Resistors value and current.

Thread Starter

Hussam Mj

Joined Oct 25, 2018
17
We know that the total current is 20A and the total resistor is 5 Ohm and the voltage is 100 Volt.
How to calculate resistors and the current on R1 , R2, R3

Thanks

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Last edited by a moderator:

ericgibbs

Joined Jan 29, 2010
9,517
hi Hussam,
Welcome to AAC.
As this is homework please post your attempt at answering the question, so that we can check your work.
E
 

WBahn

Joined Mar 31, 2012
25,217
We know that the total current is 20A and the total resistor is 5 Ohm and the voltage is 100 Volt.
How to calculate resistors and the current on R1 , R2, R3

Thanks
Based on just what you have given, let's look at a couple possible scenarios.

To make things simple, let's stipulate that R1 and R2 are equal.

Now let's say that R3 = 1 Ω. What are R1 and R2?

Now let's say that R3 = 4 Ω. What are R1 and R2?

Do you see the problem?
 

WBahn

Joined Mar 31, 2012
25,217
resistor (R1) emits an effect twice the size
like the other (R2). The third (R3) emits a power of 1.2 kW.
Now you seem to be throwing out additional information that wasn't in the first post? You can't expect much meaningful help if you don't include all the information.

What "effect" is "emitted" by a resistor? Voltage? Current? Power?

Are you saying that R1 dissipates twice the power of R2 and that R3 dissipates 1.2 kW?
 

WBahn

Joined Mar 31, 2012
25,217
In your work, you somehow conclude that the parallel resistance of R1 and R2 is 2/3 (presumably 2/3 Ω since you have to add this to R3 to get Rtot).

What is your basis for this conclusion?

If you know that the total current is 20 A and that the power dissipated in R3 is 1.2 kW, what can you conclude about R3?
 

Thread Starter

Hussam Mj

Joined Oct 25, 2018
17
R3 = 8,3 Ohm

No basis on what I gave R1 the value 1 ohm, but only to get close to find the values of the other resistors.
 

WBahn

Joined Mar 31, 2012
25,217
R3 = 8,3 Ohm
That's not correct, but I can't tell you where you made your mistake unless you show your work.

No basis on what I gave R1 the value 1 ohm, but only to get close to find the values of the other resistors.
Engineering is not about guessing. By guessing a value and then basic the rest of your work off that, you pretty much guarantee that all of your results will be wrong.
 

bertus

Joined Apr 5, 2008
20,314
Hello,

I have enlarged the image a bit for better readability:

Hussam_resistors.png

You know that the power dissipated in R3 is 1.2 kWatt.
You also know that total current is 20 Amp.
What would be the resistance of R3?

Bertus
 

Thread Starter

Hussam Mj

Joined Oct 25, 2018
17
I was mistaken, I did the calculation for the complete circuit.
The total should be 2000 Watt, and since R3 dissipate 1200 watt so R1 and R2 dissipate 800 watt!
 

bertus

Joined Apr 5, 2008
20,314
Hello,

You will probably know the Ohms laws Wheel:

Ohm\'s_Law_Pie_chart.svg.png

Now use the given power and current for R3 to calculate the value of R3.

Bertus
 

bertus

Joined Apr 5, 2008
20,314
Hello,

Correct, the value of R3 is 3 Ohms.
The value of R1 and R2 in parallel is 5 - 3 = 2 Ohms.
What is the relation between R1 and R2?

Bertus
 

WBahn

Joined Mar 31, 2012
25,217
The calculation is based on Ohm law, but the total resistance is 5 Ohm
The problem is that you don't understand Ohm's Law or the power equation.

Ohm's Law related the resistance of a resistor to the current through THAT resistor and the voltage across THAT resistor.

Similarly, P = V²/R relates the power dissipated by a resistor to the voltage across THAT resistor and the resistance of THAT resistor.

What you are doing is blindly throwing the nearest voltage value at the nearest V in a formula without any regard whether or not that voltage is actually the voltage across THAT resistor.
 
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