Simple circuit question

Discussion in 'OT Technology' started by terminator1010, Nov 7, 2005.

  1. terminator1010

    terminator1010 Eld

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    I've got a 100Kohm resistor connected to the posive terminal of a 9V battery. This is an open circuit. The voltage across just the battery is 8.6V, the voltage from the resistor to the negative terminal of the battery is ~7.5V. I'm wondering why there is a ~1.1V voltage drop across the resistor if there is no current flowing through the resistor (cause its an open circuit)
     
  2. High Voltage

    High Voltage Guest

    r u trying to get me to do your homework for you :rofl:
     
  3. Tills

    Tills Lets Go Flyers

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    When you are when you are taking the measurement, what are you doing to the circuit?
     
  4. terminator1010

    terminator1010 Eld

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    LOL, I wish my homework were this simple.

    I'm not doing anything to the circuit, it remains an open circuit. (i'm completing the cicuit with the voltmeter when taking the voltage at the open end of the resistor and the negative terminal of the battery. Getting 7.5V) This seems to defy the most basic laws of circuit anaylsis. Open circuit = no current flowing = no voltage drop across resistor. In theory I should be getting 8.6V when i'm actually getting 7.5V.
     
  5. Tills

    Tills Lets Go Flyers

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    Current flows positive to negative. If you are measuring from the "Open" end of the resistor (the closed end being attached to the positive side of the source) to the negative side, you are measuring the voltage after the resistor hence the drop in voltage.
     
  6. ScrapinSi

    ScrapinSi New Member

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    DING! That is Correct!
     
  7. twistid

    twistid Banged By Super Models Moderator

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    DC current always flows from negative to positive, where AC current flows back and forth between positive and negative.
     
  8. terminator1010

    terminator1010 Eld

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    I don't quite understand this. There is no current flowing in this circuit though. So what causes the voltage drop? Also when I put the voltmeter across just the resistor, I get zero volts. meaning no voltage drop. Why is this?
     
  9. Tills

    Tills Lets Go Flyers

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    I have a DC power source with a positive +5V and a +12V, which way is the current flowing?
     
  10. Tills

    Tills Lets Go Flyers

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    When you are placing the meter across the neg terminal and the open end of the resistor you are completeing the circuit hence current is flowing. The voltage drop that you see is from the 100kOhm resistor.

    If you attach the resistor to the neg term of the battery then take a measurement from the positive side to the open end of the resistor you should see full voltage.
     
  11. terminator1010

    terminator1010 Eld

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    Nope, tried putting the resistor on the neg side, same voltage, sign reversed of course. Current does actually flow from negative to positive. Current is the flow of electrons. The negative terminal or ground provides the electrons and they flow to the abscence of electrons (positive voltage). There is a current convention though that states you can take current flow from positive to negative as most people use.
     
  12. Tills

    Tills Lets Go Flyers

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    Very Good...now the question is why the voltage drop is still there? (I expected it to be)

    We are both right and wrong depending on which viewpoint that you take but I am right Pos to Neg :p. Lets save this debate for a later time :)
     
  13. twistid

    twistid Banged By Super Models Moderator

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    http://easyelectronics.tripod.com/dcac.htm
    Conventionally, dc current is regarded as being from 'positive' to 'negative' of a battery or any other dc source (such as a dynamo). It is a 'stream' flow, just like the water flow in the hydraulic circuit, but the 'stream' is actually composed of sub-atomic particles or electrons. Unfortunately, after convention had established the 'positive to negative' flow definition it was found that this electron stream flow was actually from negative to positive. This does not matter for most practical purposes, but for an understanding of how transistors and other solid-state devices work it is necessary to appreciate this 'reverse' working

    http://www.school-for-champions.com/science/dc.htm
    Direct current or DC electricity is the continuous movement of electrons from an area of negative (-) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire.

    http://www.xtant.com/html/techSupport/electronics.cfm
    The movement of free electrons from one atom to another is called current flow. Current always flows from negative to positive. A good analogy can be made with a water hose. The hose may be considered a conductor, and the water moving electrons. The current, measured in amperes, would be the volume of water passing by a point along the hose in a given amount of time. The higher the volume, the more current.

    i could go on...
     
  14. terminator1010

    terminator1010 Eld

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    I suspect it has something to do with the internal resistance of the source, or the non-idealties of the voltage source.
     
  15. ScrapinSi

    ScrapinSi New Member

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    how about drawing up some schematics to show us exactly where you are measuring,and what your talking about?
     
  16. terminator1010

    terminator1010 Eld

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    Too much work. Its not that hard to understand, resistor attached to battery.
     
  17. Tills

    Tills Lets Go Flyers

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    You are ignoring the electon holes which travel from positive to negative and produce current. As I said before we were both right and wrong depending on the viewpoint. If you have any questions on the matter feel free to ask them and I will explain it.
     
    Last edited: Nov 12, 2005
  18. Tills

    Tills Lets Go Flyers

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    You need to make sure the 9v is an energizer ;)

    In a properly working battery, I don't believe you should see a voltage drop that large due to the source.

    Have you calculated what the voltage drop over the resistor should be in your circuit?
     
  19. twistid

    twistid Banged By Super Models Moderator

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    here's a simple example: when you wire an amp, you put the fuse on the + side close to the battery. this one protects from a short in the wiring... if the wire from the fuse to the amp shorts out, it will pop the fuse to stop from completing the circuit because dc current flows - to +... if it flowed the other way you'd have to fuse the ground instead.
     
  20. Tills

    Tills Lets Go Flyers

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    Actually you put the fuse on the positive side in case you short to (an alternate in this case) ground.

    I am not arguing that electrons don't flow neg to pos, what I am saying is that there are "electron holes" (Empty spaces in the atoms valence) that move as well and create current.

    What happens if I am powering a light bulb with 2 negative sources?
     

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