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Electricity: series and parallel circuit

  1. Dec 16, 2006 #1
    I have a question about series and parallel circuits
    We made a simple series circuit using three light bulbs
    when we measured the current with the ammeter at each end of the light bulb, they were 30mA at the source, then it began to change to 40,40,60,60,1.6, and 2.3 Do you think this result is right? aren't they all supposed to stay the same??
    also, we made a simple parallel circuit using three light bulbs
    we measured the voltage at each end of the light bulbs to find the potential difference. Here, too aren't the numbers supposed to stay the same throughtout the whole circuit? My results were 0, 0, 2, -2(?), 2.1, and 2.2
     
  2. jcsd
  3. Dec 16, 2006 #2

    berkeman

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    Staff: Mentor

    You intuition is correct, so there must have been some problem with the meter or the experiment. The current in a series circuit is the same, and the voltage across parallel elements is the same.

    Take a look back at how the measurements were made, and look for sources of error. For example, remember that a voltmeter in current measurement mode has an internal resistance which is not always negligible. When you measure currents in the mA range, the resistance can be many Ohms.

    And on the voltage measurements, it looks to be a combination of bad connections (0V) where something wasn't making good contact, and one case of reversed leads (-2V).
     
  4. Dec 16, 2006 #3
    Then we probably did the experiment wrong or somethin eh?
    but about the 'internal resistance that's not negligible', does this mean the current can fluctuate even in series circuit?? but even with that, it wouldn't fluctuate this much right......? - -;; ------> 30, 40,40,60,60,1.6, and 2.3mA
     
  5. Dec 17, 2006 #4
    Did you connect the ammeter in series or in parallel ? The former is the right arrangement.
    Also what sort of a voltage source are you using ?
     
  6. Dec 17, 2006 #5

    vanesch

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    Did something smoke ?
    :biggrin:
     
  7. Dec 17, 2006 #6
    well we connected the ammeter parallel to the series circuit
    and i'm not sure what the name of our voltage source is called but it was just some kind of a power source that u could turn on and off......:confused:
     
  8. Dec 17, 2006 #7
    :uhh: uhhh no nothing smoked. . .
     
  9. Dec 17, 2006 #8
    Ammeters are supposed to be connected in series along with the other circuit elements so that they can read of the entire series current.
    Also, have you tried changing the power source to check ? You could use a simple dry cell for the experiment.
     
  10. Dec 18, 2006 #9

    berkeman

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    Staff: Mentor

    The series resistance of an ammeter (or voltmeter in current measurement mode) can alter the overall current in the circuit, compared to when the meter is not in the circuit. But the current will not fluctuate depending on where the ammeter is placed in a series circuit. Take the example where a series circuit consists of three 1 Ohm resistors, and the series resistance of the meter is 1 Ohm also. If the meter is not in the circuit and you connect up a 3V power supply (I'm just using round numbers here for illustration), then you get 1A of series current through the three resistors. But if you break the connection between either pair of resistors and insert the ammeter, now the total resistance of the series circuit is 4 Ohms, and you get less than the 1A of series current. The insertion of the meter has altered the current in the circuit, but it doesn't matter where in the circuit you insert that extra 1 Ohm of resistance, you will still get the same (altered) current.

    Make sense?
     
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