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Old Jun 19th 2009, 12:40 PM   #1
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Calculating the resistance

question
When a resistor of resistance 40 ohm is connected in parallel with the resistance of a circuit,the current is tripled.What is the resistance of the circuit in the absence of the 40 ohm resistor ?
(answer given is 80 ohms)


Ok,so when the 40 ohm resistor is directly connected to the circuit ,a current 'I' passes through it .
When the resistors are connected in parallel,the total current is now 3'I' so... a current 'I' would pass through the 40 ohm resistor and a current of 2I through resistor R,right?
Assuming it is so,
(In the 2nd situation )by equating the potential differences of the resistors in parallel ,
V= IR
=I*40 = 2I*R
I get R=20 ohm

I can't seem to figure out where I've gone wrong.hope you can help.
Thank you.
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Old Jun 20th 2009, 04:22 AM   #2
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the question asks for the intial resistance of the circuit

So the tripling of current is with respect to the original circuit and not the 40 ohm resistor

If the initial current were I and final current 3I, the current in the circuit would be I and that in the resistor would be 2I as the I was the initial current through the original circuit the value of which doesnt change since the voltage applied is still constant.

So, using your method now.

Resistance of the circuit=2(40)=80 ohm

You interpreted the question differently I guess.
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Old Jun 20th 2009, 06:40 AM   #3
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Originally Posted by Akshay View Post
You interpreted the question differently I guess.
Yes,Akshay.Seems like I've misinterpreted it.How silly of me.
It's such an easy question.
Anyway, thanks for the help.
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