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I Electric potential. In circuits. + -

  1. Nov 4, 2016 #1
    Electric potential energy in circuits.

    Here is what I know: The electric potential between two points is the work done against electric forces to move that mass of charge between those two points.

    What does this mean in a circuit? I know basic things about electric field, the test charge concept..and some basic physics for EE (even though our syllabus for electrical engineering did not teach us this!)

    Could anyone explain WHY when positive current or conventional current (positive charge) enters through the positive terminal of a circuit element with A+ and B being -, and the voltage of A WITH RESPECT TO b is positive, we say that the circuit element absorbs energy?

    I have asked many people but my friends and tutors werent able to answer, perhaps I am asking something irrelevent? or what? I just am not able to put the physics into my mind.

    All I have come up is this: The signs do not indicate direction of current or voltage drops/rises. The passive sign convention states to always ensure positive current flows out through the positive terminal.

    ALl I see is some analogies about water and hose and pipes which doesent fit in with

    D:<

    is this the only way circuit analysis can be learned?
     
  2. jcsd
  3. Nov 4, 2016 #2

    Dale

    Staff: Mentor

    Think of what you are describing here. You are describing a charge which moves from a lower voltage to a higher one. That movement of charge is current. This is exactly the same voltage and current arrangement as the active sign convention.

    Notice that the work done "against electric forces" means that energy is leaving the matter and going into the fields. This flow of energy from matter to fields is also consistent with the active sign convention.
     
    Last edited: Nov 4, 2016
  4. Nov 5, 2016 #3

    CWatters

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    Science Advisor
    Homework Helper

    Does it help examine the opposite situation? For example where you have a battery supplying energy to a circuit? In that case positive current or conventional current (positive charge) leaves the positive terminal of the battery. It's not hard to understand why we say the circuit element (the battery) is emitting energy.
     
  5. Nov 5, 2016 #4
    Hello. What is the active sign convention...as in what do you mean by that i google it but do not understand..also having a hard time grasping your original post. I really want to understand but so far am not able to

    1. When it moves from a lower voltage to a higher one, work is done against the electric field?

    2. When it moves from a higher voltage to a lower one, negative work is done against the electric field?


    the book I was reading said this type of analysis is really complicated when many particles are in a system, but how does this translate to work done and energy absorbed? with two particle system it is easy to see where the electric field eminates from, but in a circuit what is the zero point? the positive terminals of voltage sources? or should one just forget this type of thinking.

    Also I have often seen positive terminals having the lower potential/
     
    Last edited: Nov 5, 2016
  6. Nov 5, 2016 #5
    It is all right to think about all this at the beginning and I did too (alot) but now I have a more clear vision of the situation. You always have to think about a concept and where it came from before trying to apply it.

    You seem to not fully understand what is potential energy. I am going to explain what it is in simple terms. Imagine you throw a ball in the air, the force is downward movement is upward. Now you see the ball slowing down. We defined a quantity Energy to be force times distance.

    Okay, so you see it slowing down then when it reaches its highest point it speeds up downward. So from the kinetic energy perspective, It first had energy then all that energy (Kinetic) went away then it started to speed up again. Something is taking the energy, storing it then giving it back. It is gravity.

    So lets get back to the situation where it was going up, Force is down and displacement is up. This we say is negative work, it means that we are taking away its energy. Where is it going? It is stored in the field as a potential for the system of earth and the ball. Now the fact is that energy represents kinetic energy which relates to speed then it must be conserved. Imagine if it isn't then you would find a guy pushing a rock up and down and saying that he is making more Energy.


    Similar thing happens with electrostatics, It is a force that act on 2 objects and is equal in magnitude just like gravity. We can say also we have potential because of the electric forces

    Once you grasp that, You can easily answer the questions you listed above. If I go from lower voltage to a high one then the potential is increasing which means I am going against something. There is a noncoloumb force doing positive work on the charge and a negative work is done by the electric field on the charge.

    Notice the term By and on... When you are going from a higher voltage to a lower one then potential is decreasing which means I am going with the force in the same direction. So the electric field is doing positive force on the charge. (You can also say that the a negative work is applied on the electric field not sure of the words of the statement though)


    About point zero, You can set anything to zero and measure the potential from it. Exact potential is not important concept. What is important is the change in potential energy. Example I can say 6 - 4 = 2 and I can also set distance 4 = 0( Not a good sign though, dont interpret it wrong) and 6 = 2 as a reference point and say 2-0 = 2.. The distance between 6 and 4 is the same.

    Here is a paper (Credit to Dale) about how the electric field is formed in a circuit: https://www.tu-braunschweig.de/Medien-DB/ifdn-physik/ajp000782.pdf ( The pictures are quite good, You dont have to understand it all just look at what you know)
     
  7. Nov 5, 2016 #6
    ''I will read your post later, please gve me time, bit busy doing a lab assignment. I know basic physics definitions xD but I am weak in physics or just plain dumb, I guess.
     
  8. Nov 5, 2016 #7

    Dale

    Staff: Mentor

    You mentioned the passive sign convention in your post, so I assumed that you would be aware of the active sign convention also.

    In the passive sign convention the positive direction of I is from the + terminal to the - terminal, so VI is power going from the field into the circuit element (e.g. a resistor). The active sign convention has I point from the - terminal to the + terminal, so VI is power going from the circuit element (e.g. a battery) into the field.

    What you described in the opening post showed that you were thinking of the active convention, which may be the reason that the passive sign convention bothers you. They are both just conventions, and you can use either or both, as long as you are careful.

    Yes. Energy is transfered from the charges into the field.
     
  9. Nov 5, 2016 #8
    I wonder if an analogy might help. You are probably familiar with a marble track. The marbles roll down a hill from top to bottom. On the way down they can do all sorts of tasks, ring bells, turn wheels, etc. You may have to do the work of moving the marbles back to the top, but on good tracks there's an elevator to lift the marbles back to the top. On these tracks the marbles go around continuously in a closed loop. They flow from top to bottom, and thanks to the elevator they flow from bottom to top. They go both directions.

    At the top the marbles have potential energy. Potential energy is the possibility, the potential, to do work. By the mere virtue of being at the top of a hill, the marbles have the potential to cause things to happen on the way down. Spinning wheels and ringing bells etc.

    However notice that they didn't get to the top of the hill for free. The elevator had to push them there. On one half of the circuit the marbles flow due to their own potential energy and on the way down they give up their energy doing things= work. On the other side of the circuit, the marbles must be pushed up hill, from negative to positive. Energy has to be expended to impart potential energy. The elevator pushes the marbles uphill against gravity imparting potential energy and "adding energy" to the circuit.

    So the electrical circuit is just like this. However, the potential energy isn't lifting marbles against the force of gravity. Instead potential energy is stored by pushing the positive charges against their mutual repulsion to crowd together at the positive terminal. (Ignoring for the moment that it is usually actually negative charges that do most of the moving.) This is the battery (or generator or whatever) adding power into the circuit. When allowed the positive charges are driven by mutual repulsion away from the positive terminal and flow through various components imparting their energy to accomplish things like lighting light bulbs etc.

    So there is energy put into a circuit by the battery or dynamo or whatever, and there is energy being taken out of the circuit being expended by electrical components.

    If you wanted to keep track of the energy being put into and taken out of the circuit, you might say the energy being put into the circuit is positive, and the energy being taken out is negative. However the bookkeeping is exactly the same if you call what is being expended positive and what is being generated as negative so long as you change the signs of both. People got used to talking about the expended energy in positive terms, so that's the convention.
     
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