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Need help with understanding resistance

  1. Aug 26, 2013 #1
    so i was studying resistance and there is this something that i cant really understand
    imagine a circuit with Potential difference V and resistance R
    it has like say 3 resistors , now How is the current intensity constant ?
    if the resistors decrease the potential energy , after the 3rd resistor shouldn't electrons actually stop moving because there is no potential energy anymore ?? since all the potential energy has been used up by resistors ?
    the only way to think of this is if the whole circuit design is not physically accurate , and that those resistors drawn are just symbols for the resistance across the whole wire not just those places , this would make sense . is this true ?
     
  2. jcsd
  3. Aug 26, 2013 #2
    Potential and kinetic energy are different.

    The electrons in the circuit are all moving at the same speed. If something, like a resistor, tries to slow them down, some of their potential energy has to be converted to kinetic energy to bring them back to the original speed. After the electrons leave the last resistor, they have no* potential energy remaining, but they still have the small amount of kinetic energy they need to keep moving at the right speed.

    *In reality the wire has some resistance, so the electrons still have a little bit of potential left -- just enough to compensate for the energy they lose in the wire. But even if the wire were genuinely of zero resistance, then the electrons could simply coast back to the battery on their kinetic energy, so they wouldn't need any potential energy.
     
  4. Aug 26, 2013 #3
    i have read some kind of an explanation , would you care to assure me that i am right ?
    i have read that for an electron to move it has to push all the electrons in the wire , so it pushes electrons through the 3 resistors , thats why an electron at the beginning of the circuit faces the resistance of the resistors that it hasn't reached , now when it passes the 1st resistor , it has less speed , but it also has less resistance to push electrons through , and so on , in the end , when electrons have passed three resistors , they have no resistance anymore , so they can easily be pushed by electrons behind them who still have some potential difference * stating that the wire is super conducting *
     
  5. Aug 26, 2013 #4

    nsaspook

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    The circuit design is physically accurate it just your idea of how electrical energy moves in a circuit is not accurate. The potential and kinetic energy of the circuit is not carried by the electrons but is in fields that surround the wires and resistors but for now let stick to basic circuits and the flow of charge.
    http://www.physicsclassroom.com/class/circuits/u9l4a.cfm
     
  6. Aug 27, 2013 #5

    meBigGuy

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    Assume that as many electrons enter as leave the resistors. The current flow is constant throughout the circuit. The applied potential can force a limited number of electrons through the resistors, and so there is a potential drop distributed across each resistor (resulting in dissipated power). The two extremes would be an infinite number of resistors, or 1 resistor. There is no difference. The number of coulombs per second is constant at all points in the circuit and determined by the ratio of potential and resistance. The electrons don't really know what the voltage is at any point in the circuit. They just flow.

    Be careful about the difference between electrons and current. Your model of an electron entering causing an electron to leave is somewhat correct. Like pingpong balls in a pipe (people here probably hate that analogy).

    Resistors do not slow down electrons. Resistors just increase the potential required to force a given number of electrons per second (current) through a circuit.
     
  7. Aug 27, 2013 #6
    wouldn't that mean that before the electron reaches the resistor , it would have infinite intensity since there is no resistance *in the hypothetical circuit where the resistance only exists in the resistors * ? i understand that its constant across resistors because as the resistance increase the voltage drop also increases , but what bothers me is not the resistor , but the length of wire where the electron travels before reaching the resistor . if there is no resistance there then how could electrons still travel with the same intensity unless the electrons really push each other as i said before ?
    i also wonder why would the potential drop affect the current if the current is already travelling by the potential difference between the two terminals * which is the voltage of a battery * , it seems like a potential drop is only because the potential energy of the current was used due to resistance
     
  8. Aug 27, 2013 #7

    meBigGuy

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    Ok, I'm forced to use the water/hose analogy. Water flows in a hose, but the nozzle limits the flow. The more pressure, the more water flows out the nozzle. How does the water know there is a nozzle down stream. It's an analogy with flaws, but will help you think about it.

    "unless the electrons really push each other as i said before ?" <--- That's a valid analogy
     
  9. Aug 27, 2013 #8
    well they know because it builds up pressure , the nozzle builds pressure on water next to it and the water next to it apply pressure to nearby water and so on , my analogy was right thank god :P
     
  10. Aug 27, 2013 #9
    Compare with water flow in a channel or pipe.

    If you send water to sea, then even if you send huge amounts of water there, the sea will not rise noticeably. Sea generally away from the rivermouth will not rise much because the sea is big and it would take a lot of water to raise it; sea in the immediate vicinity of rivermouth will also not rise much, because being wide and deep it has very little resistance to flow - there will be slow current away from rivermouth in all directions, but the slope of water will be very small.

    If you look at a channel where no water is lost or added, and the water flow is steady, then there is a constant rate of water flow. The linear speed of flow gets fast where the channel narrows or shallows, and slows down where channel is widened or deepened into ponds, but the amount is the same everywhere.

    If the water flow is unsteady, like suddenly increased, then there might be different amounts of water flow along the channel, because some water is, e. g. filling ponds and not flowing on at the same rate. This applies mainly in case there is a lot of capacitance to fill. If you are dealing with circuit of little capacitance, like closed pipes full to the ceiling (even no air headspaces to compress) then the capacitance is very small - water is hard to compress, and attempting to do so causes a strong water hammer.

    In electric circuits, wires do have small capacitance, but it is relatively small - piling up electrons anywhere causes strong electric field potentials. So, the electric current has to be mostly constant along a series. And if the current is steady then no capacitances are charging and discharging, so the current in series has to be constant even if there are appreciable capacitances present.
     
  11. Aug 27, 2013 #10

    nsaspook

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    Concentrate on 'charge' and current not electrons and current. The electron fixation comes for trying to understand circuits as mechanical devices. The mechanical function of electrons at this point is a side show and a major distraction from having a intuitive view of basic circuits. I'm not saying the mechanical details of charge-carrier movement are unimportant, it's just that you have to know when it is important.
    http://www.physicsclassroom.com/class/circuits/U9l4c.cfm
     
    Last edited: Aug 27, 2013
  12. Aug 27, 2013 #11

    sophiecentaur

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    Electrical resistance is not something that you can "understand". It's just the ratio of Potential Difference and Current. Any alternative attempt at describing it (like 'how hard you have to push to get so much current through') is doomed to failure. Why bother waving your arms when you have such a rock hard definition? It tells you everything.
    You can give examples of Resistance values, in the same way as you can give instances of Velocity, Acceleration, Density etc. but it's only chatting around the topic. Also, it really doesn't help to talk about electrons going round a circuit and encountering a resistor. With low current or with AC of any value, the electrons don't get anywhere significant. The show may be over before they've had time to go more than a few mm around the circuit.
    The quantities and equations that are used in Electrical calculations were developed over the years for very good reasons. Try to think that way if you really want to get to grips with any of Physics. Once the mathematical stuff becomes familiar, it begins to be constitute understanding and you won't need to rely on inadequate quasi physical analogies.
     
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