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.
If terms like 'magnetostriction,'
mu-metal,' and 'D-ring' arouse your technostimulus receptors, then this quiz on magnetics
should be just what you've been waiting for. It appeared in a 1962 edition of Popular
electronics, but the principles therein have not changed since then. I must admit that
I had never given thought to the orientation in which bar magnets should be stored when
in close proximity to each other.
Magnetic Phenomena Quiz
By Robert P. Balin
Neither magnets nor magnetism are mysteries to the experimenter. But this quiz will
test your knowledge of the basic principles of magnetic phenomena. Mark each statement
"True" or "False" and check your answers at the bottom.
1 - The north pole of a compass points to the earth's north
magnetic pole.
TRUE FALSE
2 - If the separation between two unlike magnetic poles is reduced
by half, the attraction between them will become four times as great.
TRUE
FALSE
3 - If a compass is placed beneath a wire passing electrons from
A to B, its north pole will point to the right.
TRUE
FALSE
4 - Bar magnets should be stored by placing them so that like
poles are side by side.
TRUE FALSE
5 - There is no insulator for magnetic fields. Some metals
simply offer more resistance to magnetism than others.
TRUE
FALSE
6 - A "D-ring" is usually found on d.c. electromagnetic relay
coils.
TRUE FALSE
7 - When a nickel-iron rod is magnetized, it will grow shorter in length.
TRUE FALSE
8 - The electromagnet shown here will have its north pole located at the top of the coil.
TRUE FALSE
9 - An electron passing through the deflection yoke magnetic field and out of the page
will be deflected to the right.
TRUE
FALSE
10 - A "keeper" is placed across the poles of a horseshoe magnet
to prevent the magnet's field from passing through nearby ferrous objects.
TRUE
FALSE
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.
1 TRUE. The north pole of a compass points
to the earth's north magnetic pole which is actually the south pole of a large magnet
inside the earth.
2 TRUE. The force of attraction between unlike magnetic poles
varies inversely as the square of the distance between them.
3 TRUE. The north
pole of a compass always indicates the direction of the magnetic field in which it lies.
To determine the direction of the magnetic field, grasp the wire with your left hand
with the thumb in the direction of electron flow, from A to B. Your fingertips will point
in the direction of the magnetic field.
4 FALSE. Bar magnets should be stored
so that opposite poles lie adjacent to each other. The magnetic field from each bar will
then have a closed magnetic circuit lying entirely within the bars themselves. Hence,
the magnetic fields are least likely to go into nearby metallic objects.
5 TRUE.
There are no materials which resist magnetic fields. However, magnetic shields made of
high-permeability materials such as mu-metal are used to bypass magnetic fields around
the devices to be isolated from the effects of the magnetic fields.
6 FALSE.
The D-ring is a shorted turn of copper used on a.c. relay coils to prevent armature chattering.
When the magnetic field set up by the coil starts to collapse on alternate half cycles,
a circulating current in the D-ring builds up a magnetic field which holds the contacts
closed.
7 TRUE. This is the principle of "magnetostriction" used in ultrasonic
transducers for sonar and in ultrasonic cleaning devices.
8 TRUE. Electrons will
enter the coil from the bottom and exit at the top of the coil. Grasp the coil with your
left hand with the fingers wrapped in the direction of the electron flow. Your thumb
will point to the north pole.
9 FALSE. Use your left hand to determine the magnetic
field around a moving electron. The thumb points in the direction of electron flow and
the curled fingers point in the direction of its magnetic field. Hence, the electron
coming out of the page will have a clockwise field around it. The magnetic field to the
right of the electron will have the same direction as the field of the deflection coil.
Since magnetic lines which have the same direction repel each other, the electron experiences
a force to the left.
10 TRUE. Almost all of the magnet's magnetic lines of force
will pass through the soft iron bar. The "keeper" is usually employed when storing permanent
magnets in order to preserve the magnetic strength.
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