1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Voltmeter: Open circuit

  1. Dec 28, 2016 #1
    1. The problem statement, all variables and given/known data
    What is the reading on voltmeter 1 and 2 when K is open?

    http://blob:http://imgur.com/2d84e21a-63e2-4021-b4d9-e37231816d38 [Broken] 4IvHNY5.png
    2. Relevant equations
    There isn't any.

    3. The attempt at a solution
    Okay so I have two answers for ##V_1## and ##V_2##:
    What I am thinking is that it depends on how I define that source. If they define "Ideal" as to as a capacitor that has a specific voltage and a huge charge which makes the capacitor doesn't change its voltage as the current starts.
    If that is what they mean then I would suppose the answer would be 12,12

    Is this right?
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Dec 28, 2016 #2


    User Avatar
    Gold Member

    The question says "when the switch is open". I think you have misread it.
  4. Dec 28, 2016 #3
    I didnt? What did I misread?
    If the ideal voltage is like a capacitor then anything connected to any terminal must have the same voltage. However the terminals they dont have to have the same voltage they are not connected by anything. However in case of a real battery. terminals are connected. Everything must come to a halt. So the potential must be the same everywhere.
  5. Dec 28, 2016 #4


    User Avatar

    Staff: Mentor

    It's okay to state at the outset the assumptions you wish (or need) to make in order to arrive at your answer. For example, do you wish to assume the voltmeters are ideal or not? Do you wish to assume the voltage source is ideal or not ideal.

    Ultimately, it is up to you to choose, but it's generally advisable that you choose reasonably, choose what will make it most straightforward to answer definitively, and/or choose what it's likely your examiner expects you to assume. :smile:
  6. Dec 28, 2016 #5


    User Avatar

    Staff: Mentor

    An ideal voltage source maintains a constant potential difference regardless of the amount of current it has to supply. That includes negative, positive, or zero current.

    This problem is intended to get you to think about what effect., if any, resistors have on potential when the current through them is zero.

    Do some investigative calculations: Choose a value for R (maybe 11 Ω so that when the switch is closed the total resistance including the battery's internal 1 Ω will be 12 Ω and the circuit current will be 1 amp). Replace the switch with another resistance, say R1. Write expressions for the voltages with R1 as a variable. How do the voltages vary as R1 → 0 (equivalent to switch closed)? How about as R1 → ∞ (equivalent to switch open)?
  7. Dec 28, 2016 #6


    User Avatar

    Staff: Mentor

    I don't know whether this applies to your musings or not, but I'll spell it out just in case. In electronics, when we say a switch is "open" this is saying the copper contacts in the switch are not touching and the switch is not allowing current to flow through the circuit. When the switch is "closed" then the copper contacts touch so the circuit is completed and current can freely flow through the switch.

    Confusion occasionally arises because switch terminology seems to be opposite to that of a farmer's gate which when open allows animals to pass freely, and when the gate is closed it blocks their passage.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - Voltmeter Open circuit Date
Potential of hollow open ended cylinder Feb 20, 2018
Question about Full scale deflection Dec 5, 2017
Voltmeter Ammeter and percentage error in R Nov 1, 2017
Resistors and voltmeters Aug 21, 2017