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Polarity Quiz
March 1968 Popular Electronics

March 1968 Popular Electronics

March 1968 Popular Electronics Cover - RF CafeTable 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

Polarity Quiz, March 1968 Popular Electronics - RF CafeElectronics 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.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes




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 (VA = 35/80 = 0.4375, VB = 30/50 = 3/5 = 0.6; VA < VB)

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



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