Friday, May 28, 2010

The Rear-View Mirror

Ah the rear-view mirror... such an unassuming but useful object.

I have recently been using the night position a lot, due to the fact that I am driving on sparsely travelled roads in the middle of the Kern County oilfields. We usually travel slower than many cars on the road because our truck (the "logger" not the pickup) is large and sometimes a bit unwieldy. I drive behind the logger in my pickup. This means that people end up following me way too closely on the two-lane roads before getting the opportunity to pass! So at night, flipping the rear-view mirror really comes in handy when these bright lights are right behind me.

I used not to understand how this worked, but it is a simple principle. The mirror has two layers. The outer layer is mostly transparent but partially reflective. The inner layer is reflective and is pointed toward your eyes in the day position. In the bright light of the day, it reflects like a normal mirror, because the outer layer lets most of the light through. The outer layer is set at an angle to the inner layer, the same angle that the mirror rotates when you flip it. In the night position, the light from the car behind you gets mostly transmitted through the outer layer to the inner layer and is reflected away from you toward the ceiling. But because the outer layer is slightly reflective, it reflects just a little of the light back to your eyes, making it much easier to see in front of you but still keep tabs on what's behind you.

If you flip the mirror during the day, you'll notice that all you get is a reflection of the ceiling. This reflection is so bright that it obscures any dimmer reflection coming from the outer layer.

This is the same principle used for two-way mirrors. The two-way mirror both transmits and reflects light. The room on one side is kept very bright, so the people in this room only see the light being reflected from the mirror, which obscures anything being transmitted from the other side. The other side is kept dark so that the people on this side can see the light being transmitted through the mirror without it being obscured by any reflections.

So next time you're driving at night, take a second to appreciate your rear-view mirror!

Welcoming a new addition to my family of Apple products

I am currently writing from a coffeeshop in San Francisco. I have quite a few days off/vacation days, so I drove up here to visit a college friend. I am also writing from my new iPad!! I really like it so far. There are a few things about it that are not ideal, but I still have my old laptop too. Using the iPad all the time makes me think about the physics of touchscreens, so I will give a very brief explanation here. Circuits and E&M were not my strongest parts of physics, so it really will be brief.

From what I have read, the iPhone and iPad use something called capacitive sensing. In the screen there is a layer of conductive material covered by a layer of insulating material. A voltage is applied to the conductive material, creating a static electric field inside the insulating material. When you touch the screen with your finger, your body's ability to conduct electricity will change this electric field. This is why touching the screen with your fingernail or a stylus doesn't work. They are not conductive and therefore cannot alter the field that has been created in the screen. The iPhone and iPad screens are broken up into a grid that can sense touches at multiple points. The changes in the electric fields can be processed to figure out exactly where you are touching and what motions you are making with your fingers.

Thank you Apple for making such good use of this technology!

Sunday, April 4, 2010

Physics and Food - Two of my favorite things!

After an extremely long break from writing, I am finally back. I've been busy with work, and when I'm not working, I'm "busy" recharging from my days on duty. Christine mentioned recently in her blog that she doesn't know how people with "real lives" have time to blog. I guess I'd qualify as one of these people, and she is absolutely right that blogging is hard to fit in to "real life" sometimes!

One day recently when I wasn't out on a job, I had some of the other engineers over for dinner. I made my family's favorite pasta sauce with angel hair pasta, and I made creme brulee for dessert. While I was waiting for the water to boil for the pasta, my friend Will and I were discussing the physics of boiling water.

For water to boil, the liquid on the bottom of the pot must get hot enough to change into gas and form a bubble, and this bubble must stay hot enough to float to the surface of the water without popping under the pressure on its way up and without cooling down enough to turn back into liquid. This means that the pressure inside the little air bubble has to balance the pressure of the surrounding water, which depends upon the pressure being put on the surface of the water. And the temperature of all of the water has to be high enough to keep the bubble from cooling down, (not just the water on the bottom).

I hope some of you will remember the ideal gas law from high school chemistry or physics:

PV = nRT

where P is the pressure of the gas, V is the volume, n is the number of moles (related to the number of grams of the substance), R is a known constant, and T is the temperature. Technically this equation only applies to gases that are under perfect conditions, but we can approximate our boiling water situation to one that can be explained by it for simplicity.

The question that Will and I were discussing is whether or not putting the lid on the pot helps the water to boil faster. You probably realize that putting the lid on will keep more heat in that would otherwise escape into the air, right? But does putting the lid on also increase the pressure on the top of the water? If this were the case, looking at the ideal gas law, the increase in pressure and temperature would balance out, meaning that the water would not boil any faster. However, putting the lid on does not increase the pressure because the lid does not seal tightly enough; air can still escape fairly easily. So the answer: putting the lid on the pot does help the water to boil faster because it traps the heat but doesn't increase the pressure.

As I was writing this, I wondered to myself: if the temperature of the water and the water vapor is increasing, why does this not in turn increase the pressure anyway? I think the answer to this is that the lid will still allow the volume to increase instead of the pressure. The water vapor will transfer its heat back to the water, increasing the temperature even further, but it is still allowed to escape from under the lid.

Once water comes to a boil, the temperature of the water will not increase any further. All of the heat coming from the stove will go into converting the liquid into gas. This means that whatever you're cooking in the water can't get any hotter than 212 degrees Fahrenheit, the boiling point of water. Now a pressure cooker is a different story. The lid on a pressure cooker seals tightly and keeps the vapor from escaping. This increases the pressure on the surface of the water, so it has to get even hotter before those bubbles on the bottom can form and rise. This means that the boiling point of the water is higher in a pressure cooker, so whatever you're cooking can be cooked at a higher temperature.

What about a double boiler, what's the purpose of that? A double boiler consists of two pots; the first is filled part way with water and sits on the stove, and the second sits on top of the first. The water should not be high enough to touch the top pot. The water is brought to a boil. Whatever you're cooking in the top pot is heated indirectly by the steam from the boiling water. This is a way of cooking the food more slowly and evenly. It's used to do things like melt chocolate; doing this in a pot sitting directly on the burner can heat the chocolate too fast, causing it to burn. It's also used to make custards and sauces for the same reason. I actually used mine recently to make the custard for a banana pudding.

It's interesting to write about the connections between two of my favorite things. Maybe I'll write some other physics of cooking posts soon. As always, let me know if there's anything specific you'd like me to write about, food related or not!