It's a great interview question, since the only right answer is to reject the question. Recognizing when you cannot and should not supply an answer, just because your boss is asking for one, is true wisdom. That's a valuable skill few possess.

 

It's a great interview question, since the only right answer is to reject the question. Recognizing when you cannot and should not supply an answer, just because your boss is asking for one, is true wisdom. That's a valuable skill few possess.

It is NEVER a good move to prove to a prospective employer his mistake.

On one interview I obtained a different answer on a question posed by the man who would be my superior. It was some resistor network with 3 sources so I hit it with a Thevinin equivalent calculation to give me one source driving the point of interest. He stated that while I got the wrong answer "my first time thru" he was impressed with my method and was sure I would later "correct my error." I just nodded in agreement and smiled.

After I left I worked the problem 3 different ways and always got the same answer: the one I had given on the interview.

I ultimately got the job. That supervisor was canned about a year later.

Karma can be a bitch.

Last comment: the short wins and the meter passes no current, though I admit there is a singularity at the point of infinity that I do not possess sufficient analytical skills to compute the transfinite quantities. Specifically, can infinity times zero equal some constant? If so, the meter reads current.

(I'm going to pose that question in the Mathematics forum.)

 

It's a great interview question, since the only right answer is to reject the question. Recognizing when you cannot and should not supply an answer, just because your boss is asking for one, is true wisdom. That's a valuable skill few possess.

I agree. Were I to give such a problem (and two of my standard interview question are in this direction), I'm primary interested in seeing if the person recognizes that an issue exists and then seeing how they go about dealing with it. I'm not looking for any specific answer, just an approach that is rational and consistent while keeping in mind that the person is being asked to think on the fly in a stressful situation.

 

Last comment: the short wins and the meter passes no current, though I admit there is a singularity at the point of infinity that I do not possess sufficient analytical skills to compute the transfinite quantities. Specifically, can infinity times zero equal some constant? If so, the meter reads current.

(I'm going to pose that question in the Mathematics forum.)

Ah, but now you are reading information into the problem that is not there.

There is no basis to claim that the short imposes 0V across the middle branch more strongly than the supply imposes 50V across it. The answer is indeterminate and, in this case, is nonremovable.

The question of whether (0*∞) can be a constant is not really the relevant question. The relevant question is whether the value at the point that results in this situation limits to a constant value as you get arbitrarily close it.

 

An often mentioned and slightly more subtle problem is the case of two ideal capacitors connected by an ideal switch closed at some time. One capacitor is initially charged before the switch closure and the other is not. An "analysis" results in a dilemma wherein the total redistributed charge is (of necessity) conserved but the initial energy is not conserved.

Most folk resort to the "plausible argument" that one cannot have an ideal circuit and argue the wire resistance or switch arcing accounts for the energy loss. Again they introduce a piece of information not included in the problem definition.

 

Yes, but this is contradictory.
If the short is zero resistance the power supply cannot read 50.00000 volts!

That's what I got!

It depends if you believe if the "irresitable force" (50v) wins the fight, or the "immovable object" (short circuit).

It cannot be answered as an electronics question, it is a philosophical question (which also is unlikely to have a "right" answer).

The best electronics answer would be that a typical 50v supply, into a typical wire short, would drag the voltage down to a typical max current of the supply (a few amps) and a final voltage of a few mV.

So there would be a few mV (lets say 100mV) across the 10 ohm resistor, so the ammeter would read about 10mA.

BUT, if this is a pure theory question with perfect theoretical components, then the circuit is in error and has no correct answer.

 

...I'm primary interested in seeing if the person recognizes that an issue exists and then seeing how they go about dealing with it. ....

Excellent summary of the intent of the question.

Coincidentally, the topic of goofy interview questions came up on the Bob and Tom (radio comedy morning) show today. My daughter happens to work at one of the companies they mentioned, Epic Systems, a company famous for challenging interviewees with problematic questions.

I can't seem to find a link to the list they were reading on the radio. Some were quite wacky, others just require quick mental math, like "How many quarters does it take to reach the top of the Empire State building?".

Anyway, Google did some studies and determined that success with brain teasers did NOT correlate with later job performance, so they've dropped them from the interview process.

 

I'm not surprised that "brain teasers" don't correlate, but I don't consider quesitons like how many quarters need to be stacked to get to the top of the empire state building to be a brain teaser. I consider brain teasers as questions that require figuring out the trick or the subtle, hidden point in order to get it. The quarters question is just seeing if you can approach a problem with limited information and determine what information you need and then coming up with some kind of a reasonable estimate for them. And what is a reasonable estimate doesn't have to be anywhere close in many cases. You don't know how tall the Empure State Building is? Fine. Guess. Pick something that seem in the ballpark for really tall buildings. You don't have any idea how tall really tall buildings are? Fine. Guess. If you guess 10ft, that is unreasonable. If you guess 10,000ft, that is off by about an order of magnitude, but someone that just doesn't pay any attention to that might not know that. So go with it. If you guess 100 miles, that is probably into the unreasonable category, but we can still proceed with the estimates you have.

If I were given that, I would say, "Really tall buildings are in the range of 1000ft. A $10 roll of quarters is about 3" long and contains 40 quarters. So 160 quarters would be a foot and, therefore, somewhere around 160,000 quarters would be needed.

 

"How many quarters does it take to reach the top of the Empire State building?".

That question doesn't mention stacking quarters.

 

"How many quarters does it take to reach the top of the Empire State building?".

That question doesn't mention stacking quarters.

If they are looking for something like, "None, it's free to go up," or "No amount because the top isn't open to the public," then that I would put into the category of useless "brain teaser" type questions.

 

Twenty, if it costs $5 to ride the elevator.

That would have been my first answer.

I put that into "Learning how to think outside the box".

 

And I tend to put it into "pointless word games". If were interviewing someone for a legal position where I want them to find loop holes in contracts through creative interpretation of the specific words used so that I can get out of my obligations and screw the people that entered into those contracts with me in good faith, then that might be valuable.

 

Similar to:

Given the following items

thermometer
barometer
stopwatch
length of rope
metre rule

How would you determine the height of the Empire State Building?

 

Yes. I know. It's an old, old joke. You take the barometer to the building custodian (true custodian, not janitor with a feel-good job title) and offer to give them this nice, shiny barometer if they will tell you how tall the building is.

So has someone that has heard the joke demonstrated that they can "think outside the box" better than someone that hasn't heard it?

 

I always liked this answer:

Take the shiny barometer to the top of the building and drop it over the edge. Measure the time to hit the sidewalk; calculate the height of the building using the local acceleration of gravity.

 

How about:

Tie the stopwatch to the end of rope, lower the stopwatch to the ground and measure the length of the rope.

 

To get back to the original question and a different way of looking at the problem (this question has got a controversy in the math forum going), rather than thinking about the infinite current from the ideal source versus the zero ohm short, consider the internal resistance of the ideal source.

The internal resistance of an ideal 50 volt source would be zero ohms, and the short has zero ohms. So what we have is a battle between those two zero ohm resistances. Which zero ohm entity is more "powerful"?

If the zero ohms of the short is more "powerful", it will load down the source and keep the voltage at zero. If the internal zero ohm resistance of the source is more "powerful", the short won't be able to hold down the voltage.

By comparing things with the same units, we can see clearly that it's a silly question, a truly unanswerable question. The answer is indeterminate; there is no answer.

 

Ah, but some would argue that since zero is a real and well defined quantity, that we have a voltage divider with to equal resistors and thus the voltage across the branch with the resistor and ammeter obviously has 25V across it.

 

Ah, but some would argue that since zero is a real and well defined quantity, that we have a voltage divider with to equal resistors and thus the voltage across the branch with the resistor and ammeter obviously has 25V across it.

 

There's a slight bias towards assuming the source wins over the "short", and that is the fact that the diagram is clearly labelled "50V". We have to assume that is correct. In a sense it is telling us that the supply has won out over the short, or at least is holding 50V in the face of the short.

The resistance of the "short" is not specified. That "short" could be a mile long, thin wire and in that case the diagram would not be showing any problem at all. Of course we're no closer to knowing the voltage across the poles of the ammeter leg, so it remains an impossible question.