All Great Things Must Come To An END

Just like any other GREAT journey, our long, arduous, and stressful trek through the world of PHYSICS is OVER (well, maybe not completely over since these blogs have proven how much Physics have corrupted my worldview -- I will not look at ordinary everyday things the same way again without thinking about the underlying physics that must be behind them). I can honestly say that despite the heart-attack inducing stress, panic attacks, and the merciless blows to my self-esteem that I received everytime I say my pathetic and horrendous quiz / test (without the curve) scores, I am glad that I took this demanding class and have no regrets in losing whatever sense of sanity and sleep that I had left to this daunting yet FUN class.

In my first blog, the picture was nothing but an illustration of not only my atrocious paint skills, but also of my scattered and wildly arrayed thoughts that I tried to collate and form into something more than just a blob of goo that my brain could not comprehend. The goo was what PHYSICS was to me in the beginning, but after surviving a whole year of brain torture, I can now safely say that my understanding of PHYSICS now mirror the image above. Not only has it taken a more pleasing (and less disgusting) form, but it also demonstrates the infinite connections that I've made between PHYSICS and the world around me. I know, for sure, that while I may never commit myself to the same type of torment again anywhere soon (aka take another physics class in the future), the concepts that I learned will never leave me.

Finally, THANK YOU Doc for everything this year. You've certainly made the class a lot of FUN and it's a class that I will never regret taking (I hope). I'm glad that I had you as my AP Physics B teacher before you became the Dean of Studies : ).

And with that, GOODBYE! : )

The Physics of Bridges

While checking my e-mail this morning, I was intrigued by a link to a Popular Mechanics page titled "The World's 18 Stranges Bridges" and of course I went to check the website. I saw some really striking (both in architecture design and function) bridges that really seem to defy physics, but at the same time don't since these structures are still up and running in the world today. Here are some of the pics that I liked the best:

Langkawi Sky Bridge in Malaysia

Juscelino Kubitschek Bridge in Brazil

Millau Viaduct in France

Henderson Waves in Singapore

Rolling Bridge in London

Bridges are typical structures that must follow the laws of physics in order to function. While some of these bridges seem like the architects placed safety and fuctionality as secondary issues below aesthetic design, these infrastructures still follow the laws of physics in essence. Generally, bridges involve a number of physics concepts including Newton's Laws, Resonance, Forces, and more. Bridges typically come in three different groups: beams, arches, and suspension bridges. Each design has its own physics concepts and challenges behind it. Suspension bridges typically span longer distances, with arches coming in second, and beams last. The difference lies in how the three types of bridges deal with the two major forces of COMPRESSION and TENSION (like a spring). The best way to deal with these forces is to either dissipate or transfer them.

For instance, arches are good examples of dissipation. The design itself, a semicircular structure with supports on either side, naturally turns the weight of the object from the center of the deck to the abutments. Because compression lines are pushed outward along the curve of the arch and towards the abutments, the force of compression is dissipated, which means the force is not concentrated on one area, but is rather spread out to a larger region so that one part does not bear the brunt of the force. 

On the other hand, suspension bridges are great examples of transfering the effects of the forces of compression and tension. To transfer force is to move it form an area of weakness to an area of strength. In a suspension bridge, the towers, through which the cables are connected to, support majority of the bridge's weight. The cables feel the force of tension, but the force is transferred to the abutments or the towers, all of which are entrenched in the ground, in which the force dissipates into overtime. The compression force felt by the road is also transferred in the same manner.

The Physics of Photovoltaic (PV) Cells

Can't really think of a good physics concept to write about, but this rainy Sunday morning, while I was in my brother's room, I spied the solar panels that my neighbor installed on their roof from my brother's room window, so I immediately took a picture to write about it in a blog. So, here it is:

In each individual PV cells, a semiconductor component absorbs sunlight and covert energy into electricity through the photovoltaic effect. A PV cells is a good example of a p-n junction in which an n-type material is diffused into the surface of a p-substrate. When an n-type and p-type material are together, electrons spontaneously diffuse (n to p) and hole spontaneously diffuse (p to n). As photons from sunlight strike the cells, a mobile electron/hole pair is formed in which an electron moves into the n-region and the hole moves to the p-region. When a photon strikes a valence electron in the semiconductor in the solar cells, energy is increased, which promotes it to a higher band. Therefore, as photons cause electron-hole pairs, current flows to form electricity. In a home, solar cells are established in a circuit powering a home, there is a flow of negatively charged electrons out of the n-material into the circuit and a flow of positively charged holes out of the p-material into the circuit.

The Physics of A Computer Switch

Given that we just finished learning about this concept in our last chapter (yay!), I figured I'd talk about some of the things that we learned. Below is a picture of a computer switch, an essential computer component that only requires the simple action of turning it ON or OFF, but involves really complicated physics behind it (it took me a long to understand how it worked...).

A computer ON / OFF switch is the application of transitors, specifically MOSFET (Metal-Oxide-Semiconductor-Field-Effect-Transistor), which is a 3-terminal semiconductor device that can work either as strong resistors or strong conductors. A MOSFET transistor is a lightly doped p-type semiconductor whith two islands of n-type material placed avoce a substrate (one is source S, one is drain D). An n-channel connects the S and D while an insulator covers the substrate and the two islands and metal gate G is placed above the transistor. 

When the gate voltage = 0 (potential difference between source S and drain D), electrons flow from the negative terminal to the source S, then across the n channel to drain D and eventually to the positive terminal. Therefore, a current flows through the transistor when the gate voltage is 0. This is how the transistor funcitons when the the copmuter is switched ON.

On the other hand, when there is a negative gate voltage, the gate is negatively charged. The electrons are driven out of the n channel and the n channel is therefore depleted fo electrons (less n electrons). The electrons repel each other and the electric field they cause drives the n channel's electrons into the p-doped substrate to occupy holes. As the n channel becomes narrower and with a strong electric field in the gate, no electrons can flow from source S to drain D. Therefore, current cannot flow. This is what happens when the computer is switched OFF.

The Physics of Cutting Food

On Sunday, my family and I hosted a party for two visiting families (one from Canada and one from Texas) and predictably I got stuck doing kitchen chores. One of my kitchen duties included chopping up ingredients for a certain meal. While I was laboring away, I realized that, as always, physics is involved even in this menial task. While chopping carrots, for instance, I realized that Newton's Laws applied to the siutation. As I placed my pointer on top of the blade, I was applying a certain amount of force F that pushed the blade of the knife deep into the vegetable, thereby cutting it in the process. However, the force F that I applied was not constant. As each piece that I cut varied in size (and therefore mass), I applied a force in accordance to its mass. The smaller pieces were easier to cut (I applied less force) while the bigger pieces required more effort (I applied greater force). Additionally, I also did work (as afterwards my pointer felt numb after all the chopping) illustrated in the equation W=Fx.

Furthermore, while cutting food, I varied the way that I held my knife to cut the fod. For instance, when I cut carrots, I chopped with the blade straight down. However, when I wanted tomatoes in wedges, I cut sideways at an angle. The force that I applied sideways is greater  than the force that I applied straight down.

The Physics of Tobogganing Part 4

On the same day that we went tobogganing at Hamstead Park, a small French family joined in the cold fun : ). The picture below shows a little girl and her father on their way down the snowy hill.

Very simple explanation of the physics involved in this situation. The same previously explained concepts apply including Newton's Laws and Work. However, in this case, weight and normal force are greater than those of one person on the toboggan. The vertical component of the force of gravity mg cancels the normal force N. Thus, the force that causes the motion downwards is mgsin(theta). Although weight mg is constant, kinetic friction, and wind resistance slows my cousin down. Because the contact between the toboggan and the snow / ice creates a frictional force that is directed opposite (back up the hill), the force decreases their rate of acceleration. Work is still the equation W=fx where the w= [mgsin(theta) - f(friction)]x. Hence, the only difference is that the value of mg is greater because instead of one person, there are two.

The Physics of Standing on Stilts

Back to day 1 in Canada...While my cousin and cousin-in-law were doing some last minute Christmas shopping (predictably, they are both females), we spotted one of the characters from the Cirque du Soleil cast who were set to perform on Christmas Day.

This seemingly simple trick of towering over average human beings involves a lot of physics concepts that we've already learned in the past. Of course the most obvious concept involves Newton's Forces. The performer exerts a force on the stilts she uses to increase her height and at the same time the stilts exert an equal but opposite force. These two forces must be equal to each other as the object does not move in the y direction. Furthermore, the forces the performer exerts on the two stilts (for two legs) must be equal to each other; otherwise, she would lean on one side and increase the chance of falling down.

Moreover, this situation also involves the concept of the center of gravity or the center of mass. As in my previous blog about J.K. Rowling's Tales of Beetle the Bard, the performer would be able to balance more easily if she does not bring her legs / stilts too close to each other when standing or moving around because in order to maintain one's balance, the center of mass / center of gravity must be at its lowest point and this is achieved when the weight is more spread out within the object. This is why when humans try to balance on tight rope / ledges with really minuscule width values, the arms are spread out in an attempt to get our center of mass at its lowest point and prevent our fall.