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Homework Help: Measuring Voltage Across Two Points In A Series Circuit

  1. Feb 13, 2014 #1
    1. The problem statement, all variables and given/known data

    The series circuit is connected as follows: A 12V Battery -> 8ohm resistor -> 7V Battery -> 4ohm Resistor -> Earth. The problem states that if a voltmeter was connected from a point between the 12V battery & the 8ohm Resistor and connected to a point above the earth then what would be the voltage? I do not believe that the earth plays any part in this question. I've measured total EMF to be 19v, Total Resistance (R) 12V & the current (I) to be 1.6A

    2. Relevant equations

    Rtotal = R1 + R2

    3. The attempt at a solution

    Measuring the voltage across Rtotal: V=IR = 1.6x12 = 19V - 7V (battery) = 12V
    I know this is a simple question but it's worth quite a few marks so want to be very precise. I'm just unsure how the 7v battery actually works with the resistors when using a voltmeter.
  2. jcsd
  3. Feb 13, 2014 #2
    Since you want to be precise:

    Is your circuit a loop or a line? You have described components connected in a line which would result in zero current in both resistors.

    Where is the "point above the earth" as that is not a named node in your series circuit ? If this is a new node then this would result in zero voltage being measured no matter what current is flowing elsewhere.

    Finally the polarity of the batteries matters too.


    Once you have all those clear the answer should be very simple using the two equations that you have quoted.
  4. Feb 13, 2014 #3
    Circuit is a line. On the diagram the earth comes off one of the corners of the circuit e.g


    And the polarity is set out for the 12v -+-+ & for the 7v +-+-
    Last edited: Feb 13, 2014
  5. Feb 13, 2014 #4
    Each of your components has two nodes. The resistors don't care which way around they are connected as they are symmetrical devices, is this the case with the two batteries? The voltmeter has two nodes too, does it care how it is connected?

    I suspect the reason you are unsure of the answer is because you are not completely translating the information in the circuit diagram into your text description of the circuit.
  6. Feb 13, 2014 #5
    Your updated circuit in post #3 has a different value for one of the resistors compared to post #1.

    How can current flow in your circuit? Is the 7V battery connected to one resistor or two?
  7. Feb 13, 2014 #6
    I'm not entirely sure about the polarity on the batteries (I just guessed in my schematic).

    If my schematic is correct then you have the correct current for your circuit given my Ohm's law as you calculated: 19V / 12Ohm = 1.6A -- So what can we say about the current through each resistor?

    Also a voltmeter is approximately just a HUGE resistor we insert in the circuit. A voltmeter gets it's voltage reading from the current generated through that HUGE resistor in a voltmeter. This current get's amplified and some other stuff to actually display a number. So if we have a large resistor in parallel to two really small resistors where will approximately all of the current flow be? Through the small resistors or the large resistor?

    Attached Files:

    Last edited: Feb 13, 2014
  8. Feb 14, 2014 #7
    It's an 8 ohm resistor connected to the 12V battery. Sorry I have no internet connection and am running off a hotspot on my phone, hence why my question is so rushed.
    Last edited: Feb 14, 2014
  9. Feb 14, 2014 #8


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    Staff: Mentor

    Has feldoh correctly sketched the arrangement of components?

    Attached Files:

    Last edited: Feb 14, 2014
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