engineering thread...for engineers

[clarkliu]

The NMSQT(PSAT) sucks. It was easy in one part, then hard in another.
I took it in October, and the writing part was..............
well, if the job is to find an error, you can find none.
I still haven't got my results back.

I have a question when you mentioned that: what's the difference between a LED light source and a regular bulb?

As for the marks, what can you do if you want to go to an university of another province in China?

hi, i am going to answer your question. What is a LED? LED stands for light emitting diode. I know you never studied that before, but i will tell you it is basically a semiconductor device that has p-n junction structure. The emission occurs when you apply a forward bias current through it. Usually we design a hetero structure to form a quantum well between the p-n. When the electorns flow through the p-n junction, they will "drop" into the quantum well and "jump" from higher energy band to lower energy band. This energy band is called "bandgap". When the electron loses its energy, the engergy becomes photon energy. So, the bandgap determines the energy of the photon. And we know the speed of light is a constant(in one media), and energy=h*c/lamda , here "h" is plancks constant,c is speed of light, lamda is the wavelength of the light. Therefore, the bandgap of the semiconductor material determines the wavelength of the emitted light or you can also understand it as colors. Theoretically, this energy transition process can be 100%, but due to phonon, auger phenomenas, the actual efficiency is smaller but still much greater than light bulb or fluorescent tubes. In the light bulbs, the efficiency is so low. most of the electric energy becomes heat. Thats why you can't touch a working 100W light bulb by hand. Otherwise your hand could be burned. Thats not a efficient way to get light from it. Or we can say light bulb has efficiency of 16 lm/W, and LED has efficiency of 80lm/w (current technology). A a regular LED has power scale of mW and light bulbs has tens of W. So, if we have a 10W LED light source, it could be brighter than a 100W light bulb. one example is the Sony new TX series laptop. The backlight of the screen is a LED array instead of Fluorescent tubes.

 

[sumdud]

ok american kids.. what's SAT and PSAT??? we don't have that in canada. we just got provincials here for high school worth 50% of the mark.. then go to university and feel terrible about ourselves. i can gladly say i just got 90% on my fluid mechanics final today.. feeling good about myself.. but then again, that's my easiest one so far.

SAT stands for Scholastic Aptitude Tests. It is a test that determines whether you get to enter an university(as a freshmen), score depending on your grade and the school you are applying to.
This test is not required, however, for entering community and city colleges as a freshmen.

There are 2 tests: 1 and 2
Test 1, Reasoning Test, has a writing, a math, and a reading section, with an essay portion of 25 minutes in writing section. (Which is a *****, you try writing something in 25 minutes......) And you are penalized for wrong answers.......
3 ****** hours of hell for all normal people at a ****** expense of $40.

The ACT is an alternative to the SAT1, it does not penalize for wrong answers, but it has an extra science section. (Go Engineers!!!!!!)

SAT2 is a collection of subject tests. You choose usually 2 of them to be taken as entrance requirement.

(Note: you do not have to take these tests if you transfer from Community colleges to universities at all.)

PSAT: Preliminary SAT $12
Just a practice, but the test can be used to apply for schlorships.

but i will tell you it is basically a semiconductor device that has p-n junction structure. The emission occurs when you apply a forward bias current through it. Usually we design a hetero structure to form a quantum well between the p-n. When the electorns flow through the p-n junction, they will "drop" into the quantum well and "jump" from higher energy band to lower energy band.

Thanks, Clark
I get your words, expect here: p-n structure, quatum well? Can you explain these words?

is p-n structure the semiconductor/tungsten wire, where on "p" is the start where the eletricity come out and travel to "n"?

drop? into the quatum well? How does it "drop" into a quatum well?
All I know is, when you are doing this, the electrons get excited with energy and for a nanosecond jumps up an orbital before coming back down and releasing light. Is the difference in orbital as the electrons jumps the well?

Do LED light have virtuely an infinite life?

(Wonder if I can create a stovetop with the old tungsten lightbulbs.)

 

[drunkhomer]

hey PiSigma, we got SAT and PSAT in canada..wut r u talkin about....i took mine

 

[sumdud]

Maybe not in Alberta?
I know that Vancouver have it.
TUP had it in Vancouver.
Just looking at the school systems, the BC system is different from Alberta's, which is yet also different from Quebec's.
Vancouver has 7-5 while Calgary has 6-6.

So I guess the SATs are different also?
Wonder why Canada has such a messy school system.

 

[tphuang]

hey PiSigma, we got SAT and PSAT in canada..wut r u talkin about....i took mine

you can take it, but it's not required. I took SAT, but that's because my parents forced me into it!!!!

Anyhow, it served me no good.

 

[PiSigma]

ya maybe it's just alberta then. in high school basically we got a Provincial exam. everyone have to take it. it's worth 50% of the grades for each subject. and every subject have it, the other 50% comes from normal grades. we use the final combined mark to apply for university and scholarships. provincials are usually a joke, we got them in grades 3, 6, 9 and 12. basically all the important courses in each respective grade. in 3 only math and english, in 12 math, english, social, chem/phys/bio. but since everyone in the province get it. those of us that go to good high schools can sleep through one of those and ace it.

just had 2 finals today.. longest exam i have ever written. if anyone want to go to University of Alberta. and go into chemical engineering, i feel sorry for you. i basically spend 2 hours of non-stop writing.. just for one exam... i hate finals.

 

[drunkhomer]

yeah...i'm pretty sure alberta has it 2...its optional...not required

 

[ordinary dude]

As a Chinese Canadian who grad from a Connecticut high school, I think I can clarify this SAT thing once and for all.

The SAT is an evaluation test provided by The College Board, the scores are mostly used by universities and colleges in the US, but it is offered worldwide, you can even write them in China. Its just that the College Board doesn't offer too many sittings overseas so its pretty hard for many people outside US and Canada to understand what it is.

The PSAT is used by the US federal government to deteremine which US citizen qualify for government money such as the Bell Grant.

 

[PiSigma]

ok, so it's basically a test to write to go to AMERICAN universities... i'm too poor, i can hardly afford a canadian one without scholarships, so don't need to consider american ones. therefore never need to take it.

 

[Gollevainen]

So in USA the rich ones get to go in every university which they want and poor people have to take test to proove that they are good enough? Please tell me i'm wrong in here