Rumah Fisika: Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

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All physics of the 19th century and earlier is called classical physics.

Examples are Newtonian mechanics, which we dealt with this whole term, and electricity and magnetism, which you will encounter the next term. In the early part of this century, when we learned about the composition of atoms, it became clear that classical physics did not work on the very small scale of the atoms. The size of an atom is only ten to the minus ten meters.

If you take 250 million of them and you line them up, that's only one inch. In 1911, the English physicist Rutherford demonstrated that almost all the mass of an atom is concentrated in an extreme small volume at the center of the atom.

We call that the nucleus, it's positively charged. And there are electrons which are negatively charged, which are in orbits around the nucleus, and the typical distances from the nucleus to the electrons is about 100,000 times larger than the size of the nucleus itself.

As early as 1920, Rutherford named the proton, and Chadwick discovered the neutron in 1932, for which he received the Nobel Prize. Now, let us imagine that this lecture hall is an atom.

And the size of an atom is defined by the orbits, the outer orbits of the electrons. If I scale it properly, now, in this ratio 100,000 to 1, then the size of the nucleus would be even smaller than a grain of sand.

And it just so happens that yesterday I went to Plum Island, I walked for three hours on the beach and I ended up with some sand in my pockets. And so I will donate to you one proton; make sure you hold onto it... Ooh, this is two protons, that's too generous. So keep it there this is one proton.

And there would be an electron, then, anywhere there, near the walls, going around like mad in orbit and that would then be a hydrogen atom. Just think about what an atom is.

An atom is all vacuum. You and I are all vacuum. You think of yourself as being something, but we are nothing. You can ask yourself the question, If you are all vacuum, why is it, then, that I can move my hand not through the other hand, like a ghost can walk through a wall? That's not so easy to answer, and in fact, you cannot answer it with classical physics and I will not return to that today.

But you are all vacuum. According to Maxwell's equations, Maxwell's law of electricity and magnetism, an electron, because of the attractive force of the proton, would spiral into the proton in a minute fraction of a second, and so atoms could not exist. Now, we know that's not true.

We know that atoms do exist. And so that created a problem for physics and it was the Danish physicist Niels Bohr who in 1913 postulated that electrons move around the nucleus in well-defined orbits which are distinctly separated from each other, and that the spiraling-in of the electrons into the nucleus does not occur, for the reason that an electron cannot exist in between these allowed orbits. It can jump from one orbit to another, but it cannot exist in between.

Now, Bohr's suggestion was earth-shaking, because it would also imply that a planet that goes around the sun cannot orbit the sun just at any distance. You couldn't move it just a trifle in or a trifle farther out. It would also require discrete orbits.

It would also mean that if you had a tennis ball and you would bounce the tennis ball up and down, that the tennis ball could not reach just any level above the ground, but it would only be discrete levels, and that is very much against our intuition. We'd like to think that when you bounce a tennis ball, that it can reach any level that you want to. You give it just a little bit more energy and it will go a little higher. That, according to quantum mechanics, would not be possible.

Now, all this seems rather bizarre, as it goes against our daily experiences, but before we dismiss the idea of quantization see, the quantization comes in when you talk about discrete orbits-- you have to realize that the differences in the allowed heights of the tennis ball and the differences between the allowed orbits of the planets around the sun would be so infinitesimally small that we may never be able to measure it.

In other words, quantum mechanics really plays no role in our macroscopic world. Now, atoms are very, very small compared to tennis balls, and the quantization effects are much larger in the sub-microscopic world of electrons and atoms than in our familiar world of baseballs, pots and pans, and planets. So before we continue, I would like to repeat to you one of the cornerstones of quantum mechanics. And it says that the electrons in atoms can only exist at well-defined energy levels think of them as being orbits around the nucleus, and they cannot exist in between.

Now, when I heat a substance, the electrons in the atoms can jump from inner orbits to allowed outer orbits, and when they do so, they can leave a hole, an opening, an empty space in the inner orbits. But later on, they can fall back to fill that opening. They can occupy that place again.

And when I keep heating this substance, there is some kind of a musical chair game going on. The electrons will go to outer orbits, they may spend there some time and then they may fall to lower orbits, to inner orbits. You see here a vase, a very precious vase, and when I pick up this vase, I have to do work. I bring it further away from the center of the Earth.

Now, is that energy lost? No. I could drop the vase, and it would pick up kinetic energy. I will get that energy back. Gravitational potential energy will be converted to kinetic energy. It will crash to pieces, and it will generate some heat.

In fact, the breaking itself of this vase would take some energy. In a similar way, the energy that you put into electrons when you bring them to outer orbits is retrieved when the electrons fall back.

So there is a parallel-- dropping this vase and getting your work back that I put in. It wouldn't be a nice thing to do to this 500-year-old vase, but as far as I'm concerned, perfectly reasonable to do it with Ohanian, so we can let that go, and the energy will come out in the form of heat and also in the form of, perhaps, some noise.

When electrons fall from an outer orbit back to an inner orbit, it's not kinetic energy that is released, but it comes out often in the form of light, electromagnetic radiation. Light has energy.

Pengembangan Perkuliahan

1. Buatlah sebuah Esai mengenai materi perkuliahan ini

2. Buatlah sebuah kelompok berjumlah 5 orang untuk menganalisis materi perkuliahan ini

3. Lakukan Penelitian Sederhana dengan kelompok tersebut

4. Hasilkan sebuah produk yang dapat digunakan oleh masyarakat

5. Kembangkan produk tersebut dengan senantiasa meningkatkan kualitasnya

Ucapan Terima Kasih:

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

(http://web.mit.edu/physics/people/faculty/lewin_walter.html)

b. Prof. Bernd Surrow, Ph.D.

(http://web.mit.edu/physics/people/faculty/surrow_bernd.html)