Calculate resistance for this diagram

[ritwik06]

Homework Statement

Calculate the equivalent resistance. (Diagram attached)

Homework Equations

1/Rp=(1/R1)+(1/R2)

The Attempt at a Solution

1/25+1/5=1/Rp
Rp=4.17 ohm

is it correct?

 

[Doc Al]

No. Are those resistors in parallel? Hint: What's the significance of the horizontal line in the middle of the diagram?

 

[ritwik06]

No. Are those resistors in parallel? Hint: What's the significance of the horizontal line in the middle of the diagram?

No! They are not. There are 2 paths generated, one through both the resistors and the other through only the 5 ohm resistor. So I arrived at the result:
(1/(5+20))+(1/5)=1/Rp

 

[Doc Al]

Those resistors are not in parallel, but I don't know how you got that new result (which looks like an application of the resistors in parallel formula). Please attempt to answer my other question:

What's the significance of the horizontal line in the middle of the diagram?

 

[ritwik06]

Those resistors are not in parallel, but I don't know how you got that new result (which looks like an application of the resistors in parallel formula). Please attempt to answer my other question:

well, it provides a no resistor path for current. So is the answer 5 ohm?

 

[Doc Al]

Yes! You can treat the wire as being a 0 ohm resistor in parallel with the 20 ohm resistor. Use the formula for parallel resistors to find the equivalent resistance of those two = 0.

 

[ritwik06]

Yes! You can treat the wire as being a 0 ohm resistor in parallel with the 20 ohm resistor. Use the formula for parallel resistors to find the equivalent resistance of those two = 0.

But will you please tell me why won't the current divide at the first branch (as my teacher told me)?

 

[Doc Al]

Since one of those branches has zero resistance, all of the current "chooses" that branch.

 

[ritwik06]

Thanx!