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Come to this Physics classroom

akhl IF-Rockerz

Joined: 30 March 2007
Posts: 5247

Posted: 15 November 2008 at 4:42am | IP Logged
Does it ever happen that you have some doubts in some very basic concepts in Physics? But you are not able to get any proper guidance? May be because you asked somebody, you got the answer but the answer was not clear enogh? Or may be that you hesitate or feel shy in asking such basic doubts and, therefore, you do not ask? Since you do not ask, therefore it affects your knowledge negatively? That, in turn, creates more doubts? Thus a cycle of problems is established and you are not able to see any escape?
If you are in such a situation, then be happy.Smile
Post those doubts in Physics here in this thread. Do not feel shy. Willingness to work hard and learn is the only criteria. Post your doubts here.
Is there any Physics problem on which you are getting stuck even though you have been trying to solve it a lot? If so, then may be that the members here can help. Post the problem. It is worthwhile to mention that this thread is not to get homework done but to really learn.
Is there something, which you are reading in your Physics textbooks, but you are not able to understand it well? If so, then post your doubts here. It is possible that you will get some help here.

mellisai IF-Sizzlerz

Joined: 08 June 2007
Posts: 13668

Posted: 16 November 2008 at 9:46am | IP Logged
Hi akhl! I wanted to do Physics for A-level, but my school (and most of the other schools) doesn't let you take more than 5 A-levels, so I had to drop physics after GCSE! However, one thing interests me more than others in Physics, and that is, Quantum Mechanics. If possible, can you tell me the basics of Quantum Mechanics? Thanks.
_rajnish_ IF-Dazzler

Joined: 08 February 2008
Posts: 2510

Posted: 16 November 2008 at 10:35am | IP Logged
quantum mechanics is such a vast subject that cannot be described in short. there is huge no of things which need to be studied to know quantum mechanics.
@avinash - may i take this class of quantum mechanics. this one is one of my fav part of physicsBig smile
BTW thanks for starting this classTongue
aishu_fan IF-Dazzler

Joined: 21 May 2007
Posts: 2879

Posted: 16 November 2008 at 1:39pm | IP Logged
omg!this is osmethign ive bee looking for foreva!
akhl IF-Rockerz

Joined: 30 March 2007
Posts: 5247

Posted: 16 November 2008 at 9:03pm | IP Logged
You do not need to ask. Go ahead.Smile
_rajnish_ IF-Dazzler

Joined: 08 February 2008
Posts: 2510

Posted: 17 November 2008 at 2:01am | IP Logged
here i go, brief introduction in simplified waySmile

Quantum Mechanics

In day to day life, we intuitively understand how the world works.  Drop a glass and it will smash to the floor.  Push a wagon and it will roll along.  Walk to a wall and you can't walk through it.  There are very basic laws of physics going on all around us that we instinctively grasp: gravity makes things fall to the ground, pushing something makes it move, two things can't occupy the same place at the same time. 

At the turn of the century, scientists thought that all the basic rules like this should apply to everything in nature -- but then they began to study the world of the ultra-small.  Atoms, electrons, light waves, none of these things followed the normal rules.  As physicists like Niels Bohr and Albert Einstein began to study particles, they discovered new physics laws that were downright quirky.  These were the laws of quantum mechanics, and they got their name from the work of Max Planck. 

"An Act of Desperation"

In 1900, Max Planck was a physicist in Berlin studying something called the "ultraviolet catastrophe."  The problem was the laws of physics predicted that if you heat up a box in such a way that no light can get out (known as a "black box"), it should produce an infinite amount of ultraviolet radiation.  In real life no such thing happened: the box radiated different colors, red, blue, white, just as heated metal does, but there was no infinite amount of anything. It didn't make sense.  These were laws of physics that perfectly described how light behaved outside of the box -- why didn't they accurately describe this black box scenario? 

Planck tried a mathematical trick.  He presumed that the light wasn't really a continuous wave as everyone assumed, but perhaps could exist with only specific amounts, or "quanta," of energy.  Planck didn't really believe this was true about light, in fact he later referred to this math gimmick as "an act of desperation."  But with this adjustment, the equations worked, accurately describing the box's radiation.

It took awhile for everyone to agree on what this meant, but eventually Albert Einstein interpreted Planck's equations to mean that light can be thought of as discrete particles, just like electrons or protons.  In 1926, Berkeley physicist Gilbert Lewis named them photons. 

Quanta, quanta everywhere

This idea that particles could only contain lumps of energy in certain sizes moved into other areas of physics as well.  Over the next decade, Niels Bohr pulled it into his description of how an atom worked.  He said that electrons traveling around a nucleus couldn't have arbitrarily small or arbitrarily large amounts of energy, they could only have multiples of a standard "quantum" of energy. 

Eventually scientists realized this explained why some materials are conductors of electricity and some aren't -- since atoms with differing energy electron orbits conduct electricity differently. This understanding was crucial to building a transistor, since the crystal at its core is made by mixing materials with varying amounts of conductivity.

But They're Waves Too

Here's one of the quirky things about quantum mechanics: just because an electron or a photon can be thought of as a particle, doesn't mean they can't still be though of as a wave as well.  In fact, in a lot of experiments light acts much more like a wave than like a particle. 

This wave nature produces some interesting effects.  For example, if an electron traveling around a nucleus behaves like a wave, then its position at any one time becomes fuzzy.  Instead of being in a concrete point, the electron is smeared out in space.  This smearing means that electrons don't always travel quite the way one would expect.  Unlike water flowing along in one direction through a hose, electrons traveling along as electrical current can sometimes follow weird paths, especially if they're moving near the surface of a material.  Moreover, electrons acting like a wave can sometimes burrow right through a barrier.  Understanding this odd behavior of electrons was necessary as scientists tried to control how current flowed through the first transistors. 

So which is it - a particle or a wave?

Scientists interpret quantum mechanics to mean that a tiny piece of material like a photon or electron is both a particle and a wave.  It can be either, depending on how one looks at it or what kind of an experiment one is doing.  In fact, it might be more accurate to say that photons and electrons are neither a particle or a wave -- they're undefined up until the very moment someone looks at them or performs an experiment, thus forcing them to be either a particle or a wave. 

This comes with other side effects: namely that a number of qualities for particles aren't well-defined.  For example, there is a theory by Werner Heisenberg called the Uncertainty Principle.  It states that if a researcher wants to measure the speed and position of a particle, he can't do both very accurately.  If he measures the speed carefully, then he can't measure the position nearly as well.  This doesn't just mean he doesn't have good enough measurement tools -- it's more fundamental than that.  If the speed is well-established then there simply does not exist a well-established position (the electron is smeared out like a wave) and vice versa. 

Albert Einstein disliked this idea.  When confronted with the notion that the laws of physics left room for such vagueness he announced: "God does not play dice with the universe."  Nevertheless, most physicists today accept the laws of quantum mechanics as an accurate description of the subatomic world.  And certainly it was a thorough understanding of these new laws which helped Bardeen, Brattain, and Shockley invent the transistor. 

This is very vast subject but i hope this simplified introduction wud have helped you!Smile anyways do you want to know the postulate and laws of quantum mechanics? do tellSmile

Edited by rajnish_here - 17 November 2008 at 2:06am
akhl IF-Rockerz

Joined: 30 March 2007
Posts: 5247

Posted: 17 November 2008 at 2:14am | IP Logged
Thanks, Rajnish. Let me try.
Wave-like and particle-like
In order to appreciate Quantum Mechanics, we should first understand what is meant by particle-like behaviour and by wave-like behaviour.
Particle-like behaviour means having a well-defined location at a given time.
For example, if I throw a ball, then at any given time, it has some well-defined position. If somebody asks where the ball is at a given time, then we can give the answer. We say that the ball has particle-like behaviour. In other words, it is made up of particles.
Now consider wave. For example, sound. Can we talk about exact location of sound? No we cannot because it is meaningless to talk about exact location of a wave. At the most we can say that the sound can be heard inside this room but not outside. So we can give a region of space and say that sound is within that region. But we cannot pinpoint any exact point and say, "here is sound". That will be meaningless.
For wave, there is no meaning of a well-defined location.
In reality, there are lot more differences between wave and particle. But the difference given above will suffice for now. I will introduce more differences later if needed.
Note: In reality, according to Quantum Mechanics, even a ball does not have a 100% well-defined location. But day-to-day observation says that it has. For simplicity, let us start from day-to-day observations and gradually proceed towards more comples topics.
akhl IF-Rockerz

Joined: 30 March 2007
Posts: 5247

Posted: 17 November 2008 at 2:36am | IP Logged
Light - Wave or Particle?
Electromagnetic Wave
Over the centuries, various theories have been put forth to explain light. Those theories explained some phenomena but also had some flaws.
Then Maxwell gave theory of electromagnetic waves in which electric and magnetic fields oscillate. Maxwell said that light is an electromagnetic waves. Lots of experiments were performed and the results of those experiments matched with what Maxwell's theory predicts. Light had been proved to be a wave. Scientists were happy that they had understood what light really is.
But the happiness was short-lived.
Black-body radiation
A black body is an object, which absorbs all of the the radiation falling on it. Consider the question - "How does the intensity of electromagnetic radiation emitted by a black-body depend on the frequency of the radiation and on the temperature of the black-body?"
Scientists like Wien, Stefan, Boltzman, Rayleigh, Jeans worked on this problem and gave their solutions. The solutions worked in some cases but failed drastically in other cases.
Then Max Planck tried to solve this problem. He tried various approaches. But they failed. Finally he gave a theory, which worked. According to this theory, energy of electromagnetic waves cannot be transmitted continuously but only in the multiples of some unit. Suppose the unit is E. Then energy can be transmitted only as 0, E, 2E, 3E etc. and not anything else. For example, you cannot transmit energy 7.5 E. In other words, you can say that electromagnetic energy can only be transferred in multiples of a packet.
The value of E depends on frequency. If frequency = f, then E = hf, where h is called Planck's constant.
So, energy of an electromagnetic wave of frequency f can only be transmitted in amounts
0, hf, 2hf, 3hf and so on.
This was very strange. After all, energy should be able to take any values between any two given values?
Planck was aware that this was strange. But he gave this theory because the theory explained the behaviour of black-body perfectly and he could not come up with any other explanation.
But personally Planck did not believe that energy can be transmitted only in packets. According to him, what he did was just a Mathematical trick and people should not try to find much of reality in it.

Edited by akhl - 17 November 2008 at 2:38am

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