# Puzzle: Distinguising equivalent circuits inside black boxes

Status
Not open for further replies.

#### silver_sparrow

##### Member level 1
Can someone tell if it is possible to distinguising the following two
black boxes:

Box 1 - From the interfacing box's terminnal a
1V ideal voltage source with a 1Ohm Resistor in series is connected.

Box 2 - From the interfacing box's terminal a 1V ideal current souce with a
parallel 1Ohm resistor is connected.

Both boxes are closed and you don't know which box is which. Using the
terminals to access the circuits inside can you determine which box is which
without damaging the circuits? You can us any measurment device or components
on the outside and apply it to the terminals.

PLZ- I don't want the answer, but would like to know if there is a soluition.
If the anser is 'yes' then is the solution complex or is it simple but tricky?

#### IanP

Re: Puzzle: Distinguising equivalent circuits inside black b

Answer is yes and solution is simple: lern more about voltage source and current source.
TIP: What is the current from costant current source when you change the load from 0Ω (short circuit) to, well reasonable value (here you have only 1 V level)..

### silver_sparrow

Points: 2

#### silver_sparrow

##### Member level 1
Re: Puzzle: Distinguising equivalent circuits inside black b

Sorry I made a mistake IanP,

In box2:
The ideal constant current source outputs 1 amp (not 1V), so if you
short the terminals outside the box, 1 amp will flow through that short
circuit.

However if you did this to the first box, 1 amp would also flow through
the shortcircuit.

#### steer

Re: Puzzle: Distinguising equivalent circuits inside black b

Answer is simple: second box dissipates power all the time, so it's hotter.
Measure temperature of both boxes and see the difference.
Heh-heh-heh

### silver_sparrow

Points: 2

#### silver_sparrow

##### Member level 1
Re: Puzzle: Distinguising equivalent circuits inside black b

wow cool!

steer:
did you actually get that yourself or have you encounter this question/answer
before?

Sparrow

IanP:

>That is correct.
>What will happen if you connect, say, 0.5Ω resistor and measure voltage on this >resistor?
You will get 0.33V for either box.

>Or other question:
>It is obvious that on the constant voltage source the open circuit voltage will be 1V.
>What will be the voltage on open circuit current source?
The second box with the constant current source and parallel 1Ohm resistor
will also measure 1V

#### steer

Re: Puzzle: Distinguising equivalent circuits inside black b

silver_sparrow said:
steer:
did you actually get that yourself or have you encounter this question/answer
before?

I think I saw it in IEEE Spectrum quizzes some time ago.

Status
Not open for further replies.