Explaining the World

Sometime next year, the Discovery Channel will launch a new series called Some Assembly Required. The show will take viewers into factories and assembly lines to watch everyday objects being manufactured—and will feature physics professor Lou Bloomfield explaining the science behind those objects and how they are made.

It’s all a natural extension of Bloomfield’s work at UVA. For the past 15 years, he’s taught “How Things Work,” a popular physics class for nonscientists. In the class, students learn the mysteries of how everyday objects function—from air conditioners to Frisbees to iPods.

Bloomfield originally developed the course, he says, in an effort to offer a physics overview that would appeal to individuals without a scientific bent. “My goal is to cover things that are useful and common in everyday life,” he says.

The course has led Bloomfield—who is still an active research scientist—to a second career as a kind of physics translator. He’s written a How Things Work textbook, another book called How Everything Works for the general public, plus articles for Scientific American and the Washington Post.

His office is lined with the tools of his trade. A book titled The Science of Chocolate lies next to Steels: Processing, Structure and Performance. The books are filled with intimidating equations, but from them Bloomfield may draw lessons on the physics of the kitchen—chocolate and steel knives—for next semester’s class or the Discovery Channel show.

The following pages show a typical day on Grounds as Bloomfield might see it, using his book as a guide. For more about Bloomfield, visit his Web site at www.howeverythingworks.org/.

Mowing the grass

Each semester, Bloomfield starts his introductory physics class with the same question: Would your lawnmower still work if the blades of grass were not attached to the ground, but instead just propped up next to each other? And each semester, he says, many students mull the question over and decide that it would not.

But in fact, most lawnmowers would do a fine job trimming that fictional lawn. Why? Because of inertia, the physical property that says that an object at rest will tend to stay at rest. So the blades of grass, when hit by the mower blade, would tend to stay in place, allowing the mower blade to slice right through them—as long as it was sharp and slick enough to not create too much friction between the two.

In fact, Bloomfield says, this is how mulching lawnmowers work. They chop a blade of grass into many tiny pieces after it has already been detached from the ground, then spread those tiny pieces over the lawn to act as fertilizer.

Storing sound

These plugged-in students might be listening to one of the thousands of songs stored on their audio players. Your average digital music player holds the equivalent of a library full of CDs. But what exactly is that iPod really storing?

Audio players don’t store sound, of course; they store numbers that represent sound. All sound, including music, is made up of waves that are variations in air pressure. A microphone—like an ear—is simply an air-pressure-fluctuation sensor. So when the musicians first recorded the songs stored on an iPod, microphones recorded all of the sound as series of numbers representing varying amounts of air pressure.

Audio players store that information digitally. That means that the numbers recorded by the microphone are converted to sets of ones and zeroes. Every number between 1 and 255 can be represented by a string of eight ones and zeroes in a different pattern. Each one of those eight ones or zeroes is called a bit, and all eight together are called a byte. Today’s iPods hold up to 80 billion bytes (80 gigabytes)—enough to store more than 7,000 songs.

How Frisbees fly

As a Frisbee is launched into the air, the thrower automatically tilts it so that its leading edge points slightly up. Because of this tilt and because of the Frisbee’s particular shape, air speeds up as it flows over the top of the Frisbee and slows down as it flows under the bottom. Slowed air exerts more pressure than sped-up air, so the Frisbee experiences a net lift—more force pushing up from below than down from above. That upward lift balances out the Frisbee’s downward weight and helps keep it aloft.

But that’s not all there is to the story. The Frisbee’s spin is also important—a Frisbee tossed in the air with no spin will quickly plunk to the ground. Without spin, the lift on the front of the Frisbee is slightly stronger than the lift in the back, so the front lifts up higher and the Frisbee flips over. A spinning Frisbee, however, behaves like a gyroscope and resists flipping over.

Keeping time

We rely on wristwatches to keep us on schedule, but how does the watch keep time?

Wristwatches, like all clocks, keep time with a harmonic oscillator—an object that oscillates back and forth around an equilibrium point, taking the same amount of time to go back and forth with each oscillation. When the oscillator has gone back and forth a set number of times, that’s a signal to the clock to move the second, or minute or hour hand forward a notch.

An old-fashioned grandfather clock uses a swinging pendulum as its oscillator, but an electronic wristwatch uses a tiny tuning fork made of quartz crystal. “It’s basically a little crystal tuning fork,” Bloomfield explains. The crystal, when pushed, vibrates the way a pendulum swings—taking the same amount of time for each vibration—although much faster, more than 32,000 times per second.

The watch’s circuitry uses electrical “pushes” to keep the quartz vibrating (similar to the way that pushing a child on a swing will keep the swing moving). It also senses the quartz’s vibrations electronically, then uses that information either to control a motor that advances the watch’s hands, or to provide information to a computer chip that keeps time by counting vibrations.

The photocopier

Ever touched a doorknob and gotten an electric shock? You might be surprised to learn that photocopiers work by harnessing the power of that same phenomenon: static electricity.

The core of a photocopier is a thin plate of material called a photoconductor. A photoconductor usually acts like an insulator—a material, such as plastic or rubber, that prevents an electrical charge from moving through it. But when exposed to light, a photoconductor becomes a conductor—a material, like metal, that allows an electrical charge to flow freely.

When this student presses “copy,” the photocopier begins its work by spraying an even coating of negative charges onto one side of the photoconductor plate. Then, the photocopier uses a lens to project an image of the document being copied onto the plate.

Some parts of the image are dark, while others are light. The photoconducting plate becomes a conductor in the areas where it’s hit by light, allowing positive charges to move through it and meet up with the negative charges at the top. This leaves the areas at the top of the plate that are exposed to light electrically neutral, while the dark parts remain negatively charged.

Next, the photocopier gently brushes a thin layer of toner onto the plate. Toner is made of tiny, positively charged plastic particles that contain ink. The positively charged toner particles stick to the negatively charged portions of the plate, creating an exact replica of the original document.

Then, the photocopier transfers the image to paper by pressing a sheet of paper against the plate while spraying the back of the paper with negative charges, which attract the positively charged toner particles. Finally, the machine heats and presses the paper, fusing the ink and paper together, then spits out the fresh, still-warm copy.

Cooling down

The air conditioner that keeps Alderman Library comfortable does so with the help of the basic principles of thermodynamics, or rules that govern the behavior of heat.

Air conditioners consist of three parts—a compressor and a condenser, which are outside the building, and an evaporator, which is inside—along with a chemical (such as Freon) that flows continuously between the three.

The chemical can change forms easily from liquid to gas, and as it does it either extracts heat from the air inside the building, or releases that heat outside.

The chemical comes into the compressor as a cool gas whose molecules are spread far apart. The compressor squeezes the molecules of gas together, increasing their energy and the gas’ temperature. Then, the hot, high-pressure gas leaves the compressor and enters the condenser, a series of coils that allow the gas to slowly release heat to the outside air. As it does, it condenses into a liquid.

Next, that liquid flows into the evaporator. In the evaporator the pressure on the liquid is decreased and it starts to evaporate. As it evaporates, it extracts heat from the air around it, cooling that surrounding air. The energy from the extracted heat allows the fluid molecules to spread out as it turns from liquid to gas. Finally, the fluid—now a cool gas again—re-enters the compressor and the cycle begins again.

Zapping food

“Microwave ovens are filled with mythology,” Bloomfield says—mythology about what can go in one and what can’t, and mythology about the mysterious way they cook food.

Understanding how these applicances work can help separate fact from fiction—and answer some frustrating questions, such as why it’s so hard to get your frozen burrito to cook evenly.

Microwave ovens cook food by using microwaves—electromagnetic waves with a wavelength of about 12 centimeters—to shake up the water molecules that most food contains.

Water is a polar molecule—it has a negatively charged end and a positively charged end—so when it’s placed in an electromagnetic field it tends to line up with that field. A microwave is a constantly shifting electromagnetic field, and those constant shifts cause the water molecules to wriggle back forth billions of times per second, bumping into each other along the way. This constant jostling and bumping converts some of the molecules’ energy into heat, cooking the food.

As for the burrito: Although microwaves are good at heating liquid water, they’re not as good at heating ice. Water molecules in ice are arranged in a crystal lattice that holds them in place and keeps them from twisting. So a frozen burrito will heat slowly in a microwave until parts of it turn into liquid water—at which point those parts will start to heat up much more quickly. That’s why, when you remove your burrito from the oven, one half might still be frozen while the other half is scalding.

Chocolate

While a good chocolate bar is practically

irresistible, store it at the wrong temperature and its glossy brown surface can turn an unappetizing mottled gray. Materials scientists and chocolate manufacturers call this discoloration “bloom.”

Chocolate is made up of cocoa, sugar and fat blended together at precisely the right temperature and proportions to create a crisp solid that melts at mouth temperature.

One of the keys to the process is getting the fat to crystallize correctly. “Fat in chocolate crystallizes in about six different ways,” Bloomfield explains, depending on the temperatures used in the manufacturing process. “And how it crystallizes matters a lot.” Only one of the six forms creates the smooth texture that chocolate lovers expect.

Fat bloom happens when finished chocolate is allowed to get too warm, then cools back down. The heating destroys the fat’s crystal structure. As it cools again, the fat recrystallizes, but this time in the wrong form. The result? A grayish surface that’s still safe to eat, but not nearly as delectable.

Strong spoons, forks and knives

When you’re sitting down at O-Hill Dining Hall, you want to be sure that your cutlery is equal to the task of cutting up your meal—you want metal that keeps its shape rather than bending under pressure like a paper clip.

Cutlery makers ensure this hardiness by making their products out of steel, a metal that keeps its shape even when it’s pushed, pulled and twisted in different directions.

Steel starts with iron, a metal that bends much more easily. In iron, atoms are arranged into an evenly spaced lattice, with sheets of atoms lying on top of one another. When the iron is pushed or twisted, the sheets can slide on top of each other in a process called slip, permanently deforming the metal. Usually these slips start at areas called dislocations, where there’s a glitch in the even spacing of the iron atoms.

To turn iron into steel, steelmakers add a small amount of carbon and heat the mixture to more than 700 degrees Celsius. This forms a brittle material called cementite. Bits of cementite crystals dispersed throughout the iron help stop the sheets of iron atoms from slipping, and together the iron and cementite make the new, harder material: steel.