# Help Determining ohms Value (ohms law question)

Discussion in 'Homework Help' started by kihon, Jan 25, 2014.

1. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
Hello! Need some help with my homework. Can you help me in guiding me with the answer to this equation(need R3 Ω value):

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,386
496
Is A in a circle a current source?

3. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
Current source? No. A(for amps) is simply measuring the current on that circuit.

V = 100
I(t) = 10A
R1 = resistor using 500w
R2 = resistor using 100w (in parallel with R3)
R3 = resistor using ?w (in parallel with R2)
Need ohms value of R3

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,386
496
Ok. So total power provided to the circuit is P=100*10=1000 W.

R1 consumes 500 W.
R2 consumes 100 W.
That accounts for 500+100=600 W.

That leaves R3, R3 must consume the rest, 1000-600=400 W.
The voltage across R3 is same as the voltage across R2. So. Find voltage across R2, then plug it into the formula for power for R3, that will allow you to find resistance of R3.

• ###### r3.jpg
File size:
267.6 KB
Views:
103
Last edited: Jan 25, 2014
5. ### CapeCAD New Member

Jul 16, 2011
5
0
You know your total WATTS using 100V and 10 Amps by using Ohms law I*V, and you are able to determine total resistance by using Ohms law V/I.

This will give you the WATTS in R3 and the ratio of R1, R2, and R3.

This should be enough to get you the answer.

6. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
Isn't there a voltage drop after R1? Trying to calculate the ohms value of R1 from watts. Is the value of R1 10 ohms? 100V divided by 10A = 10 ohms. But the voltage drop....stuck on that.

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,386
496
You know the current through R1, it is 10 A.
You know the power at R1.
For R1: 500 W= (10 A)(V across R1)
Solve for V across R1.

8. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
Ok, so R1 = 5Ω and R2 = 1Ω Am I correct?

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,386
496
R1 is 5 Ohm.

R2 is not 1 Ohm because before you get to R2 the current is split into two. So some part of 10 A goes through R2, another part of 10 A goes through R3.

The bottom line is that we don't give a damn about resistance and current through R2. The only thing we want from R2 is the voltage across R2. The voltage across R2 is 100 V minus Voltage across R1. Which also means that we don't give a damn about resistance of R1. So why the eF are you wasting your time on figuring out that R1 is 5 Ohm?

10. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
Ok, so if R1 is 500w, and the current is 10a, (V=W/I) then we're looking at 50v at R1.

Now, since the current is divided into 2 before R2 and R3, and R2 is 100w consumption, how do I figure out the resistance of R3?

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,386
496
Simple. I told you how to find Power at R3. I told you how to find Voltage at R3. All you have to do now is to solve for resistance of R3.

Power=(V^2)/R
Power is known.
V is known.
Solve for R.

12. ### kihon Thread Starter New Member

Jan 25, 2014
6
0
I think I got it. 50v supplied down to R2 and R3 and if R3 was 400w, then the resistance is 6.25Ω

Feb 19, 2010
3,386
496
Yep.