# Norton's theorem

#### Dritech

Joined Sep 21, 2011
869
Hi,

Is Isc of the attached circuit 2.124-j8584 ??

I am working using the current devider rule I=(I*Rtot)/(R1+Rtot)

#### Dritech

Joined Sep 21, 2011
869
Which is the correct current divider rule please:

1) I=(I*Rtot)/(R1+Rtot)

OR

2) I=(Rtot/(R1+Rtot))*I

????

#### kubeek

Joined Sep 20, 2005
5,733
1) I=(I*Rtot)/(R1+Rtot)
2) I=(Rtot/(R1+Rtot))*I
You should learn how to math, the two equations are the same. And it should be I1=(Rtot/(R1+Rtot))*I2

#### kubeek

Joined Sep 20, 2005
5,733
Is Isc of the attached circuit 2.124-j8584
There is no Isc in that picture. If you mean the current through the 3 ohm resistor, that depends on what is connected between A and B, doesn´t it.

#### Dritech

Joined Sep 21, 2011
869
There is no Isc in that picture. If you mean the current through the 3 ohm resistor, that depends on what is connected between A and B, doesn´t it.
And what would be the current through the 3 ohms resistor is there is a wire between A and B ?

#### WBahn

Joined Mar 31, 2012
26,160
Which is the correct current divider rule please:

1) I=(I*Rtot)/(R1+Rtot)

OR

2) I=(Rtot/(R1+Rtot))*I

????
1) These two equations are the same.

2) Neither of them is correct.

First off, you only have one variable for the current, namely I, and it cancels out, your equation is only correct if R1 is identically zero.

But even allowing for the probability that you mean total current on the RHS and one of the branch currents on the LHS, it is still incorrect.

What due you mean by Rtot? Normally, this means the effective resistance. Since they are in parallel, this owuld be

Ztot = (Z1)(Z2)/(Z1+Z2)

Note that you should be using Z and not R because you are dealing with complex impedance and not just simple resistance.

You shouldn't have to rely on memorizing a bunch of formulas. You should be able to derive each and every one of them.

Let's say you want to know I1 and you know Itot, Z1, and Z2. You do not know I2 or Vtot.

Q1) What is Vtot in terms of I1 and Z1?

Q2) What is Vtot in terms of I2 and Z2?

Q3) From Q1 and Q2, what is I1 in terms of the other three quantities?

Q4) What is I2 in terms of Itot and I1?

Q5) Using Q3 and Q4, eliminate I2 and solve for I1, which should now be in terms of the three knowns.