How to find the equivalent Resistance of these circuits

[Aafour]

Suppose that all resistors have a value of 100 ohm

how to find the equivalent Resistance of these circuits, the number in these circuits indicate the areas for which we will be completing calculations.

how to go over thesehttp://img155.imageshack.us/img155/8421/32990280.jpg

http://img24.imageshack.us/img24/6659/20062468.jpg

http://img69.imageshack.us/img69/453/10031901.jpg

thanks

 

[rl.bhat]

Show your attempts.
Do you know how to find the equivalent resistance of series and parallel combination of resistances?

 

[Aafour]

rl.bhat, yes i know how to find series and parallel combination of resistances,

for series just add the resistances, and for parallel, (1/R)=(1/R1)+(1/R2)+...

but I really don't know how to start on these circuits listed

 

[rl.bhat]

OK. In the case 1, two resistances are in series and this combination is in parallel with the third resistance.
So 1/Req = 1/(R1 + R2) + 1/R3.
In the same way try the other combination.

 

[Aafour]

true, thank you. it's just that i need to draw these circuits in more easy way. thank you

 

[JakotaR]

Hey! what about if i have the value of the equivalent resistance and want to work out the value of the 2 resistors that were previously there? cheers

 

[rl.bhat]

Sorry. I didn't understand your question. Give a numerical example.

 

[JakotaR]

ok
Two resistors r1 and r2 connected in parallel have an equivalent resistance of 10/3 ohms.
When the current enters the parralell circuit it divides between the two resistors in the ratio of 2 to 1. Determine r1 and r2?
hope u can help :)

 

[rl.bhat]

If 3I current flows through the parallel combination, in r1 the current is i and in r2 the the current is I2.
The voltage across the parallel combination is V = 3I*10/3 = 10*I
So the current in r1 = I = V/r1
The current in r2 = 2I = V/r2
Substitute the value of V and find r1 and r2.