam dynamics in a particle accelerator, the "hard little round things" IS the model used. Classical picture is alive and well when we deal with particle beams in accelerators.

  10. Jul 12, 2010 #9
    I'm not arguing with ya.

    Just pointing out to the OP the best way to handle it.
  11. Jul 12, 2010 #10


    User Avatar
    Science Advisor

    Was your professor talking about alternating current? In that case, electrons do not "flow" through a wire at all. If you want to use the "water pipe" analogy, think of alternating current as water flowing back and forth a very short distance. Water "here" , being pushed back and forth in the hose, will push water "there" back and forth, producing energy by friction. It's the same with alternating current in an electric wire.
  12. Jul 12, 2010 #11
    No, he was talking about direct current.

    I read all the posts and I'm trying to understand.

    Above someone told that the drift velocity of the electrons is very small but the flow velocity is big. How could it happen? What's the difference between the drift velocity and the flow velocity? Aren't they the same thing?

    And, how could the drift velocity of electrons be small if: when we have a direct current through a conductor we have an electric field and if we have an electric field we have Coulomb's force and then we have movement and then we have drift velocity... isn't that right?

    Could someone explain-me it better?

    I'll keep re-reading the posts and trying to understand.

    Thank you,
    Rafael Andreatta
  13. Jul 12, 2010 #12
    "Drift" velocity would be how fast the electrons are actually moving. Which is not very fast at all. "Flow" velocity is how fast the "signal" is moving.

    Imagine taking a looooong metal rod and pulling it on one end. Regardless of how fast or hard you pull it, the entire rod will move almost instantaneously. The speed of sound in the rod dictates how long it takes for the other end to react to your pulling on it. This same "reaction speed" in an electric current is a good portion of the speed of light.
  14. Jul 12, 2010 #13
    In the analogy of water flow,
    1) current corresponds with displaced fluid volume
    2) voltage corresponds to pressure

    This works just as well with AC or DC current. Circuit elements such as resistors, capacitors and inductors are easily modeled.

    Resistance is modeled as a restriction of flow with viscous loss of energy.

    A capacitor is modeled as a rubber diaphragm.

    Even a straight wire has inductance, and the inertia of the fluid serves in correspondence. Inductance can be modeled as a great deal of lossless pipe. Alternately, using a little more apparatus such as an turbine and flywheel, the inertia of the fluid can be transfered to the flywheel then back to the fluid just as required.
  15. Jul 12, 2010 #14
    After reading these posts, I find myself more confused than before. :surprised
    Likely just me...
    Last edited: Jul 12, 2010
  16. Jul 12, 2010 #15


    User Avatar
    Gold Member

    Say you are at the end of a long very crowed corridor. Everyone's quiet at the first moment. You push one person. Who pushes the next person and so on until all the people in the corridor have been pushed. "Current passed through the corridor".
    It's an analogy with current. People are charges (or electrons if you prefer) and the corridor is the conductor. It doesn't matter how much people will leave the corridor being pushed forward, fast enough all the people will move. The current "flows" thanks to voltage and represent the people being pushed.
    Any physicist correct me if I'm wrong, but I think that's a good analogy.
  17. Jul 15, 2010 #16
    Hum, it makes things more clearly for me...

    Then the waves that travels through a conductor that experiences a direct current are longitudinal waves?
  18. Jul 16, 2010 #17


    User Avatar
    Science Advisor
    Gold Member

    No, electromagnetic waves, with few exceptions, are transverse waves. fluidistic's explanation is a bit misleading because it incorrectly implies that the charges are bumping into each other and this provides the push that propagates down faster than the charges themselves. The charges do undergo collisions, but this results in loss of energy as heat and is the classical model for resistance known as the Drude model. In addition, the waves do not penetrate the conductor in most situations. Only very low frequency waves can penetrate through a conductor for appreciable distances. However, DC fields do not penetrate the conductor either. Instead, the applied electric field resides on the surface of the conductor. What we are doing is setting up this electric field along the length of the wire in our DC circuit. The initial creation of this electric field has to be done by an electromagnetic wave since we have to actually turn on the circuit. That is, the applied voltage looks like a step function. During the step, there is a time changing voltage which creates an electromagnetic wave. This wave will travel down the wires but will quickly settle in as a constant DC electric field.

    So we setup an electric field along the surface of the wire. This electric field exerts a Lorentz force on the charges on the wire and causes a current to flow. This electric field is created by sending an electromagnetic wave down the wire but it settles into the DC field very very quickly. It exists as a very brief transient.
  19. Jul 16, 2010 #18
    Ah... much better.
    My confusion leading to a deep depression with paranoid delusions are slipping away... :approve:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook