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Homework Help: Electric field work physics problem

  1. Feb 23, 2008 #1
    For point charges within an electric field, the natural direction a point charge takes in respect to a field would be towards lower potential energy, whereas the work done by the conservative force should be positive. So then, by W = -q*deltaV, do they refer to the magnitude of the test charge q? If deltaV was negative, it indicates that positive work was done...but if the charge is negative, the work becomes negative.
  2. jcsd
  3. Feb 23, 2008 #2
    I'm not sure if I'm answering your question but W= -qEd which is equivalent to the change in potential energy in a UNIFORM field only.

    The potential difference can be solved for using W= -Vq this is the amount of work it takes to move the test charge from a point a to point b say in between two plates

    The electric potential V in an non uniform field does not depend on the test charge and its formula is then V=KQ/R

    Electric potential energy on the other hand is defined as PE= Vq and does depend on the magnitude of the test charge and its sign.

    In sum remember that Electric Potential does not depend on q; but that Electric Potential Energy (PE) does depend on q.
  4. Feb 23, 2008 #3
    I see....but then in that sense -deltaU = W done by conservative force and deltaU/q = deltaV. That implies that -deltaV*q = W done by conservative force. For any particle in respect to a conservative force, its movement is towards less potential energy, which should also result in a decrease in electric potential (making deltaV negative). If W = -q*deltaV, then doesn't that imply that if an electron was naturally moving due to an electric force, W would be negative, whereas conceptually the actual work done by the conservative force should be positive?
  5. Feb 23, 2008 #4
    An electron moves towards a higher potential. Electric potential is defined as the potential energy of a positive test charge.
  6. Feb 23, 2008 #5
    wait a minute...you're right. So if deltaV is negative and q is negative, deltaU is positive...which means work was done against the conservative force, whereas, the conservative force does negative work in this case. So can we make this generalization?: If a negative charge is brought towards lower electric potentials, then the work done by the conservative force is negative.
  7. Feb 24, 2008 #6
    yes. If you exchange the sign of all charges and potentials, all the forces and energies should come out the same.
  8. Feb 26, 2008 #7
    Work in an electric field

    When a charged particle passes between two parallel plates connected to a voltage source, the particle is accelerated toward one of the plates by the field. Assuming that the particle continues on its way without contacting either plate, how is the work done on the particle "paid back"? Is there a mechanism that reduces the field strength E by delta E that requires a current through the voltage source to restore the field to E? If so, what is the mechanism? If not, how is the work accounted for?


  9. Feb 26, 2008 #8
    If The particle picks up speed between the plates, it must have started at a higher (or lower for a negative particle) potential. If the particle is shot between the plates from far away (where the potential is very close to 0) and then flies off again to a far away point, the field can't have done any work on it.
  10. Feb 26, 2008 #9
    Could you say that any particle that is accelerated in an electric field must have started from a higher potential, so that no work may be done by an electric field on any particle that is accelerated in the field?
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