During World War II, two scientists invented the magnetron, a tube that produces microwaves. Installing magnetrons in Britain’s radar system, the microwaves were able to spot Nazi warplanes on their way to bomb the British Isles…. The idea of using microwave energy to cook food was accidentally discovered by Percy LeBaron Spencer of the Raytheon Company when he found that radar waves had melted a candy bar in his pocket. Experiments showed that microwave heating could raise the internal temperature of many foods far more rapidly than a conventional oven.

So, what exactly is a microwave, and are microwave ovens safe? We’ll need to consider the electromagnetic spectrum, something you’ve probably seen in your high school physics and chemistry classes.

The Electromagnetic Spectrum

Put simply, a “micro”-wave is a “small” wave that is on the order of 1 centimeter, not on the order of 1 micrometer, as the name would suggest. That is, these waves have wavelengths of about 1 centimeter. (A centimeter, or cm, is 1E-2 m.) When we compare this with visible light, which is on the order between 1E-6 and 1E-7, we see that microwaves are longer than visible light. If we remember that speed of light = (frequency of the wave)*(wavelength of the wave), where the speed of light is about 2.9979E8 m/s, then we can see that frequency is inversely proportional to the wavelength. What this means is microwaves travel at a lower frequency than visible light. (Frequency is measured in cycles per second, or Hertz.)

Again, from IdeaFinder.com:

[Microwaves] are found in the non-ionizing portion of the energy spectrum, between radio waves and visible light. “Non-ionizing” means that microwaves do not detach charged particles and produce atoms with an unbalanced plus or minus charge. Microwaves can therefore safely produce heat and not cause food to become radioactive.

Now, let’s take a look at a few practical things about microwave ovens, like why radiation doesn’t escape into the kitchen, or is it safe to stop the microwave and reach in to grab the piping hot HotPocket about eat it right away? If you notice that there is a metal mesh screen in the door of the microwave. The side of the holes, on the order of half a centimeter or so, allows the physically smaller visible light waves to pass through but prevents the “larger” microwaves from leaving the microwave oven (remember that light waves are orders of magnitude smaller than microwaves). Also, you don’t need to worry about residual microwave radiation from the microwave oven because these waves always travel at the speed of light and will have been absorbed into your food long before it gets a chance to escape and hit you in the face.

More information on the history of the microwave oven can be found at Gallawa.com. Image from ScienceProg.

What do mechanical engineers do? Well, we look and carry a big meter stick (kind of).

When I first started dealing with electric circuits, I had trouble intuiting what a capacitor did or what an inductor was.

I came up with the following menomic. Imagine a tall slope, with a height H at one end. Then, imagine there is a stream of water running down that slope. Take the stream of water to be analogous to electric current, and the height H to be the cause a potential difference between the top of the slope and the bottom of the slope (remember your physics equation that mechanical potential energy = mass x acceleration of gravity x height).

Then, in basic circuits, we get that resistors are similar to a paddle wheel in the path of the downward flowing stream. Both absorb energy from the electric current or flow of water, and converts the current from electrical energy to light or heat (typically) in the basic circuit.

Capacitors are like storage tanks that is mounted on the side of the slope. Like a tank, the capacitor starts to get “filled,” or “charged,” but once the water source goes out, the capacitor “tank” starts to deplete and continue the water flow downstream of the capacitor “tank.” (Keep in mind, these are approximate analogies, and not actually the case. While a capacitor does release energy and acts like an energy source after the original energy source is removed, the current is reversed. Clearly, water does not easily flow up-hill. There’s something wrong with this analogy, but it works, to a point.) So the way a capacitor in a circuit is to basically store electrical energy.

Finally, an inductor is, fundamentally, a coil of wire around a permanent magnet. We can think of this like a weighted paddle flywheel (like in the resistor case, but this time, it does not actually lose energy). In a circuit, an inductor is capable of smoothing out any spike in current. Analogously, a weighted flywheel would maintain the spinning of the paddle even if there is a sudden spike in the water current.

Maybe this’ll help you get a more physical grasp on the invisiblity of electric circuits. It’s closely analogous, but they are hardly dependable once we move on to more advanced circuits. It’s a start!

In most cases, conductive heat transfer happens more rapidly than convective heat transfer. That is, heat transfer through solid materials is more profound than that in liquids or gases. You’ve probably experienced this in your everyday lives, knowing that we can “sense” heat transfer when we feel warm or cold ourselves. The heat or cold that we feel is known as heat flux, which is heat transfer per unit area. Therefore, in most cases if we have a metallic object and a roomful of air at the same temperature, touching the metallic object will feel warmer (be careful of burns!) than simply standing in the room and absorbing the ambient temperature.

Wear gloves when touching hot metal surfaces!

Why does this happen? Intuitively, solids are denser than liquids and gases, meaning the molecules in solids are more closely-packed. This means that it is easier for heat to be transferred from molecule to molecule in solids, which would explain why heat transfers faster in solids.

When designing an apparatus for heat transfer purposes, one must consider two things: cost and effectiveness. Natural convection (such as with air) is relatively inexpensive because air is everywhere, but it isn’t as effective as using a metallic solid for heat transfer purposes. However, metals can be expensive. Therefore, some form of middle-ground is often desireable. This can be seen in computers, where fins conduct heat away from, say, a processor, and a fan blows the heat away in a process called forced convection.

(Image from WellBake)

The Doppler effect is a phenomenon that can be observed with a moving object which is emitting waves. Many objects emit waves: cars and trains emit sound waves, and stars emit light waves, just to name a few examples. Imagine a time when you were standing outdoors and a vehicle (such as a fire truck with its sirens on) drove by you. The sound you hear from the moving vehicle becomes higher-pitched as the vehicle moves toward you, and the sound becomes lower-pitched after the vehicle passes by and drives away from you. Why does this happen?

This visual could help you picture the fire truck example.

We can imagine wave-emitting objects like a siren-emitting fire truck as “point sources” of waves. This means the object can be modeled as a single point that emits circular waves that drift outward from itself, sort of like what you see when you drop a pebble into a pond. Keeping this in mind, if the point source moves, then the circular waves become more densely-packed in the direction that the object is moving in, and the waves become more sparse in the direction behind the object’s motion. For example, if we have a fire truck traveling to the left emitting the sound of a siren, then to the left of the fire truck the sound waves are closely-packed together, and to the right they are farther apart from each other. See if you can picture this, because this is the key to understanding the Doppler effect. This is why if you’re standing in front of a moving vehicle, you hear a high-pitched sound (high frequency, sound waves closer together), and if you’re standing behind a moving vehicle, the sound is low-pitched (low frequency, sound waves farther apart).

So, how does the Doppler effect affect weather forecasting? Doppler radars are devices that emit waves, which get reflected from objects in the sky such as raindrops, snowflakes, or even dust. The reflected waves can be interpreted in several ways. The number of reflected waves can tell weather forecasters how intense a storm is, and the frequency of the reflected waves can reflect (no pun intended) which direction the storm is traveling in relative to the direction that the Doppler radar is pointing in. These things are all useful information for weather forecasters, which is why you sometimes hear the term “Doppler radar” during weather forecasts.

(Image from Wikipedia.)

Poisson's Ratio - Compression

Let’s take a look back at tensile stress, from back in February. We said that the axial pulling of a isotropic bar creates axial or tensile stress. Some definitions:

• Axial – lengthwise along the bar
• Isotropic – often a metal, where the “crystal” structure in the material is uniform (wood and carbon fiber are not prismatic because of grain and stress biases, for example)
• Tensile stress – mechanical stress that builds up as a result of something being pulled

Whenever tensile stress is applied to something as a result of pulling, that something tends to elongate. This much is intuitive and obvious. What we don’t really pay attention to is what happens in the other direction, i.e. the lateral direction.

Take out your favorite rubber eraser (a not just a remnant nub of an eraser!). I am using a new Sanford Magic Rub for this demonstration. (Note that a rubber eraser is not entirely prismatic as we require, but it does the job very well.) Now try to evenly pull the two ends of the erase apart lengthwise, as hard as you can. As you can expect, you’ll notice that the eraser elongates slightly until you stop pulling, at which time it returns to its original length. What happens in the other direction; does the width change? And what happens if we compress the eraser? Does the opposite effect occur?

The concept we use when we consider this is called the Poisson effect. Poisson’s ratio ν (nu) is a measure of the Poisson effect.

From Wikipedia:

When a sample cube of a material is stretched in one direction, it tends to contract (or occasionally, expand) in the other two directions perpendicular to the direction of stretch. Conversely, when a sample of material is compressed in one direction, it tends to expand (or rarely, contract) in the other two directions.

Poisson’s ratio depends upon the specific material and can be determined experimentally. Its technical definition is the negated ratio of transverse strain to axial strain. Poisson’s ratio ranges from -1.0 to +0.5.

A positive Poisson’s ratio means that the material contracts in the transverse direction as it stretches in the axial direction. A negative ratio means that the material bulges in the transverse direction as it stretches in the axial direction. Similarly, if the material is compressed, it would bulge (positive ratio) or contract (negative ratio).

Most metals hover about a Poisson’s ratio if 0.33. Here are a few examples: Copper is 0.33, Gold is 0.42, and Stainless steel is 0.30. What about our rubber eraser?

Did you notice that as you pulled on it lengthwise, its midsection started to contract? And when you compressed it lengthwise (which might be difficult because it cannot be allowed to bend), you should see that the eraser gets fatter.

As it turns out rubber has a Poisson’s ratio of about 0.5, which makes it one of the best showcases of the Poisson effect. Does our demo agree with the +0.5? Sure it does!

There are a few caveats for the Poisson effect to occur though, but we’ll save that for another time. For most practical purposes though, this is applicable and is quite awesome. (For the engineers… It isn’t safe to use a material’s Poisson’s ratio when we exceed the plastic region of stress. When objects start to permanently deform, we have bigger problems, to say the least.)