# Another Interview Question

Besides me, there is one other engineer in my group that used to work for Hewlett-Packard back in the old days. One day we decided to compare notes on the crazy interview questions that we were asked all those many years ago. During the discussion, I recalled one question that I have seen only once since my interview (~30 years ago). It is worth writing down here.

A young electrical engineer is handed two boxes, one labeled A and the other labeled B. It is known that one contains a Norton equivalent circuit and the other contains a Thevenin equivalent circuit (see figure below).

Norton and Thevenin Equivalent Circuits
Norton Equivalent Circuit Thevenin Equivalent Circuit

The question for the young engineer is "What kind of circuit (Norton or Thevenin) is in box A and box B? How would you go about figuring out which box contains which equivalent circuit?"

The answer is there is no way using purely electrical test methods to determine what kind of equivalent circuit is in each box. So you need to look at other characteristics. Because the Norton circuit contains a current source, there is always a current flowing. This means that the box containing the Norton source will always be consuming power and generating a magnetic field. Most solutions are based on measuring power or magnetic field strength. The box containing the Thevenin circuit, with no load on its output, will neither consume power nor generate a magnetic field.

This is a nice problem in a number of ways. Correctly solving the problem demonstrates that the young engineer understands the important concept of equivalent circuits. It requires the young engineer to analyze a non-trivial problem while in a stressful situation. Also, it is convenient for an interviewer because it does not require any computation.

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### One Response to Another Interview Question

1. David says:

One can also measure the temperature of the two boxes. The box containing the Norton equivalent circuit should be at a higher temperature than the box containing the Thevenin equivalent circuit. This is due to constant power dissipation under no load.