A question in Feynman's lectures

[nzahra_ghasemi]

you can find it in 2nd feyman's lectures book page 6-14 ,tath speak about "the field-emission microscope"that prof. feynman said"figure 6-17 is an example...the pattern of spots on the flurescent screen shows the arrengement of the individual atoms on the tungsten tip. the reason the spots apear in ring can be undrestood by visualizing a large box of balls packed in a rectangular array ,representing the atoms in the metal .if you cut an approximately sphericl section out of this box ,you will see the ring pattern characreristic of the atom structure.
i want to know more about this box that he used as an example ,i don't know why this would happen

 

[rtharbaugh1]

Hi nzahra ghasemi

I don't have the books by Feynman. I have read some others by him, but not that one.

However, perhaps I can help a little, if you are still interested in this question.

I will start with simple things because I do not know where you are in your studies. Have you studied chemistry?

First, it is thought that all material objects are made of tiny atoms, each of which is much too small to be seen by the unaided human eye. For example, if you take a rock and crush it with a hammer, the small pieces are still the same material as the rock.

Then if you take one of the small pieces and crush it again, down to dust, and look at it with a lens, you will see that the tiny pieces are still the same as the rock. If the rock you started with was a quartz crystal, the tiny pieces will still be quartz crystals.

Some materials can be broken down further into even smaller pieces, and we find that when we do so, we can separate some of the pieces and they are not the same. For example, some quartz rocks have tiny pieces of metals in them, and the metals are different from the quartz in many ways, not just shape, but density, conductivity, specific heat, and other measures.

We have found ways to look at things that are much smaller than any dust you can make by pounding on rocks with a hammer. We have powerful light microscopes that can see tiny things so small that you could never hope to see them with your own unaided eyes. Yet all of these microscopes see the same thing: material that can be divided into even smaller pieces, without losing its qualities of density, hardness, electro-conductivity and so on.

Tungsten is a metal, which is a special kind of material, different from quartz in many ways. Metals are ductile, which means that when you pound on them with a hammer they tend not to shatter into smaller pieces, but instead they flatten out. We can change their shapes but they are still metal. Because they are ductile, we can also draw many metals into wire. If we are very careful we can make wires that are incredibly small, much smaller than a human hair. The tungsten wire Feynman was talking about has been drawn down until the tip of the wire is as small as it can possibly be. It is much too small to be seen with a light microscope.

But it has been found that we can make microscopes that do not use light. Light has a wavelength, and will not reflect off of objects much shorter than the wavelength. Since the 1970’s we have had electron microscopes, which use beams of electrons instead of light waves, and they have a much, much shorter wavelength than light, so we are able to see things that are much smaller. In fact, we can see things as small as an atom.

People have suspected that all material is made of atoms for a long time. Two thousand years ago people were already arguing about atoms. But only in the past thirty years have we had microscopes that can actually give us an image of atoms. The picture you saw in Feynman’s book was one of those pictures, made by reflecting electrons off a very tiny tip of a tungsten wire.

Atoms are all tiny spheres, like balls. You can make a model of a material at the atomic scale by filling a box with little balls. It happens that atoms are all about the same size, very nearly, but there are small variations. Some are a tiny bit bigger, some a tiny bit smaller. In a pure quartz crystal, all of the atoms are exactly the same size. In a pure tungsten wire, all of the atoms are exactly the same size, but it is a slightly different size that the atoms in quartz.

Some kinds of atoms tend to stick together, like tungsten and quartz. Some kinds of atoms do not tend to stick together, like helium and hydrogen, which almost always occur as gasses. Atoms like metals, which tend to stick together but are ductile, are different from atoms like those found in quartz, which tend to stick together but shatter when struck by a hammer. Perhaps you have already studied all of this in a basic chemistry class.

When you look at the picture in Feynman’s book, you are looking at patterns made by electrons which are bounced off the tungsten tip and caught on a screen which glows when struck by an electron. It is mostly the same as a television screen. The picture made by the screen looks like a pile of tiny balls. Those are then a representation of the tungsten atoms.

Now if you fill up a box or a jar with small balls, you can look at them and identify some of the same sorts of patterns that we see on the screen. I think that is what Feynman is suggesting. The tip of the tungsten wire is made of tiny atoms, shaped like balls, which stick together in patterns that are the same patterns you see when you fill a box with small balls.

I don’t know what materials you have at hand. I would suggest you fill a glass jar with buckshot or ball bearings or marbles to see these patterns. The main point is, that at very tiny scales, metals and other materials are made of tiny spheres. These tiny spheres occur in a hundred or so kinds, each of which has different properties of density and so on. All of the things we have on Earth are made up of combinations of these hundred or so kinds of balls. We call them elements and you can find a list of them and their properties in a periodic table of the elements.

Please let me know if this helps. Perhaps you already knew all of this. If so, we might use the ideas above to go on into more sophisticated topics.

Richard

 

[jtbell]

i want to know more about this box that he used as an example ,i don't know why this would happen

Get a lot of small balls made of styrofoam or something else that is easy to cut. Glue them together in a cubical pattern. Use a knife to carve that cubical array into a spherical shape, and see how the balls on the surface of the sphere are arranged.