1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple Circuit Question

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

    [​IMG]

    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 [​IMG] and έ (the battery)

    b) The same as in a) but with resistance between [​IMG] 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
     
  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

    User Avatar
    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

    User Avatar
    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

    User Avatar
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Simple Circuit Question
  1. Simple Circuit Question (Replies: 11)

Loading...