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Homework Help: Circuit -- what is wrong?

  1. Nov 9, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-11-9_16-54-41.png

    Why does my book say this is wrong?
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Nov 9, 2015 #2

    stockzahn

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    Think about the resistances of the measuring instruments for the corrisponding values you want to determine.

    What should be the resistance of a Voltmeter to obtain the correct voltage?
    What should be the resistance of an Amperemeter to obtain the correct current?
     
  4. Nov 9, 2015 #3
    the current is probably gonna be low because the volt meter has a high resistance. But why is it incorrect?
     
  5. Nov 9, 2015 #4
    my book: it is wrong to have an amp meter parallell because its resistance is so low that the amp meter could burn (too much current flowing through it). But how can the currrent be high if the volt meter has a high resistance?
     
  6. Nov 9, 2015 #5

    stockzahn

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    If the right Voltmeter and settings respectively is used the current should be zero. What would you measure, if there is no current?

    And what about the resistance of the Amperemeter?
     
  7. Nov 9, 2015 #6
    okay now I know what you mean but my book is saying that the amp meter is gonna burn and thats the part i dont get
     
  8. Nov 9, 2015 #7

    stockzahn

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    That's true, if you put the Voltmeter in this position the Amperemeter wouldn't burn, but there is also no current, so the Amperemeter is useless. If you put away the Voltmeter and have the Amperemeter connected in parallel you would create a short circuit. A Voltmeter is always connected in parallel and the Amperemeter in series. I think the task is to measure the difference of voltage at the resistor and the current passing it, which can be realized by changing the position of the instruments.
     
  9. Nov 9, 2015 #8
    what happens if we switch places with V and A? So the amp meter is in series and the V meter is in parallell? now the amp meter is gonna burn right? cuz the current is too high
     
  10. Nov 9, 2015 #9

    OmCheeto

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    This is difficult to answer, without answering the question, which is against forum rules.

    How about this:

    Ammeters and Voltmeters are NOT supposed alter the characteristics of circuits. They are only supposed to measure what is going on.
    How have the two meters altered the circuit?


    If your textbook really says that, then it is wrong.
    Can you copy what it says, word for word. It might explain things.
     
  11. Nov 9, 2015 #10
    i cant cuz it is in swedish so i am translating it :(
     
  12. Nov 9, 2015 #11

    OmCheeto

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    Does it say the ammeter will burn up in the circuit you've posted, or is this "burning up" mentioned somewhere else?
     
  13. Nov 9, 2015 #12

    stockzahn

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    That again depends on the instrument and its settings. If you have a look at the manuals or sometimes on the instrument itself a maximal voltage / a maximal current is stated to protect them from damage.
     
  14. Nov 9, 2015 #13
    This is through googla translate:

    Common errors while connecting measuring instruments are placing parallel ammeter and voltmeter in series. placing the voltmeter in series cause no harm. because its resistance is so great will power to the circuit to be nearly zero. To place ammeter in parallel is a serious fault. because its resistance is so small it will go much current through it and ammeter may occasionally burn out .
     
  15. Nov 9, 2015 #14

    CWatters

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    Its possible the book is wrong. My guess is they attempted to show two errors on one diagram and failed to notice that one error interacts with the other. The amp meter would burn if the volt meter wasn't also in the wrong place.
     
  16. Nov 9, 2015 #15
    okay thanx! im going to ask my teacher tomorrow i have been thinking of this non stop
     
  17. Nov 9, 2015 #16

    CWatters

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    PS: I always leave my multi-meter set on a high voltage range setting for this reason. Never leave it set on a current range.
     
  18. Nov 9, 2015 #17

    OmCheeto

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    The clarification indicates your book is correct. It was NOT referring to that specific circuit.

    Remember this:
    Ideal voltmeters have infinite internal resistance.
    Ideal ammeters have zero internal resistance.
     
  19. Nov 9, 2015 #18
    okay but I still don't get why an amp meter is going to burn up if it is paralllell
     
  20. Nov 9, 2015 #19

    OmCheeto

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    What is its internal resistance?
     
  21. Nov 9, 2015 #20
    zero
     
  22. Nov 9, 2015 #21
    but we just said that the current is gonna be zero too because of the voltmeter.. so how can an am meter burn when currint is almost zero?
     
  23. Nov 9, 2015 #22

    OmCheeto

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    And if you put an ammeter across a resistor, which has a voltage across it, what does equation say the current will be?

    We, or should say I say I, have determined that the translation you posted was NOT in reference to the circuit you first posted.

    As was correctly mentioned;
    The volt meter acts as an "open" in the circuit, so no voltage is seen across the resistor.
     
  24. Nov 9, 2015 #23
    can you draw a picture to the discription that the book had? an example where the amp meter would born and not burn
     
  25. Nov 9, 2015 #24

    OmCheeto

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    Inga problem. :smile:

    Here is the circuit your book was referring to:
    upload_2015-11-9_16-54-41.rev1.png
    The ammeter will burn.
    Please explain why.

    As inferred by CWatters:
    upload_2015-11-9_16-54-41.rev2.png
    How are you going to get any current, anywhere, when the wire is broken?
     
  26. Nov 9, 2015 #25
    oh now i get it. the books picture confused me :p thanx for explaining!
     
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