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Very Basic Circuit Question

  1. Jan 7, 2008 #1
    Hi im taking a fundamentals of circuit analysis class and i was just reviewing by circuit diagrams and i found a diagram that looks like this:

    [​IMG]


    Well i know electrons are going from negative to the positive but it says

    A1 measures teh total current delivered by the battary
    A2 measures the current passing through R4
    V1 measures the battery voltage
    V2 measures the voltage across R2 and R3


    I was wondering HOW the ammeter and voltmeter measure this. Like can someone outline where the current passes through. How does the current go into the voltmeter and ammeter.
     
  2. jcsd
  3. Jan 7, 2008 #2
    Entire current goes through the ammeter, but none goes through the voltmeter (ideally).

    You can think of an ammeter as a resistor with 0 resistance, and voltmeter with infinite resistance.
     
    Last edited: Jan 7, 2008
  4. Jan 7, 2008 #3

    stewartcs

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  5. Jan 7, 2008 #4
    It may help to think of an ammeter as a RATE meter. It is measuring the number of electrons passing a given point in a given amount of time. A voltmeter is more like a pressure gauge. Ideally it does not allow any electrons to pass in order to arrive at its reading. I get the vibe that you maybe do not quite fully understand the difference between the 2 meters. So, with my given analogy and waht's info about zero and infinite resistance relating to the 2 types of meters, can you understand why A2 is measuring ONLY the current going through R4?
     
  6. Jan 8, 2008 #5
    The ammeter measures power. The voltmeter measures the amount of power but the ohmmeter is the deciding factor. Always remember ohms law, I equals E over R. They are all dependent on each other but that is why it is called Ohms law. Read it, study it and memorize it. It may save your life someday.
     
  7. Jan 8, 2008 #6
    That is not correct. Neither the voltmeter nor the ammeter measure power.
     
  8. Jan 8, 2008 #7

    stewartcs

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    What?? :bugeye:

    Surely you are kidding.
     
  9. Jan 8, 2008 #8
    Thanks for replying,
    RE: Averagesupernova
    The A2 ammeter measures the current travelling through the R4 resistor becuase the current travels through that resistor before reaching the A2 ammeter.

    I have a few more questions if anyone can help :)


    I also was wondering why is it necessary for a resistor to be connected in series with the ammeter. Why can't the ammeter measure current without a resistor in place? Without a resistor will there be like infinite current?

    by the way
    if u add more "paths" to a circuit the current distributes evenly right?
    Therefore the current decreases since less electrons are passing through an ammeter?
     
    Last edited: Jan 8, 2008
  10. Jan 8, 2008 #9

    stewartcs

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    It's not necessary per se but...

    Yes, there would be infinite current flow (theoretically) through it (a simple short circuit of the power supply - assuming nothing else were in the circuit).

    [tex] I = \frac{V}{R} [/tex]

    As [tex]R \rightarrow 0[/tex], [tex]I[/tex] would tend to [tex]\infty[/tex].

    In real life the fuse would blow on the meter to protect it from damage, and the current wouldn't be infinite, just really high (relatively speaking) due to some very small resistance in the wires.

    CS
     
  11. Jan 8, 2008 #10
    ops i edited my post a little..


    Thanks for ur reply again


    "by the way
    if u add more "paths" to a circuit the current distributes evenly right?
    Therefore the current decreases since less electrons are passing through an ammeter?
    "

    also


    for potential difference when looking at V2 in my diagram can we say V2 is measuring the potential difference across R2 and R4? Because the current which will go through R4 will be the same that will go into R3? and the voltmeter is measuring the at the point in between the end of R4 and beginning of R3.


    I guess maybe im sort of confused on how the current travels through a circuitt that has more than one path
     
    Last edited: Jan 8, 2008
  12. Jan 8, 2008 #11

    stewartcs

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    Current has a tendency to follow the path of least resistance. It does not flow evenly unless all the paths have equal resistance. The more resistance, the less current will flow. This should be intuitive based on Ohm's Law.

    The answer to your second question can be answered with the previous explaination (i.e. it depends on the value of each resistor).

    Hope that helps.

    CS
     
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