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Electricity Question

  1. May 12, 2008 #1
    A voltmeter connected in series with a battery and resistor will register:

    a: zero
    b: the terminal voltage of the battery
    c: the voltage drop across the internal resistance of the battery
    d: the voltage drop across the resistor

    Is it C?
  2. jcsd
  3. May 13, 2008 #2
    I think B.
  4. May 13, 2008 #3
    I think the question is ambiguous... what do you mean "in series"?

    1) If "in series" means that the whole circuit is just the battery, the resistor and the voltmeter (one after the other and closed in a loop) and that you have the voltmeter "closing the circuit", then the answer is B, because the voltmeter blocks any current along it (at least if you consider it a "perfect" voltmeter) and therefore there is no voltage drop around the resistor.

    2) If "in series means that there is a whole circuit around, and you "open" the circuit at some point after the voltmeter and the resistor and insert the voltmeter, the answer is A because the voltmeter would be measuring the voltage drop around itself.
  5. May 13, 2008 #4
    It seems to me that all of the voltage is dropped in either case so you are measuring terminal voltage. Unless I misunderstand...
  6. May 15, 2008 #5
    Actually we probably don't know, there may be other batteries along the circuit. However I don't think that the question was about case 2), it makes much more sense if it meant that the only components in the circuit are the mentioned battery, the resistor and the voltmeter.
  7. May 15, 2008 #6


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    Science Advisor

    Draw the circuit, first without the resistor. Remove a section of the wire where the voltmeter would be placed. You should have an open circuit with a battery and two pieces of wire. Now what would the voltmeter read if it were placed in series (i.e. you put one probe on the first wire and the other probe on the second wire)?

    Next draw the circuit with the resistor at the end of the first wire, place the voltmeter in the series again (i.e. one probe on the end of the resistor and the other probe on the remaining wire) and ask yourself the same question.

    Hope that helps.

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