Whatever Happens, Don’t Panic

So the AP PHYSICS exams are on Monday, and I for one am have already geared into panic mode. I can’t figure out if I should study some more, rest, give up, or cry off my hysteria and try to relax (My heart begins to palpitate just thinking about taking the test…actually, I think it might be good to panic beforehand, helps avoid all the shaking during the test). Anyways, lets get back to a more erudite subject matter and discuss a physics revelation that I had this week or more accurately last Friday night.

In the subconscious state of sleep, I have often been witness to experiences that defy even the most basic principles of physics but rarely any that actually stay true to life. However, in this particular dream, I fell off the top of a bookshelf and after the initial scare began to gently float downward at a constant velocity (the bookshelf might have had to do with repeated childhood attempts to climb it), a phenomenon easily explained by the drag force created by air resistance.

Fdrag = 0.5CpAv^2

Vterminal = square root of 2mg/CpA

Since the air surrounding me in the dream was rather syrupy and coagulated, the air density (p) must have been pretty high and so was my cross sectional area because I was falling flat on my front. This all explains why my terminal speed was reached so quickly and why it was low enough that I didn’t actually wake in a panic.

CAN CRUSH

After reading the Mayor of Casterbridge for three hours straight this afternoon, I decided to take a break and wake up my brain with a little physics adventure. Since I had been drinking a can of juice while enjoying the amazingly exciting and action-packed life of Mr. Henchard, I went to try out an idea (that I must have read or heard somewhere) that will help me remember forever the significance of the idea gas law (which will probably come in handy for the AP exam). So thus determined, I turned on one of the plates in the kitchen, poured in a  little bit of water into the can and set on it on high until i could see steam coming out of its opening. Ready with my bowl of cool water and a oven mitt, I grabbed the juice can and turned it over in the cold water. Immediately, before I could prepare myself, the can was crushed by the atmospheric pressure. It was quite sudden and effective and I actually jumped (and sloshed quite a bit of water over the side…) This is probably the most elementary of physics problems ever, but it still took by surprise! (I can’t believe I hadn’t tried it sooner).

Anyways, the whole point of the exercise was a to find a way for me to set down to memory the relationships between pressure, temperature, and volume and here it is: PV = nRT

As the temperature inside the container heated up and the water begins to boil, gaseous water vapor begins to take up some of the volume of the can and forces out dry air. (The vapor actually occupies approximately 1000 times more space than it did as a liquid). When the the can is turned over into the cool water the water vapor condenses back to liquid water very quickly creating a partial vacuum in the can.  Since the hole in the can is submerged in water, air is not able to rush in and fill that volume.  With very little air inside the can the air pressure inside the can is much less than the atmospheric pressure outside of it.  The can is crushed by the net inward forces exerted by the pressure difference.

Furthermore, since the air pressure pushing down on the water outside of the can was much greater than the air pressure pushing down on the water through the drinking hole of the can, water was drawn up into the can (which explains why it sank…).

CAPACITORS

I have always found laptop touch pads to be annoying and unmanageable devices. Which is why I always use a mouse instead. However, my whole view on the topic was toppled this morning when I went back to review for the AP exam and found myself reading about capacitors. I rediscovered the many many uses of the simple concept of a capacitor with two conducting plates and equal but opposite charge. Not only is it used in cameras and keyboards, it is also the basis behind the touch pads that I have now began to appreciate.

A touchpad is made up of a series of conductors arranged in in two layers. The upper layer uses vertical electrode strips while the lower layer uses horizontal strips. A thin insulation separates each layer, and the layers are connected to an integrated circuit. The electrodes are charged with AC, and the integrated circuit constantly measures the capacitance between the two layers. When someone touches the pad, the capacitance between the electrodes changes since the current wants to flow to your finger to complete the circuit. (and the dielectric properties of a finger are very different from the air). Thus, the charge builds up at the point of the grid under your finger. By measuring this change in capacitance, the circuit knows exactly where the pad was touched. As a finger moves over the touchpad, the changes in location are translated into cursor movements. The pad can be sensitive to other commands as well including taps and scrollbar in certain regions.

I would have never guessed in a million years the simple and ingenious physics behind the dull-looking touch pad (pictured below). Amazing how innovative people can be. And here I must reluctantly admit that physics is useful after all.

Pad looks something like this inside (probably not as big though…)

Here is mine (unfortunately i couldn’t find the camera, I suspect that my mom took it to Mongolia, and I don’t thing the old ones can be plugged into the comp…so i no have original pictures, sorry doc…)

Arizona Memorial & Oil Spillage

The Junior Pearl Harbor visit this Friday proved not only beneficial for our APUSH class, but also provided great review materials for AP Physics. Even the introductory movie was action-packed with references to physics principles such as momentum, buoyant  force, kinetic energy, air resistance, torpedo launchers, engines, and electricity, just to name a few. However, the most exciting part of the trip was at the actual memorial, where in the water, there still exists the remnants of the tons of oil that kept the harbor burning for two days after the Japanese attack. There, my fellow classmates and I took the time to scatter a few flowers in honor of those who died in the bombing, and we soon noticed the layer rainbow on the water caused by the oil. I solemnly reflected on the principles behind the circle of colors, thin-film interference. I concluded that the rainbow color was due to the differing thickness of the sheet of oil at different spots. White light would hit the oil film and be refracted with a phase change of 180 degrees (the index of refraction increases from air to oil) and reflected, the refracted ray is again reflected and refracted on the surface between water and oil. The second time there is no phase change (the index decreases from oil to water). Thus when the two reflected rays interfere on the on the other side of the oil, they either end up completely in phase, completely out of phase, or somewhere in between depending on the path difference of the rays, which equals about twice the thickness of the film. So when there are different thicknesses to the film, a range of different colors result.

Monkey Hanging From a Vine

The latest King Kong movie came out on tv for the first time yesterday. Although I find the story rather absurd and ridiculous, I sat down to watch it anyway and was amazed by the great graphics, some of which utterly failed to follow the laws of physics …and biology (I couldn’t help but wonder how Naomi Watts managed to survive the swings and falls without some serious internal injuries). However, I did note some great forces at work including during the scene where two T-Rex’s, King King, and Naomi all hang onto dear life, suspended from giant vines. In review (going back to the beginning of the year), I realized that this was an excellent example of tension.

The weight of the monkey is equivalent to the x-tensions on the vine.

Some of the vines also acted as a spring would, stretching to accommodate the weight of King Kong. Considering hook’s law: F_s = − kx, such a spring must have a pretty big spring constant to be able to support the giant ape’s mass without snapping.

There were plenty of other physics concepts throughout the movie as well. Static friction is a big part: People just couldn’t seem to be able to keep upright. Kinetic friction too: A lot of poor soldiers died sliding off the side of a cliff. And of course, the huge beasts themselves could put quite a lot of force behind their punches and kicks, being rather massive creatures. I though the King Kong could be very good review for any of Newton’s laws.

Junction Rectifiers

Since the last time I wrote about laptops, I have discovered a second most-amazing property of physics that has kept my poor computer from crashing (and taking with it the few precious works of my short life). There was a brief brownout last week that made me fully appreciate the power of my battery, thanks to which my unsaved paper was saved. If my battery had not been charged that day, I would have spent one very long and sleepless night. However, fortunately, my battery was in fact charged because of the junction rectifier in my computer that was turning the AC from the outlet to DC stored by the battery. It seems that the laptop battery requires DC while the usual outlets only supplies AC, therefore a diode like the rectifier is used to block the alternate current and only allow a flow of current in one direction. The diode placed in circuit with the AC cuts of half of each of its cycle so that the remaining current is all in the same direction.

GRAPH OF AC CONVERTED TO DC

Diffraction Grating

After reading this morning that CD’s can actually act as a diffraction grating, I decided to test my new learnings by conducting a mini-experiment at home. Using an old CD and a laser pointer (borrowed from my mom), I tried shining the laser light through the CD. For a few seconds, I was extremely bewildered to discover that nothing happened. I saw no fringe patterns or bright lines as the textbook had claimed. However, I soon realized that the CD did not actually have slits for the light to pass through and instead it reflected light. Thus by inverting the CD, I received these result:

So using data gathered from the first photo (which  had the most accurate/horizontal lining) shown above, I went ahead and calculated the approximate groove spacing of my CD:

arctan(.15m/.32m) = theta

sintheta=mlambda/groove spacing  ;         m=1  and wavelength of laser=640nm

groove spacing = 1.5 micrometers

My experiment wasn’t too off, the actual spacing of the CD turned out to be 1.6 micrometers. Only a 5.76% error! Not bad for a 5 minute home-lab….

PS: Another interesting pattern I observed about the bright maximi was that at certain angles, I could see that they consisted of a second smaller vertical fringe pattern (shown below). Is that just some effect due to the curve of the CD?

Or just the evidence of my ignorance and inadequacy as a physicist?

The Magic of the Mirage Mirror

It seems we physics students have all been sent on a quest to find the mind boggling explanation behind the mysterious and extraordinary devise known as MIRAGE, an ingenious holograph machine. And I here is my attempt to answer:

This misleadingly simple looking black disk is actually consists of two concave mirrors (some people say parabolic?) placed on top of each other so that their focal points lie exactly at the middle of opposite mirror.

(Like my penguins?)

When an object is placed on the bottom mirror where the focal point of the top mirror is located, light rays from the object travel to the top mirror and is reflected in a parallel line (since all rays from the focal point reflect parallel and vice versa). The reflected ray then hits the bottom mirror and because it is parallel to the second mirror as well the rays are again reflected but this time to the focal point of the bottom mirror, which is located at the opening of the Mirage. Thus a real image is produced at the focal point of the top mirror.

LASERS

So what exactly is the difference between a flashlight and a laser light? While watching doc use his pointer the other day, I wondered to myself how laser’s actually worked. I later found that unlike regular light, laser light is monochromatic, meaning that it emits only one specific wavelength or color of light (often red), coherent, or all the photons form waves in unison, and directional (highly concentrated and emits light in a beam rather than in all directions). All of these are made possible by the two mirrors (one of which is partially coated so that the laser beams can escape) found at each end of the laser. With these mirrors, only emissions of a specific phase and wavelength traveling perpendicular to the full mirror are reflected back to the partially mirrored surface, where the laser light leaves the tube.

This particular pointer emits wavelengths of 630-680 nm

Shown here is a laser light and its reflection in the mirror, plainly thetai and thetar are the same. We chean also see the images of the two rays in the mirror (where the cross intersects).

(This is late at night in the backyard…using water sprays in order to see the laser being reflected off the drops).

DR. HAYASHI, SPIDERWOMAN

After going to see Scott Kim just last week, I really did not expect much more impressive tricks from Dr. Hayashi. However, to my surprise, I found myself thoroughly enjoying the presentation. Her description of the uncertainty she felt in choosing her career and major identified my own dilemma. With the pressure of deciding on that one perfect college and calling in life, I have often found myself confused over what I really wanted to pursue. On the other hand if Dr. H had been just as mystified as me at the moment there seems to be some hope for me after all. Like Dr. Hayashi, I pictured myself something of a lawyer or an economists, but now I think I would like to become a scientist. (My own fickleness is probably the main reason I am having such a hard time settling on a goal). I felt quite inspired to become a biologist by Dr. H’s experience. The way her study incorporated many aspects of the sciences including chemistry, evolution, geology, mechanics, and biomimetics (just to name as few) completely changed my perspective on singling out a line of profession. I saw that a particular occupation does not have to be utterly committed to one area, but that there are many parts and facets that require all types of skills. More importantly, Dr. H’s trip to Panama for a field study reminded me that I would never be sure of my decisions unless I actually went and tried them out hands on. Only after gaining knowledge and practice, can one truly know the values of a certain field. The second part of the power point on spiders and their diversity had me wondering how she managed to keep it all on her mind. There is so much to explore and study about the spider silks! Imagining the vast number of species (some not even known) and the millions of genres of fibers they probably use is simply mind boggling, rather like an endless task. It somewhat amazed me that there still existed such huge branch of unstudied science. I had pretty much supposed that everything of real significance had already been published and established, and finding something new and exciting to do looked difficult. Well, it isn’t the first time I was wrong and caught deceiving myself. In my opinion the most rewarding part of the whole searching and experimenting with creepy animals like spiders comes with recognizing the good we can do with our results. Useful applications of our newfound knowledge could be saving someone’s life (ok maybe not so dramatically) or brightening people”s day just a little. I especially thought bullet-proof vests were a great idea and spider silk clothes would surely be more durable. It’s absolutely incredible considering everything one do….

 

Its no wonder she got the McArthur Grant….Dr. Hayashi’s passion for spiders and their silks is quite evident…

 

P.S. I hope we didn’t need a “scientific” blog this week….just the response, right doc?