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Homework Help: Simple Circuit Question

  1. May 9, 2007 #1
    1. The problem statement, all variables and given/known data

    http://img125.imageshack.us/img125/5434/asdfjr7.jpg [Broken]

    3. The attempt at a solution
    Basically for this excercise I have to fill in the gaps with "increases", "decreases" or "stays the same".

    Some of the parts I can do, others I am clueless in. It would be helpful if somebody could explain it to me in a very simple manner :smile:

    a) If R7 increases, potential difference between points A and E increases. Assume no resistance in [PLAIN]http://img82.imageshack.us/img82/4954/26005668vm6.jpg [Broken] and έ (the battery)

    b) The same as in a) but with resistance between [PLAIN]http://img82.imageshack.us/img82/4954/26005668vm6.jpg [Broken] and έ (the battery)
    [I'm unsure what the answer is]

    c) If R7 increases, voltage drop across R4 decreases

    d) If R2 decreases, current through R1 _________ [unsure of answer]

    There's a couple more, but I'll leave it at that for the time being so it's easier to "digest".
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. May 9, 2007 #2
    For a and b, you need to write Ohm's law on A-->E (the lower part which has the battery), you will see how the voltage drop over AE depends on the current.

    Hint: your answer on a is wrong

    For other parts, you base on the variation of the current to justify the drop on each part.
     
  4. May 10, 2007 #3
    So in a) resistance increases, and we want to find V but how does current change?
     
  5. May 10, 2007 #4

    Mentz114

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    Gold Member

    Use ohms law to write an expression for the current -

    current = voltage/total resistance. The voltage across any element is then easy to read off, V= current * resistance of element
     
  6. May 10, 2007 #5
    You probably think I'm dyslexic, but I'm still struggling...

    V=I*R, no worries I understand that.
    R7 is increasing and we want to find V, but what about current? Does the current remain the same? How are we to find V if I is not known?
     
  7. May 10, 2007 #6

    Mentz114

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    Gold Member

    Imagine a circuit comprising 3 resistors and a battery with zero resistance, which has voltage V. So, I = V/(r1+r2+r3). The voltage across r1 is given by V1= I*r1 = V*r1/(r1+r2+r3)

    It's obvious now that if you increase r2 or r3, the voltage across r1 goes down.
     
  8. May 10, 2007 #7
    Ok yep, I understand that now.
    So in relation to the example, r1=r7, r2=r5 and r3=r2?
     
  9. May 10, 2007 #8

    Mentz114

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    Gold Member

    I'd recommend writing out the equation for your circuit. You can combine the 2 parallel sets into one resistor which gives 4 in the current denominator.
     
  10. May 12, 2007 #9
    Just thought I'd revisit this question.
    After some thought, these are my rehashed answers

    a) If R7 increases, potential difference between points A and E DECREASES. Assume no resistance in and έ (the battery)

    b) The same as in a) but with resistance between and έ (the battery) DECREASES

    c) If R7 increases, voltage drop across R4 DECREASES

    d) If R2 decreases, current through R1 INCREASES
     
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