It is like having a simple differentiator or simple integrator and having them draw the output with respect to a step input.

R2 was a distractor. It did it's job.

 

Ohm's Law?
I = V/R?

Regards,

Nandu.

What about R2? while finding equivalent resistance you need to consider R2 also.I mean you are considering only load resistance. That's why you are caluclating 100/50=2 Amps.Am I right?

 

You aren't paying attention.

R2 can be any value except zero.

Do the calculation anyway you want to dice it or slice it.

The power in the load is still 200W.

 

What about R2? while finding equivalent resistance you need to consider R2 also.I mean you are considering only load resistance. That's why you are caluclating 100/50=2 Amps.Am I right?

R2 is in parallel with 50 Ohm. The nature of resistors in parallel is that same voltage appears across the resistors in parallel. So there is 100 volts across R2 and there is 100 volts across 50 Ohm. Since we are interested in power at 50 Ohm resistor, we use formula for power: P=VI. But! This same formula can be expressed in a couple of ways: P=VI=VV/R or P=VI=IIR. Since for 50 Ohm resistor we know V and we know R, we use formula P=VI=VV/R=100*100/50=200 Watts.

We really don't give a rat's ass about R2.

 

I think this is a great interview question. Sounds like the ones I used...

 

With R1 equal to zero, R2 can be anything from 1e-99 ohms to 1e99 ohms. It just doesn't matter.

Not quite - the resistance range was limited in the interview question from zero to 100.

 

Not quite - the resistance range was limited in the interview question from zero to 100.

True. I was speaking in general terms as the OP was worried too much about R2's resistance. R2 was the distractor in that question and not part of the stem ... the maximum power consumed by the load.

1 e-99 is still greater than zero ohms in the problem. 1e99 was there to illustrate that the 200W didn't depend on R2.

That was me being frustrated with the OP, as I went way out of bounds with respect to R2.

 

What about R2? while finding equivalent resistance you need to consider R2 also.I mean you are considering only load resistance. That's why you are calculating 100/50=2 Amps. Am I right?

Your problem stated that the two resistors can be any value from 0 to 100 ohms. You had to choose the values that would produce the maximum power in the 50 ohm load.

R2, in parallel with the load has NO EFFECT on the power in the load. You can demonstrate that by applying a voltage across the parallel combination of R2 and the load and calculating the power. You can start with R2 being 1e-99 ohms and end with R2 being 1e2 ohms. You can increment R2 by a factor of 1o, until you either realize the power doesn't change or you reach 1e2.

The question did not ask anything about the total power. The question did not ask about total resistance. The question asked about the maximum power in the load. The word efficiency does not appear in your problem statement.

The question was designed by your potential employer as one that can be answered quickly, where you chose the proper value of R1 to deliver the maximum voltage across the parallel combination of R2 and the Load, producing the maximum power in the load.

 

Not quite - the resistance range was limited in the interview question from zero to 100.

And it still does not matter.
Dude, you are so focused on the trees that you don't see the effing forest!

 

Your problem stated that the two resistors can be any value from 0 to 100 ohms. You had to choose the values that would produce the maximum power in the 50 ohm load.

R2, in parallel with the load has NO EFFECT on the power in the load. You can demonstrate that by applying a voltage across the parallel combination of R2 and the load and calculating the power. You can start with R2 being 1e-99 ohms and end with R2 being 1e2 ohms. You can increment R2 by a factor of 1o, until you either realize the power doesn't change or you reach 1e2.

The question did not ask anything about the total power. The question did not ask about total resistance. The question asked about the maximum power in the load. The word efficiency does not appear in your problem statement.

The question was designed by your potential employer as one that can be answered quickly, where you chose the proper value of R1 to deliver the maximum voltage across the parallel combination of R2 and the Load, producing the maximum power in the load.

Thanks JoeJester for helping me. I completely understood your explanation. Could you please tell me what is your choice in choosing value of R2 (0<=R2<=100) according to the given circuit ?

 

For the nth time, the value of R2 is irrelevant provided it is within the 0-100 Ohm range. If it will make you happy, let's choose R2=42 Ohms, 42 being the answer to Life, the Universe and Everything (according to the Hitch-hiker's Guide to the Galaxy)

 

Could you please tell me what is your choice in choosing value of R2 (0<=R2<=100) according to the given circuit ?

My choice is whatever satisfies the requirement.

It could be the "government" solution of having a lower value of R2 which wastes energy, or the "green" solution of "higher efficiency" with a larger value of R2. It just doesn't matter with an academic exercise. Since being "green" wasn't in the stem of the question, it just doesn't matter.

Without the constraint you have placed on you, R1 could have been 9.99 yocto ohm and R2 could have been 9.99 yotto ohm. Those are the agreed upon extremes as of 1991. There are no available commercially manufactured conductors and or resistors at those values, at least none of the popular suppliers stock them, as I'm sure the demand is very low to non-existant.

 

Could you please tell me what is your choice in choosing value of R2 (0<=R2<=100) according to the given circuit ?

Don't matter because resistors in parallel have the same voltage across them. If R2 is 0, it has 100 volts across it. If R2 is 100, it has 100 volts across it. This is the nature of resistors in parallel. Why waste your life on useless crap.

 

As pointed out before, if R2 is 0 Ω then you set up an indeterminate situation that creates all kinds of paradoxes. For instance, if it has 100 V across it, then there is current flowing in the other resistor, but if you put a 0 Ω resistor in parallel with any finite resistance, won't ALL of the current go through the 0 Ω path? Hence it can't have 100 V across it. But it is connected directly to an ideal 100 V supply which will ALWAYS output 100 V. So the answer should be modified slightly to say that R2 can take on any allowed value other than 0 Ω.

OP: As long as you provide allowed values for R1 and R2 that result in maximum power delivery to the load, you have answered the question. It doesn't matter if there are multiple acceptable answers, you only need to provide one.

 

It doesn't matter if there are multiple acceptable answers, you only need to provide one.

But at interview you should also point out to the interviewer that there are multiple answers.

 

Agreed. Not absolutely necessary, but definitely a good idea. Most likely, the interviewer will get most of the information they are looking for by how you approach the problem and less so on the actual answer you give.

 

Perhaps the OP would care to answer a supplementary question to check their understanding of a related situation. Let's change the ideal voltage source to an ideal 3A current source with all other conditions remaining the same. What's the maximum power possible in the 50 ohm load given the stated allowable ranges of R1 & R2 and what values of R1 & R2 would one choose?

 

R1 and R2 cannot be assumed zero for maximum power transfer to the load, in calculations, as it was stated (R1 and R2 are both greater than zero ohms).

OP.
Reread and study up on,the algebraic (not actual voltage values) but the equation of load voltage value with respect to the supply voltage, in terms of max. power transfer.

Vload with respect to VDC.

When you understand the relationship of Vload to VDC, then solving for R1 and R2 will logically make sense, as you can then use basic (ohms and kirchoffs) theorems to solve it.

In fact you can write up the whole equation without any voltage or current values, to solve for any given load or supply voltage, its best to understand the equation for it, then actual values asked for at that moment.

 

R1 and R2 cannot be assumed zero for maximum power transfer to the load, in calculations, as it was stated (R1 and R2 are both greater than zero ohms).

Where are you getting that claim from? Look at Post #1 and you will see that a value of 0 Ω is explicitly allowed for either resistor.

OP.
Reread and study up on,the algebraic (not actual voltage values) but the equation of load voltage value with respect to the supply voltage, in terms of max. power transfer.

Vload with respect to VDC.

When you understand the relationship of Vload to VDC, then solving for R1 and R2 will logically make sense, as you can then use basic (ohms and kirchoffs) theorems to solve it.

In fact you can write up the whole equation without any voltage or current values, to solve for any given load or supply voltage, its best to understand the equation for it, then actual values asked for at that moment.

You are wasting your breath -- I've been trying to get him to do that since Post #5 and he clearly has no desire to even attempt it so that we can discuss it.

 

yeh I noticed now the equal sign with the greater sign to make it equal or greater than.
thanks for p0inting that out.