# help ... confused not home work

#### wojnarj

Dec 15, 2013
12
hello

I need help finding an answer out.. this is self study and I have the some formulas but cant see the trees through the forest. here it goes:

Self Test question #8 from Electronics Self Teaching Guide by Earl Boysen Pg 36.

Parallel circuit:
Vs=12v
I=300mA
R1=50ohms

I need to find the following:
ohms for R2 and watts for P1

I found watts for P1 =2.88 by the P=V2/r

But I can not find R2.(which the answer has 200ohms). I keep on getting 9.18w for P2 and then my mind goes dumb.

#### davenn

Moderator
Sep 5, 2009
14,277
well you know the total current through the circuit and the resistance of R1

from that work out the current through R1, then you know the current through R2
and from the current through it and the voltage applied you can get its resistance

see if that gives you 200 Ohms

Dave

#### wojnarj

Dec 15, 2013
12
hello thank you

Can you give me a formula? My brain is hurting and I am about to erase all my electronic books and bash my computer

thank you

Feb 22, 2011
128
jc

#### wojnarj

Dec 15, 2013
12
Ohms Law

Thank you for advice.. I do have Ohms laws, The Power Laws, and others written in my quick note book.
What I am having trouble with is: I have the answers, but I am not figuring out what variable goes into what part of what formula...

this is why I am asking for an example and more then two lines for an answer.

V=I * R

I have voltage and current

12 x 3mA some how does not equal 200 ohms

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
What do you know?

If you know that V = I*R, then what does I equal? And what does R equal?

You know that there is 12V across 2 resistors. One is 50 ohms, and the total current is 0.3A.

So what is the current through the 50 ohm resistor?

Which means the current through the unknown resistor is...?

Knowing this current and the voltage, what is it's resistance?

These are simple and basic applications of Ohms law. (one is just a subtraction!)

#### wojnarj

Dec 15, 2013
12
volts - Current x resistance

12v - 300mA x 50ohm

V-IR

I=V/R

I= 12/50 = 240mA in R1

300-240 = 60mA in R2

V/I = R

12/60 = 200Ohms for R2

P=V^2/R
P=12^2/ 200
P-144/200 = 7.2w
how come my powers don't equal 12?
what am I doing wrong again
thank you for the kick in the ass and the simple example

#### davenn

Moderator
Sep 5, 2009
14,277
since you found out the current through each resistor
you can work out the power dissipated by each resistor

P = V x I

P(R1) = 12V x 0.240A = _____

P (R2) = 12V x 0.06A = _____

total power used in the circuit = V x I = 12V x 0.300A = _____

you fill in the blanks

DAve

#### Arouse1973

Dec 18, 2013
5,178
You can also use I^2*R if you don't know the voltage.
Thanks

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