Chapter 3     Living Room Science

In the living room, you enjoy your life and have relaxation due to the invention of various home appliances. Those devices are basically designed to fit people's need - the desire to have an efficient and comfortable life. Generally, people prefer to save working hours and personal efforts. Vacuum cleaner is one of the many examples that eases our task in daily life. On the other hands, the CD players provide us entertainment and fun. You can also create your personal music CDs with CD burners. In this chapter, you will know more on the working principles on most appliances which serve us in the living room.

Vacuum Cleaners

In 1907, the first electric portable vacuum cleaner was invented by Murray Spangler in Ohio, since then it has become an indispensible home appliance for most people. The right picture (Credit: The Hoover Company) shows a vacuum cleaner in the old days. Conventional vacuum cleaners use a fan to create a partial vacuum and suck dirty air from carpets, rugs, and bare floors through a porous bag. Tell it simply, the vacuum cleaner works like your sipping of juice through a drinking straw. Air passes through the porous bag but dust is left behind in the bag, which can be disposed to the outside of the vacuum cleaner. As the bag in a conventional vacuum cleaner fills with dust, its pores become clogged so air can no longer flow through so easily. This clogging reduces the power of suction. The dual cyclone cleaner has no bag to clog, and retains its cleaning effectiveness.

How Things Work
  • The Fan
    The vacuum cleaner has a good-speed fan to create the swiftly moving air which sweeps up the dust. More precisely, when you switch on the vacuum cleaner, the fan inside rotates and hence pumps air from the hose (that is the inlet, a low pressure region) to the exhaust port (the high pressure region). A partial vacuum is created (hence the name 'vacuum cleaner') in the hose and the surrounding air rushes into the vacuum cleaner through the hose. Recall that the pressure drop inside the vacuum cleaner can be explained using the Bernoulli's principle, readers are suggested to review the principle again in the section 'Shower Curtain' in chapter 2.

  • The attachments
    The size of the opening at the end of the intake port governs the speed of the air flow. The narrower attachments can increases the air speed and thus decreases the pressure there, because of Bernoulli's principle. The drop in pressure translates to a greater suction force, that's why the narrower attachment can pick up heavier dirt particles than wider attachments.

  • The Viscous Drag
    High-speed air rushes toward the opening of the cleaning attachment, it carries dust with it. This phenomenon, in which a particle or portion of fluid is carried along in the flow of another fluid, is called entrainment. Dust particles are entrained in air by drag forces. These are friction-like forces, the viscous drag force, which bring the dust particles to move along with the air.

    Unfortunately, viscous drag also slows the air as it passes close to carpet fibers or the surface of the floor. It's hard to keep air moving quickly near those surfaces. Removing really ground-in dirt from a carpet or floor requires a powerful fan and the high air speed that comes from making air pass through a narrow opening. The need for high air speed also explains why battery-powered or poor-quality vacuum cleaners don't clean well.

    In fact, the dust particles are at rest relative to the moving air. Practically, the magnitude of the viscous drag force acting on the dust particle is proportional to the particle diameter and to the difference in velocities between the particle and the air. The viscous drag is directed to bring the particle to the same velocity as the air.

  • The Filter
    A filter blocks the dust particles while permitting air molecules to pass. A typical filter is made of porous paper or cloth, with fibers that are loosely woven to create openings or pores large enough for air to pass, but too small for dust to pass. Generally, there are supplementary filters to separate the fan from the suction capacity (see the figure next to the section about 'The Fans'. However, the filtration is complicated by the viscous drag. The air passes through the filter's pores loses some of its total energy trying to move past the stationary air at the surfaces of the pores. Since the viscous drag force on the air becomes larger when the air's speed through a pore increases. The faster air moves through a pore, the more energy it loses during the trip. To minimize the energy lost to viscous drag, the vacuum cleaner must move the air slowly through the filter. It does this by using a very large filter so that the air has lots of surface area and many pores through which to flow.

  • Blockage
    Blockage is a regular problem in most vacuums. When debris builds up in the vacuum bag, the air faces greater resistance on its way out. Each particle of air moves more slowly because of the increased drag. This is why a vacuum cleaner works better when you've just replaced the bag than when you've been vacuuming for a while.

  • Bagless Vacuum Cleaner
    It is the cyclone vacuum cleaner that uses no bag as its filter. This machine, developed in the 1980s by James Dyson, sends the air stream through one or more cylinders along a high speed spiral path. The spiraling motion works something like a clothes dryer: As the air stream whips around in a circle, all of the dirt particles experience a powerful centrifugal force; they are thrown outward, away from the air stream. In this way, the dirt is extracted from the air without using any sort of filter. It simply collects at the bottom of the cylinder.

    In the future, we are sure to see more improvements on the basic vacuum-cleaner design, with new suction mechanisms and collection systems. But the basic idea, using a moving air stream to pick up dirt and debris, is most likely here to stay for some time.

Science in Depth
Strictly speaking, we shouldn't use the Bernoulli's principle since it applies only to incompressible fluids in perfect steady flow, and air certainly isn't incompressible. However, the principle is adopted due to the following facts.
  • When the air's velocity is less than about 300 km/hr, and if there are no pressure differences of more than one tenth of an atmosphere - then we can consider air to be incompressible, since its density will remain fairly constant.
  • The gravity is also ignored, since the air were flowing up and down within a small altitude.
  • Alway bear in mind that Bernoulli's effect is applied when fluid is in steady flow. In other words, the effect occurs only along a streamline.

CD Players

Compact disc - commonly called CDs - are everywhere these days, Whether they are used to hold music, data, or computer software, they have become the standard medium for distributing large quantities of information in an inexpensive, reliable package. One CD can store over 100 million words of text, the equivalent of a thousand novels.

How Things Work
  • Inside the CD
    • A CD has a single spiral track of data, circling from the inside of the disc to the outside. The spiral track starts at the center ensures that the CD can be smaller than 4.8 inches (12 cm), and you can see nowaday, there are plastic business cards that you can put in a CD player. The figure in the right shows the side of a CD which has spiral track facing the read laser.

    • Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1.2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic bumps that make up the long, spiral track. The bumps appear as pits on the aluminum side (see the second figure), but on the side the laser reads from, they are the bumps. A thin, reflective aluminum layer is then coated on the top of the disc, covering the bumps. Then a thin protective acrylic layer is sprayed over the aluminum. The CD label is printed on this layer of arylic.

    • The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound spiral. The CD player reads the bumps with a very precise laser and interprets the information as bits of data. CD tracks are so small that they have to be measured in millionths of a meter, called microns. The CD track is approximately 0.5 microns ( 0.5 × 10 -6m) wide, with 1.6 microns separating one track from the next. The elongated bumps are each 0.5 microns wide, a minimum of 0.83 microns long, and 125 nanometers (125 × 10 -9m) high. The length of CD track is about 5 km.

    • The bumps and tracks on a CD need to be very small so that the CD can store lots of data. A CD holds up to 74 minutes of music. There are 44,100 samples per second and 2 bytes per sample, and a stereo recording requires 2 channels.

      A detailed calculation is given as follows.

      44,100 samples/channel/second × 2 bytes/sample × 2 channels × 74 minutes × 60 seconds/minute = 783,216,000 bytes

      In terms of bits, there are 783,216,000 bytes × 8 bits/byte = 6,265,728,000 bits stored in a compact disc.

    • The area nearest the center of disc, known as the lead-in, contains information on how the disk is read.
  • The CD Players

    • The fascinating work of CD technology is to read all the tiny bumps correctly, in the right order, at the right speed. In fact it is not a trivial job since the movement of the laser is at micron resolutions. To archieve this, the CD player has to be exceptionally precise when it focuses the infrared laser on the track of bumps. When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off the aluminum layer, and hits an optoelectronic device that detects changes in light. The bumps reflect light differently than the flat parts of the aluminum layer, which are called lands. The optoelectronic sensor detects this change in reflectivity, and the electronics in the CD player's driver interpret the changes as data bits. The most difficult job of the CD player is to keep the laser beam centered on the data track. This centering is the job of the tracking system.

    • An elegant system which ensures a correct tracking works in the following ways. If focus and tracking are correct, the reflected light forms a circular spot spread equally over the four sensors. If focus or tracking are 'out' the spot formed is either skewed (case [1]) or uneven in intensity (case [2]). The right figure illustrates the two cases. These irregularities in the distribution of light over the four sensors are translated into movements of two tiny motors that move and refocus the beam.

    • The tracking system, as it plays the CD, has to continually move the laser outward. When the laser moves outward from the center of the disc, the bumps move past the laser faster if the angular speed of the disc if fixed. It is absolutely not acceptable, since the sensor reads data at a non-uniform rate. Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. For your reference, the revolutions per minute (RPM) of the disc reaches its highest as 500 RPM when the laser is near the disc center, but it slows down when the laser has a larger distance from the disc center. That way, the bumps travel past the laser at a constant speed, and the data coming off the disc has a constant rate. The animation on the right demonstrates the varying speed of the disc when the laser is at different radial distance from disc center. (Credit: Marshall Brain's How Stuff Works). .

    • Each pit is so finely spaced on the disc, a small particle of dust could block large amounts of data and cause major problems. Error correction - ways of eliminating or hiding errors -enables a player to give a smooth and accurate output. In the recording, extra information is added to the samples written on a disc. These parity bits can be used by the microprocessor inside the player to check if any data is missing and fill in small errors. Large errors are avoided by splitting up and interleaving. In fact, a scratch only obliterates small pieces of several samples rather than one in its entirely. With interleaving, data is stored nonsequentially around the disc's circumference. The drive reads data one revolution at a time, and it uninterleaves the data in order to play it.

Domestic CD Burners

The manufacturers produce CDs by creating a mold of the bump pattern. The mold then presses the blank CD's acrylic surface and maintain the production a cost-effective approach. But it is not practical for the casual consumer. Home CD burners come at the problem from a different angle. Nowadays, CD burners are standard equipment on new computers, and the cost of a blank CDs is lower than a floopy disc.

How Things Work
  • The CD-Recordable Disc

    • A CD-recordable disc, or CD-R doesn't have any bumps or flat areas at all. Instead, it has a smooth reflective metal layer that sits on top of a photosensitive dye layer. When the disc is blank, the dye is translucent. Light can shine through and reflect off the metal surface. But when you heat the dye layer, it turns opaque. It darkens to the point that light can't pass through.

  • The Write Laser

    • A CD burner has a moving laser assembly, just like an ordinary CD player. But in addition to the standard read laser, it has a write laser. The write laser is intense enough to darken the dye layer.

    • The burner moves the laser outward while the disc spins. The bottom plastic layer has grooves pre-pressed into it to guide the laser along the correct path. By calibrating the rate of spin with the movement of the laser assembly, the burner keeps the laser running along the track at a constant rate of speed. To record the data, the burner simply turns the laser writer on and off in sync with the patten of 1s and 0s. The laser darkens the material to encode a 0 and leaves it translucent to encode a 1.

    • By selectively darkening particular points along the CD track and leaving other areas of dye translucent, a burner can create a digital pattern that a standard CD player can read. The light from the player's laser beam will only bounce back from the flat areas of a conventional CD. So, even though the CD-R disc doesn't have any bumps pressed into it at all, it behaves just like a standard disc.

  • CD-RW
    • You can write data on CD-R once, since it is not erasable. The problem is solved by using the CD-RW which uses new materials for the recordable area. In CD-RW discs, the phase-change technology is adopted. The disc is made up of the phase-change chemical compound which has silver, antimony, tellurium, and indium. The compound changes its phase, crystalline or non-crystalline (amorphous), at certain temperatures while heating. The crystalline form of the comound is translucent while the amorphous form absorbs most light. In a blank CD-RW, the compound in the recording layer is in a crystalline state. The laser heats the compound, which then cools quickly, losing its crystalline structure, that is the amorphous area is created. When the CD-RW is used again, a new pattern of crystalline area is created by the powerful enough write laser. At melting temperature, the compound melts and then returns to the crystalline state. In playback, the other laser (read laser) reads the crystalline (high reflectivity) and amorphous (low reflectivity) areas.

    • CD-RW discs do not reflect as much light as older CD formats, so that they cannot be read by most older CD players and CD-ROM drives. Some newer drives and players, including all CD-RW writers, can adjust the read laser to work with different CD formats. But since CD-RWs will not work on many CD players, these are not a good choice for music CDs.

House on the Seashore

Imagine that you have a house on the seashore. Nature wind blows through your house all the day. Generally, sea breezes during the day and land breezes in the evening is a common seashore occurrence. You need not switch on the air conditioner and sit there having a glass of cold drink nearby the balcony. Enjoy! The following paragraphs explain the source of the ever blowing wind near the seashore.

How Things Work
During the day, the sun warms the land more rapidly than the water. This is because the land, which is mostly rocks, has a lower specific heat than the water. The warm land heats the air above it, which becomes less dense and rises. Cooler air from over the water flows in to take its place, producing a "sea breeze". The below figure illustrates such effect.
At night, the land cools off more rapidly than the water - again because of its lower specific heat. Now it is the air above the relatively warm water that rises and is replaced by cooler air from over the land, producing a "land breeze". The below figure illustrates such effect.
The above processes are referred to as 'convection'. In general, convection occurs when a fluid is unevenly heated. It is this physical flow of matter that carries heat throughout the system.
  • Consider heating up a kettle of water on a stove, water at the bottom of kettle has higher temperatures since it is close to the burning stove. Now, hot water with lower density rises, to be replaced by cold dense water descending from overhead. This sets up a circulating flow of water that transports heat from the kettle's bottom to the water throughout the kettle.
  • Consider one night in a very cold winter, you are staying alone in your house. It is so cold that you close all the doors and windows, switching on the electric oven and nothing else. Strange blows of air sweep and breeze the house. You are anxious to the horrible wind breezing in such a closed system. Make it for sure, there should be no leak to the surrounding, but how come there is always a current of air blowing around?
    The answer is again due to 'convection'.
Science in Depth
Recall that the specific heat is defined as the quantity of heat which shows the propertry of a substance. It is the heat needed for a unit mass of substance having a unit change in temperature. It depends on the substance itself but it is independent to the quantity of a substance Water has an exceptional large specific heat than other matters, so it is used as coolant in daily life.

Substance Specific heat, c (J/kg · K)


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