Polarity Quiz
March 1968 Popular Electronics

March 1968 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Here's one last thing to do before you leave work for the weekend. This "Polarity Quiz" by Robert Balin appeared in a 1968 issue of Popular Electronics. If you know your left- and right-hand rules for magnetism and induction, then a 100% score is practically guaranteed... provided you also are a whiz at diode and meter connections. Since the author did not do so, I provided brief explanations for the answers at the bottom of the page. When applying the hand-rules, assume conventional current (flow from more positive to more negative), not electron current.

Polarity Quiz

By Robert P. Balin

Electronics technicians and hobbyists must be able to determine the polarity of a generated voltage or voltage drop to really understand how a circuit operates. They must also be able to determine the polarity of the supply voltage to be applied to a circuit, and how to install correctly a polarized component such as a diode, meter, or electrolytic capacitor. Test your ability to handle polarity problems correctly by selecting either "positive" or "negative" in each of the following statements. 

1)  To produce a north pole at the top of this electromagnet, the (positive __  negative __ ) d.c. supply terminal must be applied at point A.

2)  When the permanent magnet is dropped through the coil, the polarity of the voltage generated at point A will be (positive __  negative __ ) with respect to point B.

3)  The direction of the electron flow through RL will be indicated when the polarity of the instantaneous voltage at point A is (positive __  negative __ ) with respect to point B.

4)  Voltage at point A of this rectifier is (positive __  negative __ ) with respect to ground.

5)  To bypass the emitter resistor properly, connect the (positive __  negative __ ) terminal of the electrolytic capacitor to the emitter.

6)  To bias the collector circuit of this transistor amplifier properly, a battery must be inserted so that its (positive __  negative __ ) terminal is connected to the collector?

7)  The nonsymmetrical waveform appearing on this d.c. scope indicates that most of the signal has a (positive __  negative __ ) polarity.

8)  For the plate current meter in this circuit to deflect correctly, the plate lead must be connected to the (positive __  negative __ ) terminal of the ammeter.

9)  The polarity of the voltage at point A of this bridge circuit will be (positive __  negative __ ) with respect to point B.

10)  The voltage at point A is (positive __  negative __ ) with respect to ground.

<see answers below>

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

Polarity Quiz Answers

1)  Positive (per the right-hand rule of magnetism)

2)  Negative (per Faraday's left-hand rule of induction)

3)  Positive (note 180° phase shift through winding)

4)  Negative (diodes conduct during negative half-cycle)

5)  Negative (PNP transistor has p-type base and n-type emitter, looks like question 4 circuit)

6)  Positive (NPN transistor gets positive collector wrt base)

7)  Negative (waveform shifted downward toward negative)

8)  Negative (left side must be more negative than right side connected to B+ supply)

9)  Negative (V_A = 35/80 = 0.4375, V_B = 30/50 = 3/5 = 0.6; V_A < V_B)

10)  Negative (V_A = -5 + 30/[30+80]*[5+10] = -5 + 4.0901 = -0.9091)

Posted August 28, 2020