Circuit Problem

[oiprocs]

My younger brother can't solve this physics problem. There is 20 volt generator on the left hand side, running through the drawn resistors, ending at a grounded point, with 0V. How would he calculate the current through the third resistor?

Text

 

[OdiN]

Are all the resistors rated the same?

The first two are in series so you will get twice the voltage out of them. I don't remember the formula. I've never done much with electronics. But basically find out how much one resistor would have coming out, double it, then perform the same calculation with that voltage instead of the 20V for the last resistor.

This is assuming they are all rated the same.

I'm not 100% sure btw...I've only messed around with some basic circuits.

 

[Fenixgoon]

resistors in series: equivalent resistance = sum of resistors

resistors in parallell: 1/(equivalent resistance) = 1/(R1) + 1/(R2) + 1/(R3)..etc.

resistors in series receive the same amount of current while resistors in parallel receive the same voltage.

edit: Voltage----R1----R2---R3----ground (series)

Voltage-----O----R1-----------Ground. (parallel)
.................|..............|
.................|-----R2---|

 

[hypn0tik]

Label the resistors R1, R2, and R3, with R3 being the resistor that you want the current through.

Now, you have R1 and R2 in parallel, so you can find Req,12 by using the formula for adding resistances in parallel (1/Req = 1/R1 + 1/R2).

Now, Req,12 is in series with R3, so the total resistance of those two combined is Req,12 + R3 = Rtot

Now, use Ohm's law to find the current.

 

[oiprocs]

They are not rated the same. Coming from the left, R1 is on top (in parallel) at 6ohms, R2 is on bottom at 4ohms, and R3 is 8ohms.

 

[OdiN]

Am I like totally wrong?

I should read that big book on my shelf called "Electricity and Electronics"

 

[oiprocs]

You pwn hypnotik. He bows down in your prescence.

 

[frostedflakes]

First find the equivalent resistance of the two resistors in parallel using Req=1/((1/R1)+(1/R2)), so Req=2.4ohms. Then you can use the voltage divider formula Vout=Vin(R3/(Req+R3)) to find the voltage across R3. You should get 15.4V across R3. To find current you use Ohm's law, I=V/R, so I=1.92A.

Might want to wait for a second opinion, but I'm pretty sure this is correct.

EDIT: And actually now that I think about it current is through the circuit is constant (except through the two parallel resistors, but if you consider them to be one resistor of Req it should have the same current as R3), so finding the voltage across R3 isn't necessary. hypn0tik's method of finding the total resistance and using Ohm's law will give the same answer. Been a while since my intro electrical engineering course so I haven't done any circuit analysis in a while. Will get to do plenty next semester in circuit theory, though.

 

[bobert]

Some of these Req for parallel resistors are incorrect...

 

[BradAtWork]

Edit - Stupid Calculator.

10.43 Ohms total.

V = C*R

20 = C*10.43
C = 1.91A

 

[Schfifty Five]

I got I = 1.923A