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!

Resistance of the resistor and voltmeter's readings

  1. Oct 3, 2016 #1
    1. The problem statement, all variables and given/known data
    A battery of EMF 12.6 V and internal resistance 0.1 Ω is being charged from a DC source of EMF 24.0 V and internal resistance 1.0 Ω using the circuit shown in the figure below. V1 and V2 are high resistance voltmeters and R is a fixed resistor.

    43ba58dfc733.jpg

    (a) What is the polarity of terminal A of the source?
    (b) I the charging current is 5.0 A, determine the resistance of the resistor R.
    (c) If the resistance of R were changed to 0.9 Ω what would be the reading on each voltmeter?

    Answers: (b) 1.18 Ω, (c) 18.3 V on V1 and 13.2 V on V2.

    2. The attempt at a solution
    Every part I am in doubt whether it is correct or not.

    (a) Since the battery has a polarity of + - Battery + - (of we look from the top to the bottom), I would say that the polarity of the terminal A would be negative (-). Since the current flows from the positive sign of the battery to the negative sign A of the DC source.

    (b) This is only a guess: 24 - 12.6 = 5 (0.1 + 1 + R) → R = 1.18 Ω. Maybe because the voltmeters have large resistance they don't disturb the circuit and so we just calculate the way I did (like it is a simple circuit only with two batteries).

    (c) No idea.
     
  2. jcsd
  3. Oct 3, 2016 #2

    gneill

    User Avatar

    Staff: Mentor

    When a battery is being charged, the charge that it used up while it was being used in some circuit is being replenished. Which direction should this charging current flow? Would it be the same direction as when the battery is in normal use?
     
  4. Oct 3, 2016 #3

    David Lewis

    User Avatar
    Gold Member

    Conventional current flows through the load from a point of higher voltage to a point of lower voltage. A battery being charged could be considered a load. Note that energy is subtracted from the current.

    Conventional current flows through a source of EMF (e.g. power supply or battery) from a point of lower voltage to a point of higher voltage (energy is added to the current).
     
  5. Oct 4, 2016 #4

    andrevdh

    User Avatar
    Homework Helper

    (a) When a battery is being charged we have to reverse the chemical processes normally taking place inside of it.
    This is brought about by forcing a current in the opposite direction through it, that is in the opposite direction to what it normally operates in.
    This is brought about by connecting it to another emf source with a voltage higher than it.
    This opposing emf will then push a current in the opposite direction through the battery, thereby reversing the way the chemical processes normally runs and restoring the battery being charged to its formal higher capacity state.
     
  6. Oct 4, 2016 #5
    So A is negative, it goes to a positive battery, which then becomes negative again and the point below A is positive. So A is negative is correct?
     
  7. Oct 4, 2016 #6

    gneill

    User Avatar

    Staff: Mentor

    Draw the circuit with the 24 V supply in place and indicate the direction of current flow. What does KVL around the loop say the current should be?
     
  8. Oct 4, 2016 #7
    91a335a88a99.jpg

    I actually think it should be from + 12.6 V to - 24 V because the long line is where current flows from it and not from the short line (that is where electrons flow from). But it was said that current flows from low voltage to high voltage.
     
  9. Oct 4, 2016 #8

    gneill

    User Avatar

    Staff: Mentor

    What's the total EMF for that loop, following the direction of the current you've indicated?
     
  10. Oct 4, 2016 #9
    24 V + 12.6 V = 0 V like this?
     
  11. Oct 4, 2016 #10

    gneill

    User Avatar

    Staff: Mentor

    You have indicated a current going clockwise. So do a "KVL walk" clockwise around the loop summing up the EMFs (just the sources). Remember that when you traverse a source from its - lead to its + lead it is a gain in potential, and when you traverse it from + to - it is a loss in potential. What is the sum you get? Is it positive or negative?
     
  12. Oct 4, 2016 #11
    So it's -24 V - 12.6 V = 0.
     
  13. Oct 4, 2016 #12

    andrevdh

    User Avatar
    Homework Helper

    The polarity of your 24 V source is indicated incorrectly in your drawing, but your current's direction is correct, that is the source forces current in the opposite direction to what the battery would normally let it flow, so that the source is charging the battery.
     
  14. Oct 4, 2016 #13
    How did you determine that?
     
  15. Oct 4, 2016 #14

    andrevdh

    User Avatar
    Homework Helper

    The source needs to oppose the battery in order to reverse the current throught the battery.
    PA040354.JPG
     
    Last edited: Oct 4, 2016
  16. Oct 4, 2016 #15
    In other words since we want to charge the battery, we need to oppose the current flow. And since we know that current flows from + and we have + of the battery as up, therefore we put + at A (the point of the DC source), in that case current will flow from the regular + point from the source and will inversely flow into the battery and charge it. Like this?
     
  17. Oct 4, 2016 #16

    andrevdh

    User Avatar
    Homework Helper

    Correct - I have added a "drawing" to my previous post.
     
  18. Oct 4, 2016 #17
    And what about (b) and (c)?

    Update: could you tell what is written on your graphs? The image is too small.
     
  19. Oct 4, 2016 #18

    andrevdh

    User Avatar
    Homework Helper

    Things get complicated :)
    You have to add little r's (internal resistors) for both the source and the battery to your drawing.
    Didn't the problem give the readings on the voltmeters? I get the impression that they might have for (b)?
     
  20. Oct 4, 2016 #19

    andrevdh

    User Avatar
    Homework Helper

    Ok, gneill suggested an easy way to solve for (b).
     
  21. Oct 4, 2016 #20
    If the charging source has its current going from + to the + of the battery, then the battery is charging. And when + goes to - then the battery is not being charged, right? It is being consumed / used?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Resistance of the resistor and voltmeter's readings
  1. Reading of a Voltmeter (Replies: 1)

  2. Reading of voltmeter (Replies: 13)

Loading...