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Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

Electric Bill Enigma: Newsletter Challenge (February 2018)

Posted January 31, 2018 5:01 PM

This month's Challenge Question: Specs & Techs from GlobalSpec:

A family moves into a new house in December and notices a musty smell in the basement and purchase a small dehumidifier. When the electric bill arrives a month later it is much higher than expected. The family compares it to their electrical usage at their old house and it is several hundred kWh more. The old owner confirms their bills were less as well. What is the issue?

And the answer is:

The new house has an electric hot water heater running continuously due to its having hot water heat. This is especially expensive in the winter. A leak is creating the musty smell in the basement. A quick call to a plumber resolves the issue and lowers future electric bills by several hundred dollars.

41 comments; last comment on 02/07/2018
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Fuddled Frequencies: Newsletter Challenge (January 2018)

Posted December 31, 2017 5:01 PM
Pathfinder Tags: challenge question frequency

This month's Challenge Question: Specs & Techs from GlobalSpec:

Suppose your preferred AM radio station is located at 800 kHz on your radio dial, and you have a very special, perfect radio receiver that can only tune exactly at 800 kHz, excluding all other frequencies. Would you hear the music more clearly using such a radio receiver, or there is no difference between it and a standard receiver? Can this be true if you have an FM receiver with the same characteristics?

And the answer is:

If the radio receiver is limited to exactly one frequency it cannot receive modulated waves, and without modulation no information can be transmitted – only hum. This is true for FM radios as well.

In order for a radio transmission to carry information (music in this case), the main signal (music) has to be modulated so it can make “different expressions.” AM waves are modulated by “adding” a high frequency (the carrier; this is the frequency at which you should tune your receiver) to the original signal (modulating signal). The result is a signal with highs and lows is shown in the figure.

The amplitude-modulated signal contains more than one frequency and the radio receiver must be able to receive all frequencies, including the frequency of the original signal.

176 comments; last comment on 01/23/2018
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Stumped by the Screen: Newsletter Challenge (December 2017)

Posted November 30, 2017 5:01 PM
Pathfinder Tags: challenge question lcd Screen

This month's Challenge Question: Specs & Techs from GlobalSpec:

Viewing a common LCD display from straight on, you see a figure in pink. Tilting the screen so that it is at a sharp angle in your field of vision, the figure appears to have changed to bright blue. Why has the apparent color of the figure changed?

And the answer is:

The apparent change in color of images on the screen is due to the structure of the LCD screen.

A liquid crystal display (LCD) monitor’s screen is composed of millions of elements known as pixels (picture elements). Each pixel is made up of three individual subpixels: red, green and blue. Each of these subpixels controls the amount of light that passes through them, regulating the amount of each color emitted by a pixel. The mixture of the three primary colors determines the final color that the eye perceives for a given pixel.

An LCD is comprised of layers of filters, liquid crystals, and transistors. For each subpixel, light emitted by a backlight passes through a polarizing glass filter, travels through twisted, nematic liquid crystals, and continues through another polarizing glass filter.

The first glass filter polarizes light so that all light waves except those vibrating horizontally are blocked. The light passes to the liquid crystal layer where all of the light vibrating horizontally passes through since the liquid crystals are by default twisted to the correct orientation to allow horizontally vibrating light to pass. The light then arrives at the second glass filter, which is structured to only allow light vibrating vertically to pass. Since all of the light at this point is vibrating horizontally, none of it can pass through the second filter, resulting in a dark subpixel.

To illuminate the same subpixel, all that is needed is to twist the liquid crystals to the appropriate orientation. This is accomplished through the use of transistors. Each of the subpixels is controlled by a transistor that switches on and off to control the flow of electricity through each of the liquid crystals. When liquid crystals have current flowing through them, they twist and rotate light waves passing through them by 90 degrees. Rotating the horizontally vibrating light by 90 degrees causes it to vibrate vertically, allowing it to pass through the second glass filter. The result is an illuminated subpixel. A color filter over each subpixel gives each element its color.

The reason the figure on the screen appears to change color is that by tilting the screen you are viewing light escaping before it has passed through the liquid crystals and polarizing filters in the same manner as it does from a face-on angle. Colors that would have been blocked are therefore able to leak through, resulting in the apparent change in color.

For more on how colors are displayed on computer monitors, see this article.

16 comments; last comment on 12/07/2017
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Flying in the Shadow of the Moon: Newsletter Challenge (November 2017)

Posted October 31, 2017 5:01 PM
Pathfinder Tags: challenge question eclipse

This month's Challenge Question: Specs & Techs from GlobalSpec:

A new supersonic passenger airliner is due to take flight next year. A wealthy friend of yours suggests that in 2020 you both charter a flight from Namibia to Argentina to follow the track of the total solar eclipse. What is the problem with your wealthy friend’s suggestion?

And the answer is:

Total solar eclipses track eastward, not westward. This is because the Moon moves to the east in its orbit at about 3,400 km/hour and the Earth only rotates to the east at 1,670 km/hr at the equator, so the lunar shadow moves to the east at 3,400 – 1,670 = 1,730 km/hr near the equator. You cannot keep up with the shadow of the eclipse unless you traveled at Mach 1.5.

16 comments; last comment on 11/07/2017
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Mystical Magnetic Fields: Newsletter Challenge (October 2017)

Posted September 30, 2017 5:01 PM

This month's Challenge Question: Specs & Techs from GlobalSpec:

As you know, the direction of the Earth’s magnetic field changes with time, as does “North” on a compass. Some researchers have used old paintings, such the murals in the old Vatican Library (Bibliotheca Apostolica Vaticana), or ancient clay kilns to find compass directions for specific times in the past. How is this possible?

And the answer is:

The clay in the walls of ancient kilns contains the iron oxide magnets magnetite and hematite. These materials contain individual grains in which there are “ domains”—regions in which the magnetic fields of the material are uniform.

When the clay is heated to hundreds of degrees Celsius as the kiln is used, the domains already aligned with the Earth’s field increase in size while the others shrink. When the kiln cools after using the arrangement of the domains—and also the magnetic field of the clay—is retained. This is a well-known phenomenon called thermoremanent magnetism (TRM).

Many mural paintings—including the murals of the Vatican Library—contain hematite suspended in the liquid pigment of the paint. When the pigment is applied to the wall each hematite grain rotates in the liquid until it is aligned with the Earth’s magnetic field. When the paint dries the orientation of the grains is locked in place, and therefore indicate the direction of the Earth’s magnetic field at the time of the painting.

Researchers have several methods to determine the orientation of the grains in a kiln or in the mural at any time.

13 comments; last comment on 10/10/2017
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