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B How to remember what happens to I, V and R in a circuit?

  1. Mar 15, 2017 #1
    There are two circuits, each consist of a cell and two bulbs, in one circuit they are in parallel, and in the other they are in series. What's the best way to remember which lamp has the greatest pd, current and resistance when one bulb is brighter than the other in the series and parallel circuit?
     
  2. jcsd
  3. Mar 15, 2017 #2

    sophiecentaur

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    I'm not too keen on the idea of rote learning stuff like this. There are some very straightforward 'Laws' which you can use and you can guarantee they will give you the right answers.
    Firstly, we all know that V=I/R (and the other two combinations) and that Power = VI. Then:
    Kirchoff's Voltage Law (KVL) says that the total voltage drop around a series circuit (loop) is zero. That means that the total supplied volts adds up to the total of voltage drops across all the other elements. (aka Volts are 'shared' in series)
    Kirchoff's Current Law (KCL) says that the total current into a node (connection point for two or more components) is Zero. (aka Current is shared in parallel)
    So, using KVL, what can you say about the volts across the two bulbs and the Cell voltage in a series circuit? What can you say about the Current flowing through the series circuit?
    Work it out with ant particular example you can think of - then another one. That will show you the trend.
     
  4. Mar 15, 2017 #3
    In terms of your question, the total voltage drops in the two bulbs = the emf of the cell, and the current flowing through a series circuit is constant.

    But what I mean is, what if the two bulbs are of different brightness, and are in series, what would be an easy way to visualise which one has the highest current and voltage, and the same for in parallel?
     
  5. Mar 15, 2017 #4

    robphy

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    Brightness is related to the power dissipated: ##P=I_{through}V_{across}##.
    As @sophiecentaur , asks
    in series, how are the currents through each resistor related?
    Then, if one bulb is brighter than the other, what can you say about the voltage across the brighter bulb?
     
  6. Mar 15, 2017 #5
    Greater, so...:
    In series:
    The brightest bulb has less resistance, greatest voltage and same current as the dimmest bulb.
    In parallel:
    The brightest bulb has less resistance, greater current and the same voltage as the dimmer bulb?
     
  7. Mar 15, 2017 #6

    cnh1995

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    Power dissipated in the bulb will be I2Rbulb (this comes from P=Vacross*Ithrough).

    Use this expression in series and parallel circuits and see in which case which bulb should glow brighter than the other.
     
  8. Mar 15, 2017 #7

    sophiecentaur

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    Hang on there!! Will they have different currents???
     
  9. Mar 15, 2017 #8

    davenn

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    in the same post you stated the answer to this Q .....

    and again in a subsequent post


    Dave
     
  10. Mar 16, 2017 #9

    sophiecentaur

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    How can that be true? If the bulb is brighter then it is dissipating more power. For the same current (series circuit), the VI must be greater, which means the V must be higher so R (=V/I) must be higher. The higher resistance bulb 'steals' more of a share of the volts.
    Stick to the logic and analyse your 'summing up' statements to see whether or not they can be true.
     
  11. Mar 16, 2017 #10
    NO, was just kind of quickly typing that, sorry.
     
  12. Mar 16, 2017 #11
    Had shift on for too long, the o in 'NO' was meant to be lower case.
     
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