[net.ham-radio] Thevinin/Norton equivalents

austin@UW-BEAVER.ARPA (10/08/85)

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@Ucb-Vax.ARPA (10/08/85)

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@UW-BEAVER.ARPA (10/13/85)

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 proposed suspending both boxes by a thread. The
norton box will align itself with the Earth's magnetic field!

				Thanks again, Austin Mack