Help with Resistors!

John E. Franklin · 2008-07-22 15:34:40

(V0 - V2)/R2 is current goin to the left in top wire.
(V2 - V0)/R2 is same current goin' to the right in top wire.
See how I switched the order around in the "difference" above and the current goes the other way?)
Now a negative current means the current goes backwards from the arrow you make.
So if you make an arrow to the right, and subtract (leftV - rightV)/R, then if the answer is
negative, then the current is going to the left, but if the answer is positive, then the amps (current)
is going to the right.

John E. Franklin · 2008-07-22 15:40:28

Now electrons are negatively charged, that's why this is working.
Like take a ruler with the numbers 3, 4, 5, 6 on it.
If you start at 3 and go to 6, you went to the right, and +3 is the difference.
But if you do what I said with the electronics, it won't work.
Take the left number and subtract the right number and see...
3 - 6 = what? -3, right. But if you add this -3 to 3, you get zero.
Actually you should either subtract -3 to 3 and get 6, or
you should do 6 + the minus 3 to get the 3.
But it works for the electronics because the electrons are negative charged, so
it is all backwards from regular vector things and subtraction.

John E. Franklin · 2008-07-22 15:42:38

On a number line, if numbers go up, then you add to get there.
But in electronics, if voltage numbers go down, then you add current to get there.
(At least for goin' thru resistors)

John E. Franklin · 2008-07-22 15:48:04

When you finally write these node equations, you just make sure
all your pairs of two voltages subtracted, all start with, or all end with
the node you are working on. So if you are doing node V2, you will
have something like (V2 - V?)/R + (V2 - V?)/R + (V2 - V?)/R etc,etc = 0
But if you mess up and put the V2 last, just make sure they are
all the same like this is okay:
(V? - V2)/R + (V? - V2)/R + (V? - V2)/R = 0
Now just insert the right R# and V# and it will all work.
Ask some questions, and I'll continue tomorrow...

missbossyy · 2008-07-22 16:05:25

omg! u helped me so much:-)
if u have time cn you solve all the currents so i cn match my answer....
thanks once again u r awesome

John E. Franklin · 2008-07-22 16:05:49

I just want to say one more thing.
I might be absolutely wrong about the electons flowing one way or the other, and
I think I have to do somemore research about the definition of voltage.
Because I think the electrons really flow the opposite direction from the
flow of the current. But anyway, all the stuff above is still good to understand,
because remember everything in science with numbers has "units", and these
"units" make you think you understand stuff, but the physicists know more,
a lot more... And the units might be +/- backwards sort of.

John E. Franklin · 2008-07-22 16:12:18

For node V2:
(V1-V2)/R1 + (V0-V2)/R2 + (V0-V2)/R3 + (V0-V2)/R4 + (V0-V2)/R5 = 0
For node V0:
I have to read more to know the correct method when
you are near voltage sources.

missbossyy · 2008-07-22 16:14:36

u helped me so much:-)

missbossyy · 2008-07-22 16:15:20

if u have time cn u solve currents so i cn match my answer and cn u see my other post of magnetic field? i really need help with that

John E. Franklin · 2008-07-23 03:03:33

Mesh equations loops a,b,c,d:
Notice I'm getting ready to set up matrices by including the zero terms.
Vs = Ia(R1+R5) + Ib(0) + Ic(0) + Id(-R5)
0 = Ia(0) + Ib(R2+R3) + Ic(-R3) + Id(0)
0 = Ia(0) + Ib(-R3) + Ic(R3+R4) +Id(-R4)
0 = Ia(-R5) + Ib(0) + Ic(-R4) + Id(R4+R5)

John E. Franklin · 2008-07-23 04:14:45

I'm restudying matrices, so don't expect an answer right away.
My matrices teacher totally lost the whole class and was fired the next year,
so I have to learn it on my own. He just did examples so fast, and didn't
ever stop to see if anyone got it.
Matrices is only one method of finishing this problem.
Since you don't want to show your answers, I will just continue slowly and learn.

John E. Franklin · 2008-07-23 09:29:40

Hi again missbossy.
I just made up these
answers from lots of
knowledge and teaching.
I didn't learn matrices yet.
But I think they are right.

I1=Vs/[r1 + 1/(1/r2 + 1/r3 + 1/r4 + 1/r5)]

I1 (1/r2)
I2= -----------------------
(1/r2 + 1/r3 + 1/r4 + 1/r5)

I1 (1/r3)
I3= -----------------------
(1/r2 + 1/r3 + 1/r4 + 1/r5)

I1 (1/r4)
I4= -----------------------
(1/r2 + 1/r3 + 1/r4 + 1/r5)

I1 (1/r5)
I5= -----------------------
(1/r2 + 1/r3 + 1/r4 + 1/r5)

John E. Franklin · 2008-07-23 09:55:59

Note that I just jumped straight to the answer,
and didn't even note that if you used
the mesh method, and solved the 4 linear equations,
then you need these simple do-hickies:
I1 = Ia
I5 = Ia - Id
I2 = Ib
I3 = Ic - Ib
I4 = Id - Ic
I1 goes rightward thru R1.
I2 goes rightward thru R2.
I# goes rightward thru R#.
Sorry I didn't solve it step by step yet.
Maybe I'll learn the inverse matrix method, I'm starting.
So far, I just learned how to multiply square matrices again.

Math Is Fun Forum

#26 2008-07-22 15:34:40

Re: Help with Resistors!

#27 2008-07-22 15:40:28

Re: Help with Resistors!

#28 2008-07-22 15:42:38

Re: Help with Resistors!

#29 2008-07-22 15:48:04

Re: Help with Resistors!

#30 2008-07-22 16:05:25

Re: Help with Resistors!

#31 2008-07-22 16:05:49

Re: Help with Resistors!

#32 2008-07-22 16:12:18

Re: Help with Resistors!

#33 2008-07-22 16:14:36

Re: Help with Resistors!

#34 2008-07-22 16:15:20

Re: Help with Resistors!

#35 2008-07-23 03:03:33

Re: Help with Resistors!

#36 2008-07-23 04:14:45

Re: Help with Resistors!

#37 2008-07-23 09:29:40

Re: Help with Resistors!

#38 2008-07-23 09:55:59

Re: Help with Resistors!

Board footer