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Electricity current and circuit questions

  1. Jul 11, 2015 #1
    may I know if there will be potential difference if there is no resistor in a circuit? If the positive terminal of a battery has a high potential in terms of conventional current, then in electron flow, does it mean that the negative terminal has a high potential?
     
  2. jcsd
  3. Jul 11, 2015 #2
    There are lots of ways to cause a potential difference in a circuit other than a resistor.
     
  4. Jul 11, 2015 #3
    Electrons have high potential energy (not potential) near the negative terminal compared to the potential energy near the positive terminal, which is why electrons drift from the negative terminal to the positive terminal, giving a conventional current in the opposite direction.
     
  5. Jul 11, 2015 #4
    In the case of a short circuit, batteries, capacitors, generators, ect, have their own internal resistance.
     
  6. Jul 11, 2015 #5
    May I know what do you mean by "high potential energy (not potential)"?
     
  7. Jul 11, 2015 #6

    Drakkith

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    The absolute potential is irrelevant, it is the difference in potential between two points that matters. The difference in potential between the two terminals of a battery is equal to the voltage of the battery. It doesn't matter if you think of the positive as being at +9 volts and the negative at 0, or the positive at 0 volts and the negative at -9.

    Note that it doesn't matter whether you use conventional or electron current, the potential of the terminals doesn't change.

    Potential energy is the the amount of energy a charge has by virtue of its location relative to other charges and/or sources of electric and magnetic fields. An electron at the negative terminal will feel a force that moves it towards the positive terminal. The fact that the electron is moving under the application of a force means that work is being performed on it and energy is being used up. The maximum amount of energy that can be used to push this electron from one terminal to the other is equal to the amount of potential energy the electron has prior to having this work done.

    In other words, the electron has the potential to have work done on it by virtue of its location near the negative terminal, and the amount of potential work that can be done is called the potential energy.

    In contrast, an electron very close to the positive terminal has very little potential energy because there is very little distance that the force can be applied over before the electron reaches the terminal.
     
  8. Jul 11, 2015 #7
    Thank you!
     
  9. Jul 11, 2015 #8
    is this pulling force known as the electromotive force? Also, is the energy possessed by the electron known as voltage? I thought potential difference is the amount of energy required to drive a coulomb of charge from a point to another.
     
    Last edited: Jul 11, 2015
  10. Jul 11, 2015 #9

    davenn

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    nothing is being pulled
    read again what Drakkith said :smile:
     
  11. Jul 11, 2015 #10
    is electromotive force = potential difference?
     
  12. Jul 11, 2015 #11
    is the force known as electromotive force?
     
  13. Jul 11, 2015 #12

    davenn

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    quoting from elsewhere on the www


    Dave
     
  14. Jul 12, 2015 #13

    Drakkith

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    The most accurate definition of EMF is too complicated to get into at this level, but for our purposes we can consider the EMF as the force that causes charges to move.

    Electric potential difference (also known as voltage) is different from electric potential in that the former is equal to the work done, per unit of charge, in moving that charge between two points in an electric field. Electric potential is a measure of the potential energy a charged object has in an electric field and is equal to the work done in moving that object from infinity to its current position divided by the charge of the object.

    In other words, voltage (electric potential difference) is the work done per charge in moving a charged object from point A to point B. Plain old electric potential is the work done per charge in moving a charged object from point A to infinity. When you move the charged object from point A to point B, the electric potential will change, and the difference between the electric potential at point A and at point B is, you guessed it, the electric potential difference.

    It's a little confusing if you've never dealt with potential energy before.
     
  15. Jul 12, 2015 #14
    The only that I'm struggling to understand is potential difference. So going by the equation V=E/Q , potential difference (volts) is the amount of work done (energy) by a coulomb of charge, right? Also, may i know how is potential difference a measure of the amount of potential energy that is converted to other forms of energy?
     
  16. Jul 12, 2015 #15

    Drakkith

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    Without getting into the details, yes. If you spend X joules moving Y coulomb of charges between points A and B, then the voltage is equal to X/Y joules per coulomb.
     
  17. Jul 12, 2015 #16
    May i know if ,for example, 9V passes through a light bulb in a circuit, will the light bulb use all 9V worth of energy or will it just use a certain amount and the remaining charges will flow to the negative terminal of the battery?
     
  18. Jul 12, 2015 #17
    There are several sources of confusion resulting from misunderstood and loosely used words.
    Potential energy is different form potential.
    Potential energy (symbol U, unit Joule) is a general concept, seen in mechanics, as well as in electricity. In words, the force on a particle, whether it is a point mass in a gravitational field, or a point charge in an electric field, is in the direction of decreasing potential energy. It does not matter whether the charge is positive or negative. In both cases, the force on the charge is in the direction of decreasing potential energy. Certainly, it is true that absolute value of potential energy U is not defined, only differences in U are observed. But once the zero of U is defined at a particular point, it is defined everywhere.
    Electric potential, or just potential (symbol V, unit Volt) is defined like this: At every point in space, there is a scalar quantity V, such that, if you place a point charge q (positive or negative) at that point, the potential energy is (q)(V). This is so whether the charge q is a positive or negative number. Similar to potential energy, absolute value of V is not defined, only differences in potential are observed.
    If you take a 1.5 V battery, and you define the negative terminal to be 0V, the positive terminal is 1.5V. Now, if you place a +1C charge at the positive terminal, the potential energy is (+1C) (1.5V) = 1.5 J. If you place the positive charge at the negative terminal, the potential energy is (+1C) (0V) = 0J. If you place a - 1C charge at the positive terminal, the potential energy is (- 1 C) (1.5V) = - 1.5 J. If you place it at the negative terminal, the potential energy is (- 1 C) (0 V) = 0 J.
    You can redefine the zero of potential, and can say, for example, that the positive terminal of the battery is at 0 V. Then automatically, the negative terminal is at - 1.5 V. The battery makes sure that it maintains the potential difference. The method of calculation of potential energy is the same, use U = qV, and recalculate all the values again.
    In every case, you will notice that the positive 1C charge has a high potential energy at the positive terminal and low potential energy at the negative terminal. The negative charge has a high potential energy at the NEGATIVE terminal, and a low potential energy at the POSITIVE terminal.
    Now use the rule that everything (mass, charge, does not matter) is pushed towards lower potential energy, and you will see that negative charges flow from the negative terminal of the battery, along the wire, towards the positive terminal, and positive charges flow from the positive terminal to the negative terminal
     
  19. Jul 12, 2015 #18
    In this last post, there are two concepts wrong.
    (1) Voltage does not pass through anything. A voltage difference exists (maintained by a battery) between the terminals or ends of a bulb. A CURRENT passes through the bulb.
    (2) There is nothing called 9V worth of energy, since Voltage is not Energy. The energy (actually the power) used by the bulb is the current passing through it, multiplied by the voltage across it. For example, if 1 A of current is passing through the bulb, and 9V is the voltage across the bulb, then the power dissipated in (or used up by, if you prefer) the bulb is (1A) (9J) = 9 W. The symbol W stands for Watt, which is the unit of power.
     
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