Electrical – Possible calculation error using current divider rule on a non ideal current source circuit

current measurementcurrent-dividercurrent-sourceerror

I am going through my notes in preparation to an exam, the following was taken straight from the board as my lecturer wrote it.

Notes from whiteboard

As far as im aware the current divider rule should be $$I1 = It * (Rt/R1)$$

However it is written here as $$I1 = It * (R2/Rt) $$

there must be something I am missing bit I cant figure it out.

Any help would be very much appreciated.

EDIT:

This was the formula that I thought was valid across the board : –

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However Spehro Pefhany has shown this is not valid for all cases.

Best Answer

It's identical to the current divider rule shown in Wikipedia:

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Rt = 3K here (the resistor in parallel with the current source) Rx = 6K It = 9mA

So \$ I_X = I_T \cdot \frac{R_T}{R_X+R_T} = 9ma\cdot \frac{3K}{3K + 6K} = 3mA\$

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