Doonesbury by Garry Trudeau for May 04, 2006
May 03, 2006
May 05, 2006
Transcript:
Joanie: So what'd you think of M.I.T, Alex? Alex: Wasn't too impressed. I stumped two instructors with a question... and it was simple! Two black boxes, each with 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! Joanie: Sounds like a trick question, dear. Alex: But they're faculty, Gram! They should see that!
How are they “mathematically identical”? Meanwhile, you put a variable resistor and a voltmeter across the leads, in parallel. As the resistance is increased, the measured voltage will increase across the current supply, but not change (or go up a tiny amount) across the voltage supply.