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What is the voltage at the end of the circuit?

  1. Mar 4, 2017 #1
    attachment.php?attachmentid=43390&stc=1&d=1328146169.gif

    I have a some questions that will help me understand voltage better.

    1. What is the voltage from the positive terminal to the negative terminal? Why?
    2. What is the voltage from point 7 to point 8? Why?
    3. Is the electric potential at the very end of the circuit (negative terminal) zero? Why/Why not?
    4. If you connect one lead of voltmeter to the positive terminal of the battery, and one lead to the negative terminal of the battery (no resistors), what would the voltage read? I know it'll either be 9V or 0V. I think the answer is 9V, but conceptually shouldn't it be 0V because the electric potential at the two points should be the same, because there are no resistors, just the voltmeter. So x volts minus x volts = 0 volts

    This isn't homework btw, I'm just trying to work some stuff out in my head to help me conceptualize voltage and understand it better.
     
  2. jcsd
  3. Mar 5, 2017 #2

    haruspex

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    Potentials are relative. You can define any point as zero potential, then all other points are relative to that.
    Which is why voltmeters contain very high resistors. You do not want much current going through them or the reading will be wrong.

    For your first two questions, maybe not homework, but I still think it would be useful for you to try to give your own answers first.
     
  4. Mar 5, 2017 #3
    1. 15 V is what I think
    2. 0 V is what I think.

    And this definitely isn't homework. I'll pulled a pic off google images and made these questions. I could care less about the value if the answer, I want to know the reasoning behind it. Is 1 and 2 correct and why?
     
  5. Mar 5, 2017 #4

    cnh1995

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    The battery is of 9V.
    Right.
     
  6. Mar 5, 2017 #5
    Voltage is measured as the difference in potential between two points. A voltmeter has two leads which must be connected to the two points whose potential difference is to be measured. Any two points may be chosen. One of this point is treated as a reference point. We have a very similarity situation when we try to measure a height of an object. We need a reference point. The most common reference point is "above mean sea level". But when you measure the height of the table in your house the floor now becomes your reference point.
    So, the voltage from the positive terminal to the negative terminal is equal to -9V. This means that the negative terminal voltage is 9V lower than the positive terminal voltage.

    https://www.physicsforums.com/attachments/rys0-1-jpg.69517/

    https://www.physicsforums.com/attachments/0-1-png.102221/

    Try read this
    http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf
    And watch this


    Notice that point 7 and 8 are the same point. In fact, all points starting from 5,6,7 and 8 are the same point in the circuit (represent the same point).
    This means that there is no voltage between thus points. So, the answer is 0V, no difference in potential between those two points. They are at the same electric potential.


    No, voltage can exist without the resistor. So, the answer is 9V.
     
  7. Mar 5, 2017 #6
    Just to point this out, in the stable state, you can mentally just treat parts of the circuit as a black box.

    So, imagine the Voltmeter actually showed 0V between the terminals. Where would any power come from then? That is, if the Voltmeter "feels" 0V, why would the resistors down the line (which are in parallel to the Voltmeter) feel anything? With 0V you get zero current.
    So, just by that argument alone, thee must be voltage between the terminals.
     
  8. Mar 5, 2017 #7

    davenn

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    Not quite ... you forgot that there is internal resistance in the battery ( any battery) and that is what you are measuring the P.D. across
     
  9. Mar 6, 2017 #8
    1. 9V. That is the difference in potential between the points as indicated by the battery voltage
    2. 0V. They're directly connected, thus there is no difference between them (assuming negligible resistance in the wire)
    3. The question can't be answered as constructed. You're asking the voltage "at the end of the circuit"; i.e. a single point. Voltage is always defined as the difference between two points. The negative terminal has a 0V difference to itself as well as points 5-8; a 9V difference to the positive terminal as well as points 1-4.
    4. The resistors are irrelevant to that test (at least in the moment you take the reading; if connected a circuit will form, current will begin to flow, and your voltage reading will drop accordingly). The voltage across the battery terminals is 9V. The electric potential between the two points is not the same; they are separated by the battery itself.

    It wasn't on your list of questions but just one other point, the values of the resistors are irrelevant to your questions. Positive-negative, 1-8, 2-7, 3,-6, and 4-5 are all 9V.
     
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