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Doonesbury comic strip -- on topic!

"So what'd you think of MIT, Alex?" "Wasn't too impressed. I stumped two instructors with a question.. And it was simple! Two black boxes, each with an internal circuit, okay? Using standard tools, how can you tell which is current source and which is voltage? Answer: you can't. They're mathematically identical circuits. You have to hold them in your hand. The current circuit has a resistor, so it's warmer!" "Sounds like a trick question, dear." "But they're faculty, Gram! They should SEE that!"

Reply to
kell
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Sounds to me like someone has jumped forward to the title of reigning village idiot ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Not me! I have no idea where he got that. But it was funny, even if it's wrong.

Reply to
kell

It's nice to see some nerd electronics making the comics.

Can we assume these boxes had external terminals for their current and voltage sources?

This riddle could work if stated differently, i.e., two black boxes connected to identical external (resistive) loads, having identical voltages and currents into these loads, and no further tampering or measurements allowed. (But such a restricted problem would be uninteresting.)

We know B. Trudeau isn't an E.E. He must have gotten what he thought was a hot lead from someone he trusted (who was ignorant on the subject), or he failed to properly understand the scenario that was laid out for him, and grossly erred by oversimplifying, etc.

At least in this particular case MIT's reputation is safe. Trudeau went to Yale...

--
 Thanks,
    - Win
Reply to
Winfield Hill

For those newbies in s.e.b., a little context might help to understand the mirth here:

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The joke is that any voltage source with a given output impedance can be substituted in circuit analysis, and is electrically identical to, a corresponding current source with a parallel impedance. The internal heat in the "black box" is the same. And like many precocious high school senoirs, Alex Doonesbury is apparently smug in not knowing what she doesn't know yet. Fortunately, it's usually a temporary thing.

Mr. Trudeau graduated from Yale with a B.A. and an M.F.A. in graphic design, proving again that even geniuses can be rather lost outside their field of competence. Many times, real experts in one field like politics consider themselves experts in other unrelated fields like electronics, with predictably poor results.

Always a pleasure when the gentlemen from s.e.d. grace us groundlings with their presence.

Cheers Chris

Reply to
Chris

This is an ancient riddle, somewhat garbled. The original question was how to tell a 1-volt, 1-amp Thevenin source from a Norton one. The answers are that the Norton gets warm and displays an external magnetic field.

Trudeau gets a lot of stuff wrong.

John

Reply to
John Larkin

[snip sig]

[snip]

Well, not really. The Voltage source doesn't dissipate any power when it is open-circuited, and the current source doesn't dissipate any power when it is short-circuited.

So in principle a dissipation test could distinguish the two.

John Larkin also mentioned the magnetic field issue. A field strength meter (or pocket compass) could allow you to detect the magnetic field caused by the idling (open-circuited) Norton current source. Note that when I say open-circuited, I mean that external to the current source and parallel resistor it is open-circuited. I don't mean that the internal resistor is disconnected.

--Mac

Reply to
Mac

Er... You can easily distinguish a (constant-)current source from a (constant-)voltage source. See how much voltage it delivers into 2 different resistances.

I think the idea is that every circuit has both a Thevenin equivalent and a Norton equivalent, and you can't distinguish them because they're equivalent. But that's not exactly how the question is actually stated.

Reply to
mc

Alas, so did I...

Reply to
mc

Ah! Battery with resistor in series vs. battery with resistor in parallel. That's what he was groping for.

Reply to
mc

I like it when Dilbert tries to explain the whiteboard to the pointy haired boss and its a completely logical block diagram with correct acronyms describing a fractional T1 CSU or a DSLAM and has a couple of numbers like 100 Mbps in it.

Reply to
BobG

The Norton is a current source across a resistor. It's electrically indistinguishable from the Thevenin equivalent, a battery in series with a resistor.

John

Reply to
John Larkin

It was a *cartoon* for Pete's sake. Trudeau keeps forgetting he's a cartoonist, and acts like he's a philosopher and moralist. That's why he's seldom funny any more.

John

Reply to
John Larkin

On 4 May 2006 18:40:45 -0700, Winfield Hill Gave us:

Wouldn't the same heat be in both, no matter what?

The only difference is the distribution, no?

Reply to
Roy L. Fuchs

On Fri, 05 May 2006 08:14:43 -0700, John Larkin Gave us:

In auto parts, "they" say "part is parts".

In this we say "Power is Power".

Reply to
Roy L. Fuchs

Please bear in mind that Trudeau is translating to a much less well informed audience. (PHB's etc.)

--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  
--Schiller
Reply to
joseph2k

No. The one that consists of a perfect voltage source in series with a resistance will dissipate the most power when it's shorted, and the one that's the perfect current source with the resistor in parallel will dissipate the most power when it's open.

Cheers! Rich

Reply to
Rich Grise

My favorite is Foxtrot, with Jason Fox, the genius kid. One strip showed a bowl of Alpha-bits soup, where the message was spelled out in (what else) ones and zeros. I had to decode it of course, and was delighted to read "YOU NERD".

There was another one in which there was a color by numbers picture where the color key was: divisible by 13 = green, divisible by 17= orange, divisible by 19 = red, prime = yellow.

BobG wrote:

Reply to
dcfrench

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