# Transposing Formula.

## Homework Statement

Make Rs the subject when Im = I x Rs/Rm+Rs.

## The Attempt at a Solution

Rm x Im/I = Rs. I dont think this answer is correct. Whats throwing me is the fact that the subject appears twice in the equation. Any guidance would be appreciated.

Mark44
Mentor

## Homework Statement

Make Rs the subject when Im = I x Rs/Rm+Rs.
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.

## The Attempt at a Solution

Rm x Im/I = Rs. I dont think this answer is correct.
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.
Whats throwing me is the fact that the subject appears twice in the equation. Any guidance would be appreciated.

1 person
The question iam 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.

Mark44
Mentor
The question iam 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.
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.

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?
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.
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.
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".

1 person
Mark44
Mentor
Are you going to give it a shot?