|MLA Citation:||Bloomfield, Louis A. "How Everything Works" How Everything Works 21 Feb 2018. Page 157 of 160. 21 Feb 2018 <http://www.howeverythingworks.org/prints.php?topic=all&page=157>.|
A simple way to see why that's the case is to picture the gas as composed of many little bouncing balls inside the container. Those balls are perfectly elastic so they rebound from a stationary wall without changing their speeds at all. But the walls of the container aren't stationary, they move back and forth as you shake the container. Because of the moving walls, the balls change their speeds as they rebound. A ball that bounces off a wall that is moving toward it gains speed during its bounce, like a pitched ball rebounding from a swung bat. On the other hand, a ball that bounces off a wall that is moving away from it loses speed during its bounce, like a pitched ball rebounding from a bat during a bunt. If both types of bounces were equally common in every way then, on average, the balls (or actually the gas molecules) would neither gain nor lose speed as the result of bounces off the walls and the gas temperature would remain unchanged.
But the bounces aren't equally common. It's more likely that a moving ball will hit a wall that is moving toward it than that it will hit a wall that is moving away from it. It's a geometry problem; you get wet faster when you run toward a sprinkler than when you run away from the sprinkler. So, on average, the balls (or gas molecules) gain speed as the result of bounces off the walls and the gas temperature increases.
How large this effect is depends on the relative speeds of the gas molecules and the walls. The effect becomes enormous when the walls move as fast or faster than the gas molecules but is quite subtle when the gas molecules move faster than the walls. Since air molecules typically move at about 500 meters per second (more than 1000 mph) at room temperature, you'll have to shake the container pretty violently to see a substantial heating of the gas.
To understand why light scatter depends on homogeneity, consider what happens when light pass through clear particles. Even though they are clear, light still interacts with them, as evidenced by rainbows, clouds, and even the blue sky. How best to think about that interaction depends on the size of the particles. If the particles are large, like smooth beads of glass or plastic, then they exhibit the familiar refraction and reflection effects of window panes and lenses. If the particles are small, like air molecules and tiny water droplets, then they exhibit a more antenna-like interaction with light. In effect, those tiny particles occasionally absorb and reemit the light waves, particularly at the short-wavelength (i.e., blue) end of the light spectrum.
Both types of interactions are quite familiar to us. Large particles scatter light about without any color bias and exhibit a white appearance. The more surface area a collection of particles has, the more light that collection scatters. For example, a large ice crystal is clear but crushed ice or snow is white. Similarly, a bowl of water is clear but a mist of water droplets is white. Lastly, a bowl of air is clear, but a froth of air bubbles in water is white. As you can see, the transparent particles don't have to be solids or liquids to scatter light, they can even be gases!
On the other hand, truly tiny particles scatter light about according to wavelength and color. In most cases, shorter-wavelength (blue) light scatters more than longer-wavelength (red) light. That effect, known as Rayleigh scattering, is responsible for the blue sky and the red sunset.
In a nutshell then, large transparent particles appear white and tiny transparent particles appear colored (typically bluish). And the more particles there are, the more light is scattered.
Returning to your question, a loose powder of transparent particles scatters light like crazy and appears white or possible colored, depending on particle size. As you pack the powder more and more tightly together, its surfaces join together and it starts to lose the ability to scatter light; it becomes less white and more translucent. When the consolidation is almost complete, the material acquires a slightly hazy look due to scattering by the occasional voids left inside the otherwise transparent material. Finally, when the material is fully consolidated and there is no internal surface left in the powder, it is homogeneous and clear. So sending light through a packed transparent powder and measuring the amount and color of the scattered light tells you a lot about how well consolidated that powder is.
But while the speed of light in vacuum is a constant, the speed of light in matter isn't. Light is an electromagnetic wave and consists of electric and magnetic fields. Electric fields push on electric charge and matter contains electric charges, so light and matter interact. That interaction normally slows light down; the light gets delayed by the process of shaking the electric charges. In air, this slowing effect is tiny, less than 1 part in a thousand. In glass, plastic, or water, light is slowed by about 30 or 40%. In diamond, the interaction is strong enough to slow light by 60%. In silicon solar cells, light is slowed by 70%. And so it goes.
To really slow light down, however, you need to choose a specific frequency of light and let it interact with a material that is resonant with that light. Because a resonant material responds extremely strongly to the light's electric field, it delays the light by an enormous amount. And by choosing just the right wavelength of light to match a particular collection of resonant atoms, Lene Hau and her colleagues managed to bring light essentially to a halt. The light lingers nearly forever with the atoms in their apparatus and it barely makes any headway.
If you look carefully at the string while it's vibrating in a mixture of several harmonics, you'll see that it has only one shape at any moment in time. It's just a jiggling string, after all. The parts of that shape, however, are evolving at different rates in time and those parts are actually the different harmonics going through their individual motions at their own frequencies.
When you cool a basketball, however, you reduce the pressure of its air. That's because the air molecules have less thermal energy at colder temperatures and thermal energy is responsible for air pressure. A basketball that was properly inflated at warm temperature becomes under-inflated when you cool it down. At the same time, the basketball's skin becomes less elastic and more leathery at cool temperatures. So the basketball suffers from under-inflation and from a leathery, not-very-bouncy skin.
If you cool a basketball to low enough temperature, its skin will freeze and become brittle. Just how low the temperature has to go depends on the material used in to make the basketball. I've never seen a basketball shatter on the court, even in pretty cold weather, so I doubt you can "freeze" one in a household freezer. But I'm sure that a dip in liquid nitrogen at -395 °F would do the trick. I often freeze rubber handballs in liquid nitrogen for my class and then shatter them on the floor.
Those YouTube videos were complete fakes; they didn't pop any popcorn while the camera was rolling. To make it appear that the cell phones were popping the corn, the people who produced the videos dropped already prepared popcorn into the frame and then photoshopped away the unpopped kernels. When you watch the video, it looks like the kernels are popping, but they're really just disappearing via video editing as precooked popcorn is sprinkle onto the set from above.
The reason they had to use video trickery is pretty clear: to pop popcorn with microwaves, those microwaves have to be extremely intense. Each kernel contains only a tiny amount of water and it's the water that heats up when the kernel is exposed to microwaves. If the microwaves aren't intense enough, the heat they deposit in the kernel's water will flow out to the rest of the kernel and into the environment too quickly for the kernel's water to superheat and then flash to steam.
Even when you put popcorn kernels in a closed microwave oven, it takes a minute or two for the kernels to accumulate enough thermal energy to pop. In that closed microwave oven, the microwaves bounce around inside the metal cooking chamber and their intensity increases dramatically. It's like sending the beam from a laser pointer into a totally mirrored roomthe light energy in that room will build up until it is extremely bright in there. In the closed cooking chamber of the oven, the microwave energy also builds up until the microwave intensity is enough to pop the corn. How intense? Well, a typical microwave oven produces 700 watts of microwave power. Since the cooking chamber is nearly empty when you're popping popcorn, the cooking chamber accumulates a circulating power of very roughly 50,000 watts.
Although that power is spread out over the cross section of the oven, the microwaves are still seriously intense -- thousands of watts per square inch. To put that in perspective, a cell phone transmits a maximum of 2 watts and that power is spread out over at least 5 square inches so the intensity is less than 1 watt per square inch. When I saw those videos in Summer 2008, I realized that there was no way cell phones were ever going to pop popcorn. They certainly wouldn't do it while they are ringing, because that's when they are primarily receiving microwaves, not when they're transmitting them. It's when you're talking that your cell phone is regularly producing microwaves. It was all obviously just fun and games.
So what about your disassembled microwave oven? Since there is no metal box to trap the microwaves and accumulate energy, they'll only have one shot at popping the corn kernels. The microwaves will emerge from the magnetron's waveguide at high intensity, but they'll spread out quickly once there is nothing to guide them. You could probably pop kernel right at the mouth of the magnetron but not a few inches away. Unless you use microwave optics to focus those microwaves, they'll have spread too much by the time they get through the table and reach the kernels of popcorn and the kernels will probably never pop.
If that were the whole story, the worst that would happen with your experiment would be that it wouldn't cook popcorn. But there is a real hazard here. Sending about 700 watts of microwaves into the room isn't exactly safe. It's something like having a red hot coal emitting 700 watts of infrared light, except that you won't see anything with your eyes and this microwave "light" is coherent (i.e., laser-like) so it can focus really tightly. You'd hate to have some metal structure in the room or even inside the walls of the room focus the microwaves onto you. You absorb microwave much better than the corn kernels and you'll "pop" long before they do. Actually, your eyes are particularly sensitive to microwave heating and you might not notice the damage until too late. Without instruments to observe the pattern of microwaves in the room when the magnetron is on, I wouldn't want to be in the room.
An older oven would have used a heavy transformer, a capacitor, and a diode to convert ordinary household AC power to high-voltage DC power for its magnetron microwave tube. But since your oven was made recently, it probably uses a switching power supply to produce that high voltage. That supply contains a much more sophisticated electronic switching system to convert household AC power to high-voltage DC power. The new approach is cheaper and lighter, so it's taking over in microwave ovens. Just because it's more sophisticated, however, doesn't mean it's more reliable.
My guess is that the unit in your oven has a problem. If it has an intermittent contact in it or if there is a conducting path that is sparking somewhere in the power supply or in the unit as whole, they'll be randomly fluctuating currents present in the oven and those current fluctuations will produce radio waves. A sparking wire or carbonized patch on the power supply will start and stop the flow of current erratically and that can easily cause interference on the AM band. Ordinary AM radio is very susceptible to radio-frequency interference at around 1 MHz and sparking stuff tends to produce such radio waves. A car with a bad ignition system, a lawn mower, and a thunderstorm all interfere beautifully with AM reception. And I suspect that you've got a similar electrical problem in your oven. I doubt that your oven is a microwave hazard, but you should probably have a repair person to take a look at it. It shouldn't have anything sparking inside it.
The idea of a wave that travels through space itself was a rather disorienting notion to scientists in the late 1800s. They were used to the idea that waves are disturbances in a tangible material or "medium": fluctuations in the density of air, ripples on the surface of water, vibrations of a taut string. Having observed that light and radio waves are electromagnetic waves, they set about looking for the medium that supported those waves. They were expecting to find this "luminiferous aether" but they failed. In fact, the absence of an aether led in part to Einstein's theory of special relativity.
The structure of a radio wave, or any electromagnetic wave, is quite simple. It consists only of a fluctuating electric field and a fluctuating magnetic field. An electric field is a structure in space that affects electric charge; it pushes on charge and causes that charge to accelerate. Similarly, a magnetic field is a structure that affects magnetic pole. Remarkably, changing electric fields produce magnetic fields and changing magnetic fields produce electric fields. That interrelatedness allows the wave's fluctuating electric field to produce its fluctuating magnetic field and vice verse. The wave's electric and magnetic fields endless recreate one another. Although electric charge or magnetic pole is needed to emit or receive a radio wave, that wave can travel perfectly well for billions of light years without involving any charge or pole. It travels through space itself.
In fact, turning off the power strip is likely to save energy as well. Many computer devices have two different "off" switches: one that stops them from doing their normal functions and one that actually cuts off all electrical power. Computers in particular don't really turn off until you reach around back and flip the real power switch on the computer's power supply. The same is true of television monitors and home theater equipment.
In general, any device that has a remote control or that can wake itself up to respond to a pretty button or to some other piece of equipment is never truly off until you shut off its electrical power. Our homes are now filled with electronic gadgets that are always on, waiting for instructions. Keeping them powered up even at a low level consumes a small amount of electrical power and it adds up. Last I heard, this always-on behavior of our gadgets consumes something on the order of 1% of our electrical power. Whatever it is, it's too much. So by turning off your power strip and completely stopping the flow of power to your computer, your speakers, your monitor, etc., you are saving energy. You lose the convenience of being able to turn everything on from your couch with a remote, but who cares. Energy is too precious to waste for such nonessential conveniences.
Even if the oven door has multiple layers of glass, those layers are there for your protection. If you touch the outside of the metal grid while the oven is on or get close enough to it through the last layer of glass, you'll be able to absorb some microwave power and it'll probably hurt. That's because while the holes in the grid are too small to allow the microwaves to propagate through them and truly escape from the oven, they do allow an "evanescent wave" to exist just outside each hole in the grid. That evanescent wave dies off exponentially with distance beyond the hole, so it won't travel around the room. But you don't want to put your finger in it.
For inexpensive microwave ovens, you're probably best off simply recycling the oven. I'm not happy about the modern everything-is-disposable state of appliances and equipment, but I can't say that it's cost effective to repair an oven that costs less than about $100. For more expensive microwave ovens, you can usually replace the window or the door. We have had a GE combination microwave and convection oven over our stove top for about 10 years and the door started to come apart about 18 months ago. I purchased a replacement microwave oven door over the web for $140 and installed it myself. It works beautifully. If you're not handy or are concerned about microwave leaks, you should probably have it replaced professionally. But you can look up the parts themselves online at a number of web sites and get an idea of what the cost will be.
The How Everything Works Home Page — Printer Friendly
The Complete Collection of Questions (160 prints, from oldest to newest) — Printer Friendly: