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Open electrical circuits

  1. Jun 6, 2014 #1
    I was thinking, a circuit don't need necessarily be a circuit, can be something like this:


    How compute the voltage of a circuit if the voltage of the source is different of the sink? Also, how compute the voltage, the current, resistance, etc, of a circuit when we have various sources and sinks distributed by circuit and all with different voltage!?
  2. jcsd
  3. Jun 6, 2014 #2

    Philip Wood

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    Have you ever set up a circuit with real batteries and resistors or lamps? Have you ever used a voltmeter?

    I ask these questions so we can assess your level of experience.
  4. Jun 6, 2014 #3
    My level of experience is only theoretical... :X
  5. Jun 6, 2014 #4

    Philip Wood

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    I think it's very difficult to get a feel for electrical concepts without setting up circuits and using meters. I'm sure there are some good animations of simple circuits on the internet if you can't get hands-on experience.

    'Sources' and 'sinks' are not the concepts we normally use when dealing with circuits. You ask how to compute 'the voltage'. I'm afraid the question doesn't make any sense unless you specify two points in the circuit between which you need to compute the voltage (or potential difference). I come back to the strong feeling I have that you need to gain some basic experience about voltages and currents in simple circuits.
  6. Jun 6, 2014 #5


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    Voltages are always measured between two points - any time that I say that something is at X volts, I mean it is at X volts relative to something else. When I say that something is "a 6V battery", I'm saying that the positive terminal of the battery is always 6V higher than the negative terminal.

    In your diagram, no current is flowing, the resistors aren't doing anything, and if you were to poke around with a voltmeter (which has TWO probes - voltages are measured between two points):
    - If I put one probe of the voltmeter anywhere on the wire between the two, six, and fifteen volt batteries, and the other probe anywhere on the wire between the three-volt and fifteen-volt battery, the voltmeter will read 15 volts.
    - If I leave the first probe where it is, but touch the second probe to the negative terminal of the 3V battery, the voltmeter will read 18 volts.
    - The difference between the terminal of the three volt battery and the positive terminal of the six volt battery will be nine volts.

    Check these, understand why, and try working out a few more yourself... You just need to be careful about when to add and when to subtract, and for that you just need to remeber that the voltage at the negative terminal is always less than at the positive terminal.
  7. Jun 6, 2014 #6
    But why this led no light?


    Exist the potential difference between the led, why it no light?
  8. Jun 6, 2014 #7

    Philip Wood

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    There is a p.d between the – and + of the 3 V battery, and a 15 V p.d between the – and + of the 15 V battery. What makes you think that there is a p.d. between the + of the 3 V battery and the – of the 15 V battery?
  9. Jun 6, 2014 #8


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    Without having BOTH terminals of your batteries connected to the circuit the batteries cannot operate. The chemical reactions inside the batteries will cease due to a buildup of charge and they will no longer apply a voltage to the circuit.
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