オッペンハイマーんの映画にもちょいやくで出ていた、
つまり才能を見つけるのが優れているオッペンハイマーに才能を見出されていた
若いリチャードファイマンは量子電磁気学には関係ない。
ところで
大気の電気についても、彼のかいた
feynman physics
で大気の電気についての思考を披露していた。
これ：
’（著作権なし) 図は原文参照されたし”
Electricity in the Atmosphere
| Reference: | Chalmers, J. Alan, Atmospheric Electricity, Pergamon Press, London (1957). |
9–1The electric potential gradient of the atmosphere
On an ordinary day over flat desert country, or over the sea, as one goes upward from the surface of the ground the electric potential increases by about 100100 volts per meter. Thus there is a vertical electric field EE of 100100 volts/m in the air. The sign of the field corresponds to a negative charge on the earth’s surface. This means that outdoors the potential at the height of your nose is 200200 volts higher than the potential at your feet! You might ask: “Why don’t we just stick a pair of electrodes out in the air one meter apart and use the 100100 volts to power our electric lights?” Or you might wonder: “If there is really a potential difference of 200200 volts between my nose and my feet, why is it I don’t get a shock when I go out into the street?”
We will answer the second question first. Your body is a relatively good conductor. If you are in contact with the ground, you and the ground will tend to make one equipotential surface. Ordinarily, the equipotentials are parallel to the surface, as shown in Fig. 9–1(a), but when you are there, the equipotentials are distorted, and the field looks somewhat as shown in Fig. 9–1(b). So you still have very nearly zero potential difference between your head and your feet. There are charges that come from the earth to your head, changing the field. Some of them may be discharged by ions collected from the air, but the current of these is very small because air is a poor conductor.
Fig. 9–1.(a) The potential distribution above the earth. (b) The potential distribution near a man in an open flat place.
How can we measure such a field if the field is changed by putting something there? There are several ways. One way is to place an insulated conductor at some distance above the ground and leave it there until it is at the same potential as the air. If we leave it long enough, the very small conductivity in the air will let the charges leak off (or onto) the conductor until it comes to the potential at its level. Then we can bring it back to the ground, and measure the shift of its potential as we do so. A faster way is to let the conductor be a bucket of water with a small leak. As the water drops out, it carries away any excess charges and the bucket will approach the same potential as the air. (The charges, as you know, reside on the surface, and as the drops come off “pieces of surface” break off.) We can measure the potential of the bucket with an electrometer.
Fig. 9–2.(a) A grounded metal plate will have the same surface charge as the earth. (b) If the plate is covered with a grounded conductor it will have no surface charge.
There is another way to directly measure the potential gradient. Since there is an electric field, there is a surface charge on the earth (σ=ϵ0Eσ=ϵ0E). If we place a flat metal plate at the earth’s surface and ground it, negative charges appear on it (Fig. 9–2a). If this plate is now covered by another grounded conducting cover BB, the charges will appear on the cover, and there will be no charges on the original plate AA. If we measure the charge that flows from plate AA to the ground (by, say, a galvanometer in the grounding wire) as we cover it, we can find the surface charge density that was there, and therefore also find the electric field.
Having suggested how we can measure the electric field in the atmosphere, we now continue our description of it. Measurements show, first of all, that the field continues to exist, but gets weaker, as one goes up to high altitudes. By about 5050 kilometers, the field is very small, so most of the potential change (the integral of EE) is at lower altitudes. The total potential difference from the surface of the earth to the top of the atmosphere is about 400,000400,000 volts.
9–2Electric currents in the atmosphere
Another thing that can be measured, in addition to the potential gradient, is the current in the atmosphere. The current density is small—about 1010 micromicroamperes crosses each square meter parallel to the earth. The air is evidently not a perfect insulator, and because of this conductivity, a small current—caused by the electric field we have just been describing—passes from the sky down to the earth.
Why does the atmosphere have conductivity? Here and there among the air molecules there is an ion—a molecule of oxygen, say, which has acquired an extra electron, or perhaps lost one. These ions do not stay as single molecules; because of their electric field they usually accumulate a few other molecules around them. Each ion then becomes a little lump which, along with other lumps, drifts in the field—moving slowly upward or downward—making the observed current. Where do the ions come from? It was first guessed that the ions were produced by the radioactivity of the earth. (It was known that the radiation from radioactive materials would make air conducting by ionizing the air molecules.) Particles like ββ-rays coming out of the atomic nuclei are moving so fast that they tear electrons from the atoms, leaving ions behind. This would imply, of course, that if we were to go to higher altitudes, we should find less ionization, because the radioactivity is all in the dirt on the ground—in the traces of radium, uranium, potassium, etc.
Fig. 9–3.Measuring the conductivity of air due to the motion of ions.
To test this theory, some physicists carried an experiment up in balloons to measure the ionization of the air (Hess, in 1912) and discovered that the opposite was true—the ionization per unit volume increased with altitude! (The apparatus was like that of Fig. 9–3. The two plates were charged periodically to the potential VV. Due to the conductivity of the air, the plates slowly discharged; the rate of discharge was measured with the electrometer.) This was a most mysterious result—the most dramatic finding in the entire history of atmospheric electricity. It was so dramatic, in fact, that it required a branching off of an entirely new subject—cosmic rays. Atmospheric electricity itself remained less dramatic. Ionization was evidently being produced by something from outside the earth; the investigation of this source led to the discovery of the cosmic rays. We will not discuss the subject of cosmic rays now, except to say that they maintain the supply of ions. Although the ions are being swept away all the time, new ones are being created by the cosmic-ray particles coming from the outside.
To be precise, we must say that besides the ions made of molecules, there are also other kinds of ions. Tiny pieces of dirt, like extremely fine bits of dust, float in the air and become charged. They are sometimes called “nuclei.” For example, when a wave breaks in the sea, little bits of spray are thrown into the air. When one of these drops evaporates, it leaves an infinitesimal crystal of NaCl floating in the air. These tiny crystals can then pick up charges and become ions; they are called “large ions.”
The small ions—those formed by cosmic rays—are the most mobile. Because they are so small, they move rapidly through the air—with a speed of about 11 cm/sec in a field of 100100 volts/meter, or 11 volt/cm. The much bigger and heavier ions move much more slowly. It turns out that if there are many “nuclei,” they will pick up the charges from the small ions. Then, since the “large ions” move so slowly in a field, the total conductivity is reduced. The conductivity of air, therefore, is quite variable, since it is very sensitive to the amount of “dirt” there is in it. There is much more of such dirt over land—where the winds can blow up dust or where man throws all kinds of pollution into the air—than there is over water. It is not surprising that from day to day, from moment to moment, from place to place, the conductivity near the earth’s surface varies enormously. The voltage gradient observed at any particular place on the earth’s surface also varies greatly because roughly the same current flows down from high altitudes in different places, and the varying conductivity near the earth results in a varying voltage gradient.
The conductivity of the air due to the drifting of ions also increases rapidly with altitude—for two reasons. First of all, the ionization from cosmic rays increases with altitude. Secondly, as the density of air goes down, the mean free path of the ions increases, so that they can travel farther in the electric field before they have a collision—resulting in a rapid increase of conductivity as one goes up.
Although the electric current-density in the air is only a few micromicroamperes per square meter, there are very many square meters on the earth’s surface. The total electric current reaching the earth’s surface at any time is very nearly constant at 18001800 amperes. This current, of course, is “positive”—it carries plus charges to the earth. So we have a voltage supply of 400,000400,000 volts with a current of 18001800 amperes—a power of 700700 megawatts!
With such a large current coming down, the negative charge on the earth should soon be discharged. In fact, it should take only about half an hour to discharge the entire earth. But the atmospheric electric field has already lasted more than a half-hour since its discovery. How is it maintained? What maintains the voltage? And between what and the earth? There are many questions.
Fig. 9–4.Typical electrical conditions in a clear atmosphere.
The earth is negative, and the potential in the air is positive. If you go high enough, the conductivity is so great that horizontally there is no more chance for voltage variations. The air, for the scale of times that we are talking about, becomes effectively a conductor. This occurs at a height in the neighborhood of 5050 kilometers. This is not as high as what is called the “ionosphere,” in which there are very large numbers of ions produced by photoelectricity from the sun. Nevertheless, for our discussions of atmospheric electricity, the air becomes sufficiently conductive at about 5050 kilometers that we can imagine that there is practically a perfect conducting surface at this height, from which the currents come down. Our picture of the situation is shown in Fig. 9–4. The problem is: How is the positive charge maintained there? How is it pumped back? Because if it comes down to the earth, it has to be pumped back somehow. That was one of the greatest puzzles of atmospheric electricity for quite a while.
Fig. 9–5.The average daily variation of the atmospheric potential gradient on a clear day over the oceans; referred to Greenwich time.
Each piece of information we can get should give a clue or, at least, tell you something about it. Here is an interesting phenomenon: If we measure the current (which is more stable than the potential gradient) over the sea, for instance, or in careful conditions, and average very carefully so that we get rid of the irregularities, we discover that there is still a daily variation. The average of many measurements over the oceans has a variation with time roughly as shown in Fig. 9–5. The current varies by about ±15±15 percent, and it is largest at 7:00 p.m.in London. The strange part of the thing is that no matter where you measure the current—in the Atlantic Ocean, the Pacific Ocean, or the Arctic Ocean—it is at its peak value when the clocks in London say 7:00 p.m.! All over the world the current is at its maximum at 7:00 p.m. London time and it is at a minimum at 4:00 a.m. London time. In other words, it depends upon the absolute time on the earth, not upon the local time at the place of observation. In one respect this is not mysterious; it checks with our idea that there is a very high conductivity laterally at the top, because that makes it impossible for the voltage difference from the ground to the top to vary locally. Any potential variations should be worldwide, as indeed they are. What we now know, therefore, is that the voltage at the “top” surface is dropping and rising by 1515 percent with the absolute time on the earth.
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大気中の電気
参考文献 Chalmers, J. Alan,Atmospheric Electricity, Pergamon Press, London (1957).
9-1大気の電位勾配
平坦な砂漠地帯や海上の平凡な一日では、地表から上方に行くにつれて、電位は1メートルあたり約100100ボルト上昇する。したがって、大気中には100100ボルト/mの垂直電界EEが存在する。電界の符号は地表の負電荷に対応する。つまり、屋外では、あなたの鼻の高さの電位は、あなたの足元の電位よりも200200ボルトも高いのである！1メートル離れた空中に電極を突き刺して、100100ボルトの電力を電灯に使えばいいじゃないか」と思うかもしれない。と疑問に思うかもしれない： 「鼻と足の間に本当に200200ボルトの電位差があるのなら、なぜ通りに出てもショックを受けないのだろう？
まず2番目の質問に答えよう。あなたの体は比較的優れた導体です。あなたが地面と接触している場合、あなたと地面は1つの等電位面を作る傾向があります。通常、等電位は図9-1(a)に示すように表面に平行であるが、あなたがそこにいるとき、等電位は歪み、電界は多少図9-1(b)のようになる。つまり、あなたの頭と足の間の電位差はまだほとんどゼロに近い。電界を変化させながら、大地からあなたの頭にやってくる電荷がある。そのうちのいくつかは
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