Urgent, Kirchoff question

[Peter_1]

DaBit, this is something for you and for the other smart guys here around.

My son has tomorrow an exam of electronics.
In his study book, he found a question which he can't solve.
He hopes that they don't ask this question.

See picture in attachment.

He knows that the solution should be 50 ohms.

Can someone explain why please?

Question is: what should be the value in ohms of RX so that the current through Rx should be the same as I R1 (current through the 80 ohms resistance)

[PobodysNerfect]

If we call the current through R2 for I1 and the current through R1 for I2 and the current through R3 for I3. We also know that the current through RX = I2 and the current through R4 = I3-I2. And if we set E to 10 V Then:
5I1 + 80I2 = 10
5I1 + 10I3 + 25I3 - 25I2 = 10
I1 = I2 + I3
From this three we can find the currents (I1 = 04, I2 = 0,1, I3 = 0,3)
Then:
5*0,4 + 10*0,3 + 0,1*Rx = 10
Rx = 50
Or so I think
Saludos
Jan

[Peter_1]

Marc, Jan,

Thanks in the name of my son you both did some efforts.
Mark, on the first hand, I thought... this can't be difficult, let's start and solve it in some minutes.
But it isn't that simple, anyway for me it isn't.

Jan found a solution but my son had some difficulties to see through the equations you made. You deleted some steps he thought on the first sight.

Anyway, he printed it out in a hurry this morning and while he is on his train to school (45 minutes) , he has a little bit time to try to understand the given solution)

If it has to be done with iteration, then it has nothing no to see anymore with electronics, this is pure math.

These are questions from which you don't learn that much, it is an electronics lesson. But some teachers have always to prove that they're smart.

Thanks.

[Peter_1]

Hi,

it's now noon and I just had my son on the phone. Believe it or not but that problem was one of his 5 exam questions with exact the same numbers.

He wrote it down the way Jan explained it.

Thanks Jan. I ow you a coffee and a pancake (or 'poffertjes',... what the hell is that I hear you say) :) ) in the tearoom of my wife

Poffertjes came from the country of DaBit.
I would like to hear pronouncing this by the readers.

[DaBit]

Oops, dropped in too late :(

Marc: close...
When I have access to a microphone connected to a PC I'll post a wave file :)