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Homework Help: ELectricity Question>?

  1. Apr 27, 2007 #1
    1. The problem statement, all variables and given/known data

    When I say I have a 9v battery, am I talking about the electric potential, electric potential energy, or the change in electric potential?

    2. Relevant equations

    3. The attempt at a solution

    My first thought was the change in electric potential energy. But then I thought that it was electric potential energy becuase the 9v is referring to the energy that it can produce to move electrons? I figured that the change in electric potential would be if were were talking about specific locations, I.e one node of the battery would be at 9 (the high end of electric potential) and the other end the negative end would be 0 (low end of electric potential) so the difference would be 9v.

    See where I am just confused a little>? any help in just reasoning it out some more to make it more clear?

  2. jcsd
  3. Apr 27, 2007 #2
    Voltage is defined as the difference in charge between two points, regardless of the amount of energy each point has. So the answer would not be electric potential energy.
    The answer would be the electric potential, since the change in electric potential is known as Work.
  4. Apr 27, 2007 #3
    OK that is confusing to me still....so the 9v battery would be called electric potential. Meaning that the 9v is the amount of of electric potential is has? Can you describe a bit more? I just want to understand the differences between the three and I think I am getting a few things mixed up? Let me know when you can

  5. Apr 30, 2007 #4
    Sure -- one thing you need to remember is that the voltage in an object can only be measured relative to the voltage of another object. This is comparable to velocity in kinematics -- say you were walking at 3mph on a train moving 50mph. You are only moving 3 mph relative to the train, but you are moving 53 mph relative to the ground.
    For typical circuits on earth, we are comparing the voltage of a power source to the voltage of ground. For a 9V battery, the "+" lead of the battery has a voltage difference of 9 volts relative to the "-" lead, regardless of how many extra electrons each side has (they want to balance out -- the electric potential is how hard the electrons push eachother towards the lower potential).

    Compare this to change in electric potential: now we are adding "work" to the system (sort of like acceleration). An object is pushing electrons from "+" to "-" harder and harder. When you are increasing the voltage between two bodies you are adding more and more push, and this change in push over time is the "change in electric potential".

    When you refer to the electric potential energy, you are referring to the actual capacity of extra electrons in a body. This value isn't especially useful unless it's compared to something else (the electric potential). Sort of like saying we are moving at millions of miles per hour around a sun or galaxy, but we don't see a difference because we are not moving at all relative to the Earth.
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