1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Voltage drop difference in closed and open circuits

  1. Oct 8, 2011 #1
    So the voltage drops across closed circuits I get that is P = V^2/R to get the power and then you will use P=I^2/R to get the current running through the circuit and in the case the current flowing through the closed circuit is equal in every resistor and so is the voltage drop. The part that I don't get is how come the voltage drop in an open circuit would change in every resistor? and is the current changing in every resistor as well?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Oct 8, 2011 #2


    User Avatar
    Homework Helper

    In the second example, you have not so much an open circuit as a set-up with repeated reference to the Earth point. The Volt meters are not connected to measure just across a single resistor; rather from a point on one side of the resistor, all the way to Earth.
  4. Oct 8, 2011 #3
    So, how would I go about solving for the voltage drops? in the specific circuit?
  5. Oct 8, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Which voltage drop(s) are you trying to solve for ?
  6. Oct 9, 2011 #5


    User Avatar
    Homework Helper

    Has anyone ever asked you your meterage or footage, or you poundage?

    I think not, they would normally ask your height and weight, and not include the units in the description.

    My point is that people often refer to the Potential Difference as the Voltage - presumably because PD is measured in Volts.
    At least you had the good sense to say Voltage Drop.

    But what you are after is the Potential difference.

    Because each Voltmeter had one terminal connected to Earth, the reading is the Electric Potential of the point the other terminal is connected to - since Earth is defined as zero Volts.

    The difference between the potential to one side of a resistor, and the Potential to the other side of that resistor IS the Potential difference.
    We generally accept that there is no Potential Difference across the hook-up wires, so when two resistors are connected by a wire, the Potential at each end of the wire is the same - so the Potential at the end of one resistor may be the same as the potential at the start of the next one.
  7. Oct 9, 2011 #6
    The one for the open circuit picture
  8. Oct 9, 2011 #7


    User Avatar
    Homework Helper

    The only difference between the two circuits is the way the Volt meters are connected.

    In Circuit 1, they are connected across 1 resistor each.
    In Circuit 2, they are connected across 1 only, two combined and all three combined resistors.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook