Homework Help: A practice problem to find R

1. Feb 14, 2012

Saeed.z

Hi all ,

there is a practice problem in my text book which say :

http://im17.gulfup.com/2012-02-14/1329248560881.jpg [Broken]

i found after the load is attached that the current through 10 ohm's is 0.2 ampere and through R_L is 0.25 ampere, therefore , the value of R_L is 8 ohm's but the correct answer is 24 ohm's !

i checked again with the multism program and found my answer is correct !

i used the current divider method here .

so i'm a little confused ! which one is correct ?

BTW , sorry for my bad english :)

thanks

Last edited by a moderator: May 5, 2017
2. Feb 14, 2012

Staff: Mentor

The current drawn from the source power supply is greater when the Rl resistor is connected.

To figure this out, just write the equation for the output voltage with the known resistors 40 and 10 and the unknown Rl in parallel with the 10 Ohm resistor.

Last edited by a moderator: May 5, 2017
3. Feb 14, 2012

DragonPetter

I get 8 Ohms too.

4. Feb 14, 2012

Staff: Mentor

Interesting. I didn't even bother to figure out the answer, but you are right. Plugging in 8 Ohms for the load resistor does give 2V at the output. Hmm...

5. Feb 14, 2012

DragonPetter

I tried to figure out where they could get 24 ohms from, as if I misinterpreted the problem, but I didn't bother with spending the time and assume its an error in the answer book.

6. Feb 15, 2012

Saeed.z

@berkeman
@DragonPetter
thank you all of you guys , i really appreciate your help !
thanks !