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How can Voltage have polarity? How can they be negative or positive?

  1. Jun 18, 2014 #1
    Voltage is potential difference between two terminals of a power source that causes the electrons to flow, right? How can that be positive or negative? What is positive voltage and what is negative voltage?

    Thank you,
     
  2. jcsd
  3. Jun 18, 2014 #2

    NascentOxygen

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    Potentials are relative. So if one wire in a circuit is of a greater voltage than a second wire, we can say the first one is positive with respect to the second. Or, equivalently, we can say the potential of the second wire is lower than that of the first, i.e., the second wire is negative with respect to the first.
     
  4. Jun 18, 2014 #3
    Do you understand how, in general, a difference can be positive or negative?
     
  5. Jun 18, 2014 #4

    UltrafastPED

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    Current flows from the + terminal to the - terminal; if you have a galvanometer hooked up in the middle, and switch the leads at the terminals, it will read the same current, but with the opposite sign.

    This is the test of polarity. For an AC generator, the sign changes 60 times per second, so you better put the meter into AC mode!

    Batteries are also labeled + and -.


    PS: Note that "standard current" is the flow of positive charge carriers; electrons actually flow in the opposite direction, but that is simply a confusing fact which is due to the history of how + and - charges were defined by Ben Franklin!
     
  6. Jun 18, 2014 #5

    rcgldr

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    You could also consider the voltage between two points near some charged object that generates an electrical field, using a third point as a reference point.

    For the potential from a charged point or sphere (for the field outside the sphere), a convenient reference distance from that charged source would be ∞, with the potential defined as zero at ∞ distance, and increasing (towards +∞) as distance from the charged source decreases. Note that voltage is potential energy per unit charge, so for a negatively charged particle, the electrical potential energy would become increasingly negative as distance from the charged source decreases.
     
    Last edited: Jun 18, 2014
  7. Jun 18, 2014 #6
    You mean that the potential difference will be positive between any two points in that field?
    How can this be? The sign and value of the potential difference between the two points depends on the two points, including the order of the points.
     
  8. Jun 18, 2014 #7

    rcgldr

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    No, only that the potential of both points relative to a third "reference" point could both be positive. The potential difference could be positive, zero, or negative. Note - I cleaned up my previous post.

    For the potential from a charged point or sphere (for the field outside the sphere), a convenient reference distance from that charged source would be ∞, with the potential defined as zero at ∞ distance, and increasing as distance from the charged source decreases. Note that voltage is potential energy per unit charge, so for a negatively charged particle, the electrical potential energy would become increasingly negative as distance from the charged source decreases.

    For the potential between two oppositely charge plates, the surface of the negatively charged plate could be used as a reference where the potential is defined to be zero, and potential would increase as distance from the negative plate increases, until the positive plate was reached.

    For the potential from an infinite plane or disc with a uniform charge per unit area, the surface of the plane or disc could be used as a reference point where the potential would be defined as zero. For a positively charged infinte plane or disc, the potential would become more negative as distance from the plane or disc increases.

    For the potential from an infinitely long wire with uniform charge per unit length = λ, the potential at radius r versus a reference radius r0 is - 2 k λ ln(r / r0), so choosing r0 = 1 depending on the unit of distance (meter, foot) would result in a potential of zero at r = r0.
     
    Last edited: Jun 18, 2014
  9. Jun 18, 2014 #8
    Yes, it's true that potential of all the points in the filed can be positive in respect to a reference point.
    But you mentioned the "voltage between two points" in your previous post. It was a little confusing.:smile:
     
  10. Jun 18, 2014 #9

    rcgldr

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    I cleaned up that previous post.
     
  11. Jun 20, 2014 #10
    Thanks, but could you explain as to how a wire in the circuit creating potential difference could have more voltage than the other?
     
  12. Jun 20, 2014 #11
    If you attach two wires to opposite ends of a battery, and complete the circuit by attaching the other ends of the wires across a resistor, then one wire will have a higher voltage than the other. This is because one end of the battery attracts electrons more strongly than the other end. This has to do with the chemistry of the elements that make up the cells of the battery.
     
  13. Jun 20, 2014 #12

    sophiecentaur

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    Consider the situation of Gravitational Potentials. Take a roller coaster track. the GP difference between 'top and bottom' is the same, whatever the altitude of the site it happens to be built. If you lift the end and lower the start points, the GP difference will be the negative of the GP difference you started with.
    The term Potential Difference is better than Voltage because it stresses the aspect of a 'Difference', which the somewhat sloppier term 'Voltage' doesn't. Think about Electrical PD in the same way as you'd think of Heights and Height Differences and the answers should come naturally and accurately.

    Two batteries connected to a metal plate, which is connected by a big copper spike to the ground. One battery is 3V and the other is 12V. Both '-' terminals are connected to the Earth plate. The voltage between the 12V+ terminal and the 3V+ terminal will be 9V, if you are talking with respect to the 3V terminal (black lead from the meter on the +3V)and -9V if you are talking with respect to the 12V terminal (black meter lead on the +12V terminal). The 12V is 'higher' than the 3V and the 3V is 'lower' than the 12V.
     
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