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Equivalent Resistance

  • Thread starter Ballin27
  • Start date
  • #1
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Homework Statement


The equivalent resistance between points A and B of the resistors shown in the figure is 27Ohms.

Find the value of resistance R.

Here is the diagram: http://session.masteringphysics.com/problemAsset/1122575/1/Walker.21.36.jpg

Homework Equations



Rseries = R1+R2+R3...

The Attempt at a Solution



Not really sure where to even start, having a real hard time with equivalent resistance. If someone could point me in the right direction it would be greatly appreciated.

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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Hi Ballin27! :smile:

(have an ohm: Ω :wink:)
Not really sure where to even start, having a real hard time with equivalent resistance. If someone could point me in the right direction it would be greatly appreciated.
The two on the right are in parallel, so find their equivalent resistance first.

Once you have that, the 12 Ω on the left is in series with it, so find the total equivalent resistance (which you can then put equal to 27 Ω) …

what do you get? :smile:
 
  • #3
5
0
Thanks for the help tiny-tim! :biggrin:

I ended up getting 21 Ω which turned out to be right :D
 
  • #4
754
1
Thanks for the help tiny-tim! :biggrin:

I ended up getting 21 Ω which turned out to be right :D
Yes, that's the rounded value. What is the exact value?
Show us your work.
 
  • #5
5
0
Yes, that's the rounded value. What is the exact value?
Show us your work.
Here it is:

1/Req1 = 1/55 + 1/R = 1/55(R/R) + 1/R(55/55) = R/55R + 55/55R = (55+R)/55R

Req1= 55R/(55+R)

12 + 55R/(55+R) = 27
12 + 55R = 27(55+R)
55R = (27-12)(55+R)
55R = (15R) + (15 x 55)
40R = 825

R = 20.625
 
  • #6
754
1
here it is:

1/req1 = 1/55 + 1/r = 1/55(r/r) + 1/r(55/55) = r/55r + 55/55r = (55+r)/55r

req1= 55r/(55+r)

12 + 55r/(55+r) = 27
12 + 55r = 27(55+r)
55r = (27-12)(55+r)
55r = (15r) + (15 x 55)
40r = 825

r = 20.625
perfect!!!!
 

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