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Homework Help: Transposing Formula.

  1. Oct 30, 2013 #1
    1. The problem statement, all variables and given/known data
    Make Rs the subject when Im = I x Rs/Rm+Rs.

    2. Relevant equations

    3. The attempt at a solution Rm x Im/I = Rs. I don`t think this answer is correct. Whats throwing me is the fact that the subject appears twice in the equation. Any guidance would be appreciated.
  2. jcsd
  3. Oct 30, 2013 #2


    Staff: Mentor

    I'm almost certain that you are missing parentheses.

    Is this your equation?
    $$I_m = I * \frac{R_s}{R_m} + R_s $$

    If it isn't, then add parentheses to make your meaning clear.
    Probably not, but I can't tell what you did to get this.

    Start with the equation you're given and show what you're doing at each step to get a new equation.
  4. Oct 30, 2013 #3
    The question i`am given is is this: Determine the required value of the shunt resistance if the maximum value of the current I is 200A. The meter can read a maximum of 1mA and has a resistance of 0.1Ω.
    The relevant equation I think is this : Im = I x Rs/ Rm + Rs.
    The question asks for the shunt resistance therefore i am required to transpose and make Rs the subject.
    Now, my maths is not yet to the required standard and to make Rs the subject is confusing me due to the fact that it appears twice in the equation. Do i start by cancelling both Rs out of the equation? My first step would be to move I over so it would be I/Im = Rs/Rm+Rs. Not sure on my next move though.
  5. Oct 30, 2013 #4


    Staff: Mentor

    Please carefully read what I wrote earlier. Your equation probably doesn't what you think.

    This is what you wrote:
    $$I_m = I * \frac{R_s}{R_m} + R_s$$

    Is that what you meant? If not, use parentheses as necessary.

    No, and it would probably be useful if you removed "canceling" from your vocabulary. It has a very specific meaning that you apparently don't understand.
    Is this your starting equation?
    $$I_m = I * \frac{R_s}{R_m + R_s} $$

    If so, you need parentheses around the two terms in the denominator, like this:
    Im = I * Rs/(Rm + Rs).

    For this type of equation, you can use these operations:
    • Add the same quantity to both sides of the equation.
    • Subtract the same quantity from both sides of the equation.
    • Multiply both sides of the equation by the same quantity.
    • Divide both sides of the equation by the same quantity.

    I'm hopeful you won't use the word "cancel".
  6. Oct 30, 2013 #5


    Staff: Mentor

    Are you going to give it a shot?
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