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Noob questions about electricity and magnetism

  1. Nov 7, 2008 #1
    I'm a little confused about "voltage". We learned that voltage is somewhat similar to electric field: a way to refer to the potential energy of a charge A when put near charge B without actually knowing the charge of charge A. That made sense to me, but then suddenly in class (this is college physics by the way) we're refering to the voltage in a battery as a source of current... and i guess i didn't really get the transition from the voltage of a point in relation to a point charge, and voltage as some abstract value of a circuit thats equal to the current times the resistance. I mean i can do the math and solve circuit problems, the whole E=IR thing is simple enough to use, but i don't get how "voltage" applies to the situation. I kind of see circuits being similar to a river with a mill in it- as the water flows by the mill some of it's kinetic energy is transfered to the wheel. While the current of the water is the same before and after the water interacts with the wheel, the velocity(and energy) of the water changes. Is this a good way to think of it or is this a horrible comparison? And if it's a sound comparison, what would the "voltage" be, the difference in gravitational potential energy between the source and mouth of the river?
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  3. Nov 7, 2008 #2

    Ben Niehoff

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    The voltage in a circuit is exactly the same kind of voltage as in electrostatics.

    A battery is a device which maintains a voltage difference between its two terminals. If you let a battery sit in empty space, then there will be an electric field around the battery that looks like a dipole field.

    When you connect a wire to the battery, the electric field tends to concentrate along the direction of the wire. This is because the wire has a very high dielectric constant (a perfect conductor is the limit as you take the dielectric constant to infinity). Thus, when you complete a circuit with a wire, most of the electric field goes along the wire, and pushes charges around the circuit.

    Compare to taking a magnet and putting an iron loop between the two poles: because the iron has a very high permeability, the magnetic field lines tend to concentrate inside the iron loop.
  4. Nov 7, 2008 #3
    Thanks for the response :) What do you mean by the voltage between the two terminals? is it the difference in charge (which should be twice the charge of either terminal, right?) divided by the distance between them? and does this mean that if you took the terminals of a battery and pulled them apart you would change the voltage?
  5. Nov 8, 2008 #4


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    The "voltage between two points" is by definition the work (energy) it takes to move a charge between those two points, divided by the amount of charge. One volt equals one joule per coulomb.
  6. Nov 8, 2008 #5


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    I hope this doesn't sound too simplistic, but voltage in an electrical system can be thought of like pressure in a fluidpower system. It is the potential energy of electricity. Like pressure (the potential energy in a fluid), voltage exists because of a difference in potential between two parts in the system. When a path or outlet for this energy is available, it is this potential energy which drives the current (whether fluid or electrical).

    I think your analogy of a river turning a waterwheel only fails in that a river uses gravity for its potential energy. So, after passing by the wheel, the water can begin accelerating again, as gravity acts like a constant input of new energy. Replace your fluid source with a pressure pump feeding enclosed tubes or hoses, and you're there.
  7. Nov 8, 2008 #6
    I've heard this definition before and i get it, but when applied to points with charges it confuses me because isn't the voltage AT a point of charge infinite?
  8. Nov 10, 2008 #7
    I've been going through my (crappy) textbook and i still don't really "get" what's actually happening in a circuit. The whole voltage thing is beginning to make a little more sense to me, though now i have a few more questions that the book doesn't really explain: is the current constant throughout a circuit, or does it change after the electrons run through resistance? If current is constant, then how is energy stored in the flow of electrons? Also, what determines the voltage of a battery? I would imagine that one of the major factors would be the rate at which the chemicals inside the battery can react and create ions, is that it or are there other larger factors at work? TIA
  9. Nov 11, 2008 #8


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    When figuring the potential at the location of a point charge, you don't include that charge itself. The potential is calculated from all the other charges in the system.
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