# Circuit- Solving for Req

Discussion in 'Homework Help' started by J.live, Sep 12, 2010.

1. ### J.live Thread Starter Member

Sep 10, 2010
35
0
Solve for Req

Attempt:

The resistors in the middle can be added to total of 5v. So then then 5v +5v = 10v

Then they are all in series 1.5+10+ 1.5= 12.5? What I don't understand is what do i do with
Rx? or 12.5 is Req?

Here is the circuit:

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Sep 10, 2010
35
0
errr anyone?

3. ### Ghar Active Member

Mar 8, 2010
655
72
It's been 20 minutes man, people don't just sit here waiting to answer.

Why are you using 'v' as ohms?
Anyway, Req seems to be the resistance in parallel with Rx, so Rx and the 1.5 ohm resistor aren't included.

You made a mistake, the 4||4 + 6||6 branch is indeed 5 ohms, but that's in parallel with the 5 ohm resistor.
Req = 1.5 + 1 + (4||4 + 6||6)||5

4. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Please clarify the question. If possible, copy it.
Also, the unit of measuring the resistance is Ohm, or Ω, as its symbol is, not V, for Volts.
Finally, while you successfuly added in parallel the 4 and 6 Ohm resistors, you added in series the 5 Ohm equivalent with the 5 Ohm resistance next to it, coming up to 10 Ohms. The correct way to do it is add them in parallel getting 2.5 Ohms.

Give it another try!

5. ### J.live Thread Starter Member

Sep 10, 2010
35
0
Thank you Ghar and Georacer.

Ghar, sincere apologies for being so impatient. Idk ,why I used v for ohms. As you can see I am a total noob at this stuff. Thanks again.

Wait a minute.

If Req = 1.5 + 1 + (4||4 + 6||6)||5 and (4ll4+6ll6)ll5 = 5 so isn't 5ll5 = 10? Since it is 1/5 + 1/5 = 2/5 = 5*2 = 10?

Last edited: Sep 12, 2010
6. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Please use the correct notation. We cannot understand what you are trying to pass along when you alternate / with * in the "2/5 = 5*2". And that's just one example.

Other than that, you seem to have successfully calculated that (4||4+6||6)Ohms=5 Ohms, but you can't do 5||5 Ohms= 2.5 Ohms. Did you do the first conversion yourself or did you have another do it for you? Do you know how to add in parallel two resistances? It is not wrong to ask for help, but we cannot help you properly if we don't know the level of your understanding of the problem.

7. ### J.live Thread Starter Member

Sep 10, 2010
35
0
No, I did it on my own. / as in dividing * as in multiplying.

So Req = 5?

Nvm now I know how you got it. I was not using the definition for Req correctly. My level of understanding is awfully low in this area. But, I hope to comprehend it sooner or later. Thank you.

8. ### Ghar Active Member

Mar 8, 2010
655
72
Parallel resistors are :

5||5 = 1 / (1/5 + 1/5) = 1 / (2/5) = 5/2 = 2.5

You wrote 2/5 = 5*2, that can't ever be true.

9. ### J.live Thread Starter Member

Sep 10, 2010
35
0
No, I know. I was under the impression that we have to drag the denominator up and multiply it by numerator. Totally discard my pre-concived notion. Like I said I was being impulsive and wasn't looking at it closely.

Just flunked my first circuits quiz. I need to put in more time understanding this stuff. Thanks again

10. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Yes Req=5 Ohms, but seriously, I advise you out of pure camaraderie alone: If you have any interest in electronics, do read a book on linear circuit analysis. It's the very basics you can start from. Any attempt to learn anything beyond that will be vein, because of lack of a steady base of knowledge. And a little work on math wouldn't hurt too. You just can't write 2/5=2*5!

11. ### J.live Thread Starter Member

Sep 10, 2010
35
0
So, what should i do? Drop and take this class next semester? Or should I study simultaneously ? There is another quiz next week. I won't be getting my refund since it's pass deadline but i wont get a grade penalty just yet.

You are right it will be pointless in proceeding with a weak foundation.

Do you know who is the author and edition ?

12. ### Ghar Active Member

Mar 8, 2010
655
72
Personally I made sure I understood the most basic examples in the course as thoroughly as I possibly could, ignoring the hard questions. Once I got those down I'd go onto the hard ones... and I would find they no longer seemed so hard.
It's going to come down to personal differences but that worked for me rather well.

You don't want to carry forward bad habits. By doing it like this you convince yourself your problem is with new material rather than old which lets you move on.

Circuit theory was one of those courses where I was always behind. I somehow scraped by and over time I completely filled in the stuff I didn't catch while taking it.

Edit:
I should also say that this often meant I was studying old chapters instead of the ones that I was about to be quizzed on...

13. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
I can't tell you if you should drop the course or not. I don't know how western educational institutions and their point systems work, since I (sadly) don't study in one of them, I 'm greek as my info page says. So I can't be of any help to your dillema and I wouldn't like to be held responsible for the final outcome. What I will take responsibility for is this: read any textbook that your college provides for the course we are talking about, ask the classmates that seem to do better for advice on study material and for any questions on exercises, well, you know where to find us.

14. ### J.live Thread Starter Member

Sep 10, 2010
35
0
That is very true but rather hard to apply for me. I have a bad habit of being impulsive and not get out of my comfort zone to actually understand the material.

Yeah, I have never done this before and started to study for the quiz two days prior. Not a good choice I know. Now that I have a failed my first quiz and the next one coming up I feel like i am in an endless loop. Not sure if I should carry on with this
class this semester.

Thanks once again Georacer. Appreciate your help.

Ghar, I will surely apply that technique.

Last edited: Sep 12, 2010
15. ### Ghar Active Member

Mar 8, 2010
655
72
I know that personally I wasn't even allowed to a drop course and if you failed it you ended up with an extra course the next year... an awful situation.

Another big thing about doing easy questions is that you can usually manage them to some degree. You figure them out, get the right answer and gain motivation. Motivation is huge, probably the biggest thing.
I did pretty poorly initially because courses like circuit theory are extremely tedious and my motivation was always interest level so I would barely look at the homework etc.

When I got to the more advanced courses I was very interested and actually did homework ahead of time... I never thought I would do that.