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Work Done By Elelctric field in a circuit

  1. Jun 2, 2012 #1
    hello, i've been searching the forum here, many websites and books and i just dont get it. The potential difference between two points in a circuit is the work done AGAINST the electric field so who's making the work in a circuit? isnt suppose to be the Electric Field? and in a circuit the electrons flows from lower potential to higher potential so electrons is moving with the direction of the electric field no? and when we say there's a drop of potential across a resistor is it means that the work done is negative? thank you

    And i want to add something what is the relation between the energy gained or lost for an electron in a circuit and the work done by the electric field?
     
    Last edited: Jun 2, 2012
  2. jcsd
  3. Jun 2, 2012 #2
    It seems you do grasp the fundamentals. Within a simple circuit, electric field lines run along the length of a resistor from positive to negative, it can be thought of as field lines going from the positive to negative terminal of the cell. Therefore to move an electron from negative to positive, we are doing work against this field.

    Also, where I believe you are getting slightly confused is by thinking about electron flow rather than conventional current, unfortunately circuit theory began before the discovery of the electron. Thinking in terms of positive charges can make things a little easier.
     
  4. Jun 2, 2012 #3

    sophiecentaur

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    If you look at the Definitions of Work, Potential and Field (etc.) you find that Field is the gradient of Potential / Work done on a unit charge. There is no field inside a perfect conductor so there is little point in bringing electron movement into this. Electrons need (virtually) no energy to make them flow (albeit at a snail's pace) through a conductor. No work is involved because the Potential change is zero.

    An Electric field does no work. It is a mis-use of terms to say that.
     
  5. Jun 2, 2012 #4
    so what's the physical(real) meaning of a potential difference in a circuit?
     
  6. Jun 2, 2012 #5
    is it a kind of energy per charge(gaining or losing?)
     
  7. Jun 2, 2012 #6

    Dale

    Staff: Mentor

    This can either be some chemical which is doing work "against" the electric field at the molecular level, or a generator which is mechanically doing work "against" the electric field.

    That is just a convention. You very rarely need to actually think about the electrons, just think about the current for the most part and don't worry about the direction of the charge carriers.

    Yes, the work done ON the circuit BY a resistor is negative.
     
  8. Jun 2, 2012 #7
    it's not the generator that "creates" the electric field in a circuit?
     
  9. Jun 2, 2012 #8

    sophiecentaur

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    Moving one coulomb of charge through a potential difference of one volt involves the transfer of one Joule of work / energy.
    Bear that in mind at all times and it will help you to get things right.
     
  10. Jun 2, 2012 #9
    thanks:D
     
  11. Jun 2, 2012 #10

    Dale

    Staff: Mentor

    Not really. Generators need an excitation, particularly larger or older generators. The excitation gives the generator a field to work against and thereby provide power to the circuit. Of course, that can be cumbersome to arrange, so modern small generators typically are "self excited" meaning that they have the exciter built-in as part of the design.

    You can think of the exciter as providing a small field, the generator does work against that field and makes it bigger and adds power to the circuit. Then that power is used by the attached electrical devices to accomplish the purpose of the circuit.
     
  12. Jun 2, 2012 #11
    aha ok! i didnt know that! thanks
     
  13. Jun 3, 2012 #12
    sorry, but is it the electric field that moves the electrons so how come there's another force doing work on the electrons?
     
  14. Jun 3, 2012 #13

    sophiecentaur

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    It's obvious that it takes a force to move an electron but a force doesn't "do work" because a force in itself is not an energy source (in strict terms). Also, which force / field are you discussing? There is virtually no field inside a wire and no work is done in there. In a resistive circuit, even, where would you say an overall field exists? Just across the terminals? The wires could be looped all over the place. So it is better just to think in terms of Potential, which is a scalar and doesn't care about the geometry.
     
  15. Jun 3, 2012 #14
    So who's responsible for moving electrons in a circuit? Is it the force= thecharge x electric field?
     
    Last edited: Jun 3, 2012
  16. Jun 3, 2012 #15

    Dale

    Staff: Mentor

    Short answer: the "other force" moves the charge in the opposite direction of the field, the field itself obviously can't do that.

    Long answer: A lot of people on these forums don't like the "plumbing" analogy, but I do. For a lot of practical purposes you can think as electricity running through wires in a similar way as water running through a pipe. In that analogy, pressure is analogous to voltage and flow is analogous to current.

    So, let's say that we have a set of water pipes. Attached is a turbine (motor) which we want to drive to do some external work. At the turbine, the power is determined by the pressure (voltage) times the flow (current). The pressure difference (voltage difference) across the turbine (motor) is what drives the flow (current) through the turbine (motor), but how did we get that pressure (voltage) in the first place?

    On some other part of the pipes (circuit) we installed a pump (generator). This pump (generator) takes energy from an external source and uses it to push water (charge) in the opposite direction of where it wants to go naturally, i.e. from the low pressure (voltage) to the high (pressure) side. If the pump (generator) didn't do that then the turbine (motor) would use the little bit of energy stored in the pipes (wires) and then all of the pressure (voltage) would be equal throughout the pipes (wires) and no more work would be done on the turbine (motor).
     
    Last edited: Jun 3, 2012
  17. Jun 3, 2012 #16
    Just last question... In a circuit if there'sa dc source and a resistance the electrons flow from lower potential to higher potential and across the resistance its the same so we can say also that the resistance is doing work against the electric field?
     
  18. Jun 3, 2012 #17

    Dale

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    No, the current flows through a resistor from a region of high potential to a region of low. I.e. The current is moving with the field through a resistor.
     
  19. Jun 3, 2012 #18
    Yes so the electrons flow from lower to higher potential:p no?
     
  20. Jun 3, 2012 #19
    And how about the work done across the resistor is it against the electric field?
     
  21. Jun 3, 2012 #20

    Dale

    Staff: Mentor

    Work is a scalar, it doesn't have a direction.
     
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