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Electric potential is separation of charge?

  1. Feb 4, 2014 #1
    I am having some troubles wrapping my head around the electric potential concept. From what I gather it is a separation of charge? Is this true? On a more formal level electric potential energy is the amount of energy in joules it takes to move a charge against an electric field or the amount of energy in joules released in the form of kinetic energy when in the same direction as an electric field. Then the electric potential is found by dividing by the charge. So we are essentially saying it takes a certain amount of energy per unit charge to move a charge in an electric field. Then electric potential difference is the difference between the joules per coulomb to some arbitrary reference point or any other point with an electric potential.

    Could someone please clarify these concepts. Why do we define potential as joules per coulomb, what's the point?
     
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  3. Feb 5, 2014 #2
    Its a definition I think. If an Electric field is required to cause an electron to convey energy then the potential for it to do work it must essentially measure the amount of Joules (work) it will convey to each Coulomb (which is part of how Electric current is measured).
     
  4. Feb 5, 2014 #3

    Simon Bridge

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    The electric potential is the electric potential energy of a particular charge divided by the amount of that charge.

    By analogy with gravity - the gravitational potential energy (near the Earth) would be U=mgh, the gravitational potential would then be U/m = gh.
     
  5. Feb 5, 2014 #4
    so electric potential is related to energy and the location of where it is in an electric field. So here is a question; if we have a positive and negative charge, according to our definition the electric potential should increase as you move them apart but if you move them to infinity I would think the electric potential should decrease to zero as the electric field would be almost non existent.
     
  6. Feb 5, 2014 #5

    jtbell

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    The potential energy of a positive and negative charge together is a negative number. As you separate the charges, the potential energy increases towards zero, and decreases in magnitude (absolute value).
     
  7. Feb 5, 2014 #6

    sophiecentaur

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    It is true that there's something much more 'intuitive' about Forces than about Work and Energy - so people often prefer to talk about Electric Fields and try to appreciate all electrical stuff that way. However, using the idea of Potential (which is what Volts are all about) is totally equivalent and tells you just as much about the system. It also allows you to talk about the energy involved in taking charge all round a circuit without needing to bother with the way the circuit is laid out in space. (You need to know the metres in order to know the Volts per Metre). That makes Potential a real favourite in most cases and pretty much all electrical problems are done using Volts.
    Whenever you come across Volts, say to yourself
    One Volt is One Joule per Coulomb
    That tells you the energy needed to shift charges around and Electricity is all about making things happen with energy.
     
  8. Feb 6, 2014 #7

    Simon Bridge

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    Have to be mindful of the sign when you have positive and negative charges together.
    You also appear to be confusing potential with potential energy. I'll try to explain:

    A function that approaches zero as distance increases, but, at the same time, gets bigger, must always be less than zero. But it is the potential energy that does that ... check back with what I said about the relationship between electric potential and electric potential energy and do the math.

    For a charge "-Q" a distance r from a "+Q" one, the potential is V=U/(-Q) =kQ/r
    For the +Q charge, same distance from the -Q one, it is V=-kQ/r
    The work done taking the charges from a separation of r1 to r2 is the difference between the potential energies not the potentials. Therefore: $$W=q(V_2-V_1)=-kQ^2\left(\frac{1}{r_2}-\frac{1}{r_1}\right)$$ ... in either case.
    Notice that if r2 > r1, then the work is positive?

    The potential energy gets bigger as it approaches zero (for increasing separation) by becoming less negative.


    I also want to spell out some distinctions:
    The potential energy of a system of charges is the amount of work needed to assemble it.
    The potential energy of an individual charge at a particular place is the work needed to get that charge to that place.
    The potential of a particular place is the amount of work-per-unit-charge needed to get positive charge to that place. The place does not have to have any charge in it to have a potential.
     
  9. Feb 21, 2014 #8
    So why does negative charges tend to flow toward higher potential and positive charges toward lower potential? Rephrasing this question, how do we define high potential and low potential... because if we bring an electron toward a negative charge it has high potential and if we bring a positive charge near another positive charge it has high potential.

    On a separate note just to clarify I understand. Lets use a real world example. A wall outlet has 120 volts across its terminals. So applying the electric potential difference concept to this situation, we could say that it takes 120 Joules per coulomb of charge to move a charge from the lower potential side of the outlet to the higher potential side of the outlet?
     
    Last edited: Feb 21, 2014
  10. Feb 21, 2014 #9

    Simon Bridge

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    Because that is how the conventional potential is defined.
    The charges behave in opposite ways to a potential because opposite charges attract.
    If gravity had an opposite charge, the negative-grav objects would roll up the underside of ramps.

    By using the work-energy relation to get the potential energy. You can look up the definitions for potential and potential energy and see.

    Wall outlets are AC - so that is 240V rms.

    If a battery has 9V between the terminals then it would take 9J energy to move a one coulomb negative charge from the positive terminal to the negative one. Falling (from rest) the other way, it gains 9J kinetic energy before it hits.

    Dealing with real charges inside solids is a bit trickier - real life is messy.
     
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