# Find R1 and R2

Discussion in 'Homework Help' started by joe809, Mar 4, 2018.

1. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
Find R1 and R2 when i1=0.5 A and V2=50 V
https://ibb.co/ijVdF7

I believe I got the right answer for R1.
10^6 MV = 1,000,000 V
V=iR --> 10^6/0.5 = 20 M ohms = 2,000,000 ohms
Now I am not sure what to do to find R2.

Last edited: Mar 4, 2018

Mar 31, 2012
23,384
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3. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
Here it is.

Moderators note : shown image full size

Last edited by a moderator: Mar 5, 2018
4. ### panic mode Senior Member

Oct 10, 2011
1,641
456
wow... 10^6MV=10^12V=1000000000000V
what is the current? are you certain it is 0.5A and not 0.5mA or even 0.5MA?

5. ### panic mode Senior Member

Oct 10, 2011
1,641
456
btw even if we ignore units and values... your attempt of using Ohms law is not correct. all values (V,I,R) must apply to one and SAME component.
if you are trying to solve for R1, you need to determine correct V,I values for R1 before you can plug them into equation...
for example why did you choose 10MV as R1 voltage and not 50V? are you sure that either of them is correct?

6. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
I never said mA. It's Amps.

7. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
I think I got something. The total resistance is 10^6/0.5 = 2M ohms
R1 + R2 = 2M ohms
100 = 10^6 (R2/(R1 + R2)) --> 100 = 10^6 (R2/2 M ohms)
R2 = 200 ohms
R1 + 200 = 2M ohms
R1 = 2M ohms. (This is where I am confused. I'm not sure if this is right).

8. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
10^6 MV = 1,000,000 MV = 1,000,000,000,000 V since 1 MV = 1,000,000 V all by itself.

Your annotation on the schematic that i = 0.5 is meaningless since it has no units and since 0.5 is a number and not a current. If you mean 0.5 A, then put i = 0.5 A. Units and scaling prefixes are extremely important.

You are making the very common mistake of grabbing the nearest V, the nearest I, and the nearest R and throwing them at Ohm's Law. Ohm's Law is very specific. It relates the value of a resistor to the current through THAT resistor and the voltage across THAT resistor.

Also, your schematic says that V is 10^6 MV. IF it were the voltage across R1 (which it is very close to being), then if you track your units you would see that

V = iR = 10^6 MV / 0.5 A = 2·10^6 MΩ = 2,000,000 MΩ = 2,000,000,000,000 Ω.

Even if the answer were 20 M ohms, that is NOT the same as 2,000,000 ohms.

But first things first. Take a step back and clearly indicate what the actual voltage of V1 is. Is it 1 MV (i.e., 10^6 V)? Or what?

9. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
IF V1 = 1 MV and the current out of V1 is 0.5 A, then the resistance seen by V1 is 2 MΩ.

This does not follow since R1 and R2 are not in series.

Where does this "100" magically appear from? Apparently it's a voltage of some kind.

It's not.

Is this the full and complete problem you were given?

10. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
The voltage is MV and the current is A. That's the part I am confused; I don't know if the total resistance I got is correct which was 2 MΩ or what you wrote, 2,000,000,000,000 Ω.

We were not told what to do and we never worked with anything like that.

11. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
How MANY MV?

Do you understand that this is

10^6 MV = 1,000,000 MV = 1,000,000,000,000 V

If the voltage is actually 10^6 V, then that is 1,000,000 V which is 1 MV.

There's no point even talking about resistance until everyone is on the same page regarding what the supply voltage is.

As an aside, even 1 MV is an unbelievably high voltage. Very few systems work at those kinds of voltage levels. It is in the realm of the high-voltage electrical power transmission systems.

I don't think we are likely every to see 10^6 MV here on Earth. Lightening tops out at a bit over 100 MV and the highest man-made voltages are about a fourth of that.

12. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
That's what the professor said. The 10^6 is in MV. I thought that would be 1,000,000 V. It isn't and you are right because it is one trillion V. That's why I been having a lot of problem with this particular exercise. Nothing was told except that the 10^6 is in MV.
So, for the resistor should I work with MV or I have to convert it to V?

13. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,577
1,281
10^6 volts is indeed equal to 1 ,000,000 = 1 MV but 10^6 MV is equal to 10^6 MV = 1,000,000 MV = 1,000,000,000,000 V.

So, your source voltage is 1 MV

14. ### MrAl AAC Fanatic!

Jun 17, 2014
5,494
1,157
Hi,

Am i seeing this schematic right?

[Temporarily removed reference to R2]

R1 is based on the difference between the first source voltage and the second voltage source and so has nothing to do with R2. You should be able to calculate R1.

Last edited: Mar 5, 2018
15. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
How do you come to that conclusion?

They were told "10^6 is in MV" and the schematic says 10^6 MV. Both of those mean that the voltage is 10^12 V (as absurd at that may be -- but then 1 MV is pretty absurd, too, in almost all situations).

Now, if they were actually told "10^6 is a megavolt," THEN it would be quite reasonable to conclude that the source voltage is 1 MV (even as sloppy as such a statement would be). But we don't have any indication that that is what they were told.

16. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
Yes, you are seeing it right. Was hoping to get the TS to a point where they could realize that themselves.

17. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
That's exactly what we were told. The voltage source is in fact 10^6 "MV". I've never done anything with that unit before, that is why I got confused and wanted to see if my result for R1 is 2MΩ.

18. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,577
1,281
It is more likely that TS misunderstood the professor words than the professor said "10^6 is a megavolt".

19. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
Then I think you have to proceed with a value for V1 of 10^6 MV which is 10^12 V.

With that in mind, and figuring that 10^12 V - 50 V is essentially 10^12 V (the 50 V is negligible in comparison), what would R1 be if the current is 0.5 A?

Put in Ω first. Worry about scaling prefixes later.

joe809 likes this.
20. ### WBahn Moderator

Mar 31, 2012
23,384
7,096
Be that as it may, WE only have what the TS tells us to go on and the TS is telling us that they were told that the 10^6 was expressed in MV.