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

Charges and energy transfer in a circuit

  1. Nov 25, 2012 #1
    I've been trying to understand what happens on a microscopic level - in terms of charges and EM fields - in a simple circuit (say a battery with wires to a lightbulb), and I'm finding it pretty difficult. I read these articles that try to untangle the flow of charge from the flow of energy, and claim that the energy from the battery to the lightbulb is carried not by electrons, but by the EM field running _outside_ the wire:


    I still think I don't understand how the current gets going. Please help me fill in the gaps (or fix what I already understand wrongly)!

    Here're three pictures.

    1. We have a battery, two pieces of wire running to its poles, but not connected to it, and a lightbulb and an open switch in series on those wires. My understanding is that the wires are uncharged, and there's no electric field inside the wires (nor magnetic field outside). Because the battery does create an electric field around it, there's a distribution of surface charge on the wires that cancels it out inside the wire.

    2. Now we connect the wires to the battery, but the circuit is still open. What happens? Do charges from the battery flow into the wires, making them oppositely charged? Or do the wires remain uncharged on the whole?

    3. Now we close the circuit. I know, from articles above, what happens *some time after* this point: there's a steady state in which a surface charge density on the wires creates an electric field both inside and outside; the electric field inside creates the current, the current creates the magnetic field outside, and the electric + magnetic field outside get the EM field going alongside the wire (please correct if anything's wrong). But how does this situation come to be, as a result of closing the circuit? What causes the surface charge densities on the wires? Are the wires as a whole charged or uncharged as the current flows?

    Links to any articles/books that describe the whole picture on such a microscopic level would be great, too.
  2. jcsd
  3. Nov 25, 2012 #2

    OK, let me take a crack at it. First, let's define some things. Voltage is the energy density of the charge. It takes energy to bring some isolated electrons together into one place. It takes more energy to bring additonal electrons together in the same place. It also takes even more energy to pack the electrons together into a smaller space. That energy divided by the number of electrons is the energy density of the charge, also called voltage. It takes two points to define a voltage; a reference point and the point to be measured. If one point has a higher voltage, that means that the energy density at that point is higher. Charge carriers like electrons are going to flow from the higher energy density to the lower energy density. Along the way, the electrons are going to encounter resistance, which robs them of their energy and converts it into heat. So at the end point, the electrons have less energy density than at the start point, and the voltage measured at the end point shows a drop compared to the start point.

    When electrons move in a wire, their speed is very fast. But they bounce back and forth off each other and the ionic cores of the wire atoms, so their forward drift velocity is about the speed of cold molasses. Metal wires contain an exceeding great number of electrons, so even at a slow drift velocity, considerable current can be maintained. Now, here is what is important to know. The electrons act like a hose filled with marbles. If you cram one marble into the end of the hose, another pops out at the end almost immediately. If you successively insert marbles into the end of the hose, it might take all day for the first marble to come out, but there could be a pail of marbles collected at the end. So the electrons that pass through and light the bulb in your example might have taken several minutes to arrive there, but the lamp lighted immediately, at the speed of light. When the wires were connected and the switch closed, a voltage difference was established between the ends of the wire, and an electric field was established at the speed of light. Charge carriers are attracted or repelled by an electric field. A steady moving charge sustains a magnetic field. It takes energy to first build the field, but the energy is returned to the system when the field collapes from current cessation. You don't have to worry too much about energy in the electromagnetic (EM) fields of straight wires. If you try to wind a transformer and "steal" magnetic energy from your circuit, then that has to be taken into account. So for the most part, the energy is transported in your light circuit by just the charge carriers.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook