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Voltage in a circuit

  1. Nov 28, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-11-29_0-18-33.png
    What happens with the voltmeter if we close s? How am I supposed to think here? I am guessing that the total resistace is oing to decrease and then what? btw is the voltmeter measuring emk or the voltage in the circuit?
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Nov 28, 2015 #2
    An analogy, the battery like a water dam. All the components are turbines where we can see the rotating blades. The wire just water pipe. A switch is a valve to pass/stop water flowing.
    The voltmeter is just a pressure meter where we assume the height of the water in the dam is constant.
     
  4. Nov 28, 2015 #3
    well I kinda have to know what the voltmeter is measuring. any ideas?
     
  5. Nov 28, 2015 #4

    gneill

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    Staff: Mentor

    How does the voltmeter reading change with the amount of current supplied by the battery (what influences the voltage reading)? Consider that the voltmeter is connected directly across the battery: What comprises the battery?

    How does the battery current change when the switch is closed?
     
  6. Nov 28, 2015 #5
    If we short-circuited a components, it means we bypassed the the water going thru that component render it invisible to the whole system.
    If you short-circuit the battery, the water dam will burst.
     
  7. Nov 28, 2015 #6
    the current increases and the total resistance decrases so it depends by how much they change. Is te voltmeter measuring emk?
     
  8. Nov 28, 2015 #7

    gneill

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    Staff: Mentor

    Can you define "emk"? I suspect that this is a language/translation issue.

    From what I can tell from the circuit diagram the voltmeter is measuring the potential across the battery. The batter apparently has some emf (electromotive force or inherent potential difference) in series with some internal resistance Ri.
     
  9. Nov 28, 2015 #8
    electromotor something

    U = ems - RI

    is U constant?

    if so then ems is going to increase
     
  10. Nov 28, 2015 #9
    ems or emk is emf
     
  11. Nov 28, 2015 #10

    gneill

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    Staff: Mentor

    ems (or emf or emk) is not going to change. It's an inherent property of the chemistry of the battery. Assuming that R remains constant for that battery, then U will change when the current I changes. U is the potential measured across the battery's terminals.

    So the voltmeter is measuring the potential across the battery's terminals (U). When the switch closes, how will this affect the current drawn from the battery?
     
  12. Nov 28, 2015 #11
    when the current increases the voltage decrases right?
     
  13. Nov 28, 2015 #12

    gneill

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    Staff: Mentor

    Yes, when the load current increases the potential across the battery will decrease. So what are your conclusions about what happens when the switch closes?
     
  14. Nov 29, 2015 #13
    but why is emf constant?

    emf = (Ri + R)I

    The current increases and the resistance decreases so the emf shouldn't be constant .
     
  15. Nov 29, 2015 #14

    gneill

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    Staff: Mentor

    The emf is constant, it's a built-in property of the battery. Other measured values (voltages, currents) in the circuit will vary.

    Writing 2 = x + y does not make 2 a variable :smile:
     
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