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Circuits-equivalent resistance

  • Thread starter smoics
  • Start date
28
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circuits--equivalent resistance

1. Homework Statement
What is the equivalent resistance between points a and b in the figure ?


2. Homework Equations
Resistors in series add up, resistors in parallel are 1/(1/R + 1/R...)


3. The Attempt at a Solution
I tried 1/(1/16 + 1/12 + 1/4) + 5, but I get 7.5 and the answer is 7 ohms....where did I go astray?
 

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berkeman

Mentor
55,724
5,805
Re: circuits--equivalent resistance

1. Homework Statement
What is the equivalent resistance between points a and b in the figure ?


2. Homework Equations
Resistors in series add up, resistors in parallel are 1/(1/R + 1/R...)


3. The Attempt at a Solution
I tried 1/(1/16 + 1/12 + 1/4) + 5, but I get 7.5 and the answer is 7 ohms....where did I go astray?
It's a 6 Ohm resistor in the figure, and 16 Ohms in your calculation...
 
28
0
Re: circuits--equivalent resistance

Why wouldn't you add the 6+10=16? Because they're in series and the rest are in parallel.....Yet when I add them all up as if they are in parallel, plus the "Extra" 5, I get the correct answer.....
 

berkeman

Mentor
55,724
5,805
Re: circuits--equivalent resistance

Why wouldn't you add the 6+10=16? Because they're in series and the rest are in parallel.....Yet when I add them all up as if they are in parallel, plus the "Extra" 5, I get the correct answer.....
The 6, 12 and 4 Ohm resistors are in parallel. See how they all are connected at their tops with solid wire lines? And the same for their bottoms -- all connected with solid wire lines.

And as for the 10 Ohm resistor -- can you see how there is a solid wire line all the way from its top to its bottom? What is the parrallel combination of a resistor and a wire...?
 
28
0
Re: circuits--equivalent resistance

So it takes the path of least resistance, which is NOT through the 10 ohm resistor?
 

berkeman

Mentor
55,724
5,805
Re: circuits--equivalent resistance

So it takes the path of least resistance, which is NOT through the 10 ohm resistor?
Correct-amundo. When you short out a resistor with a wire, you get zero Ohms (for ideal wire -- for real wire, it's still close to zero Ohms).


EDIT -- So do you get the right answer now?
 
28
0
Re: circuits--equivalent resistance

Yes--merci beaucoup!
 

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