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Electrical Circuit Problem- Results when you remove a resistor from a parallel circui

  1. Aug 11, 2012 #1
    Hey I came across this question in my textbook and the answer provided just does't make sense.

    A circuit consists of a battery and 4 identical resistors in parallel. The current in each resistor is 12 A. If one of the resistors burns out, what will be the current in each of the remaining 3 resistors?

    Book's explanation:
    The battery remains the same and will provide the same voltage both with the 4 resistors and with the 3 resistors. Since the resistors are in parallel, the voltage drop on each must be the same as the voltage provided by the battery, and will be unchanged when one resistor burns out. Since both V and R are unchanged for each resistor by the burned out resistor, I is also unchanged for each resistor, and the current in each resistor is still 12 A. Note that what does change is the total current on the circuit, from 48 A with 4 resistors (4 × 12 A) to 36 A with 3 resistors (3 × 12 A). Since the resistors are in parallel, fewer resistors will mean a higher equivalent resistance, so the total current is reduced, even though the current on each resistor is unchanged.

    I don't understand how the total current could change without any change to a single resistor's current. Can someone please clarify. Thanks very much in advance.
     
  2. jcsd
  3. Aug 11, 2012 #2
    Re: Electrical Circuit Problem- Results when you remove a resistor from a parallel ci

    Let total resistance of parallel circuit is Rt.

    With 4 identical resistors of resistance RΩ,
    1/Rt =1/R1+ 1/R2+ 1/R3+ 1/R4
    Rt=R/4

    With 3 identical resistors of resistance RΩ,
    Rt=R/3
     
  4. Aug 11, 2012 #3

    BruceW

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    Re: Electrical Circuit Problem- Results when you remove a resistor from a parallel ci

    Hi, welcome to physics forums Durfys! Normally on this forum, people need to show their attempt at figuring out the problem first, and write that down, so that we know how to help. But this is your first post, so maybe you didn't know that. No worries.

    I'm not certain what you're stuck on. Is it this sentence from the book: Since the resistors are in parallel, fewer resistors will mean a higher equivalent resistance, so the total current is reduced, even though the current on each resistor is unchanged. Um, It is true that the current on each resistor is unchanged, but one of the resistors has been removed, due to it being burned out! So the circuit does change. Maybe the sentence in the book made it seem like the circuit did not change at all. But the circuit does change.
     
  5. Aug 12, 2012 #4
    Re: Electrical Circuit Problem- Results when you remove a resistor from a parallel ci

    Hey, sorry about that. Thank you very much for the replies, after spending more time thinking about the question I now understand what I missed.
     
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