[net.puzzle] Thevinin/Norton equivalents

austin@nsc-pdc.UUCP (Austin Mack) (10/04/85)

	This is an old one but most people I've met have not seen
	it. If you solve it in less than an hour, give yourself an
	'A'.

	The problem is as follows:

	You are given 2 black boxes. One contains a perfect 1 volt 
	voltage source with a 1 ohm resistor in series connected to
	2 external terminals ( assume 0 resistance wiring). The
	other box contains a perfect 1 amp current source with a
	1 ohm resistor in parallel with it connected to the external
	terminals. Can you identify (without opening) which box is
	which?

	I'll post the correct answer(s).




				    /\	      /\
				   /  \      /	\
	  |------------------\	  /    \    /	 \-------------X terminal
	  |		      \  /	\  /
    /-----|-------\	       \/	 \/
   /		   \
  /		    \		    1 ohm
 /		     \
 |		     |
 \   ------------    /
  \/		 \  /
  /\		  \/
 /  \		  /\   1 volt source
/    -------------  \
|		    |
\		    /
 \		   /
  \		  /
   \------|------/
	  |
	  |-----------------------------------------------------X terminal






	  |-------------------------------------|--------------X terminal
	  |					|
    /-----|-------\				|
   /		   \				|
  /		    \				\
 /		     \				 \
 |		     |				  \
 \   ------------    /				  /
  \/		 \  /				 /
  /\		  \/   1 amp source		/  1 ohm
 /  \		  /\			       /
/    -------------  \			      /
|		    |			     /
\		    /			     \
 \		   /			      \
  \		  /			       \
   \------|------/			       |
	  |				       |
	  |------------------------------------|----------------X terminal


					Austin Mack

karn@petrus.UUCP (Phil R. Karn) (10/05/85)

You can't, if you're allowed only to see how the two terminals behave to
external circuits; they're equivalent.

However, with the terminals open-circuited, the black box with the current
source will be dissipating 1 watt of power, while the the one with the
voltage source will not be dissipating anything. Short the terminals and the
situation reverses. Stick each box into a picnic cooler with a thermometer,
and wait. This is the only way I see to solve the puzzle.

Phil

austin@nsc-pdc.UUCP (Austin Mack) (10/07/85)

------


	Thanks to everyone for all the replies to my previous posting.
	Many of you answered (correctly) that the boxes are indeed
	electrically equivalent, but the heat generated by current
	flowing through the resistor will cause the Norton box to be
	warm. I think Jonathon E. Quist (ihnp4!laidbak!jeq) had the
	best summary. In addition to the one box being warm, he pro-
	posed suspending both boxes by a thread. The norton box will
	align itself with the Earth's magnetic field!

					Thanks again,

					Austin Mack