For the Love of Physics (24 page)

Lightning happens when thunderclouds become charged. Generally the top of the cloud becomes positively charged, and the bottom becomes negative. Why this is the case is not yet completely understood. There’s a lot of atmospheric physics, believe it or not, that we are still learning. For now, we’ll simplify and imagine a cloud with its negative charge on the side closest to the Earth. Because of induction, the ground nearest the cloud will become positively charged, generating an electrical field between the Earth and the cloud.

The physics of a lightning strike is pretty complicated, but in essence a flash of lightning (electric breakdown) occurs when the electric potential between the cloud and Earth reaches tens of millions of volts. And though we think of a bolt as shooting from a cloud down to Earth, in truth they flow
both
from the cloud to the ground and from the ground back up to the cloud. Electric currents during an average lightning bolt
are about 50,000 amps (though they can be as high as a few hundred thousand amps). The maximum power during an average lightning stroke is about a trillion (10
¹²
) watts. However, this lasts only for about a few tens of microseconds. The total energy released per strike is therefore rarely more than a few hundred million joules. This is equivalent to the energy that a 100-watt light bulb would consume in a month. Harvesting lightning energy is therefore not only impractical but also not too useful.

Most of us know that we can tell how far away a lightning strike is by how much time elapses between seeing the bolt and hearing the thunder. But the reason why this is true gives us a glimpse of the powerful forces at play. It has nothing to do with the explanation I heard from a student once: that the lightning makes a low pressure area of some sort, and the thunder results from air rushing into the breach and colliding with the air from the other side. In fact, it’s almost exactly the reverse. The energy of the bolt heats the air to about 20,000 degrees Celsius, more than three times the surface temperature of the Sun. This superheated air then creates a powerful pressure wave that slams against the cooler air around it, making sound waves that travel through the air. Since sound waves in air travel about a mile in five seconds, by counting off the seconds you can figure out fairly easily how far away a lightning strike was.

The fact that lightning bolts heat the air so dramatically explains another phenomenon you may have experienced in lightning storms. Have you ever noticed the special smell in the air after a thunderstorm in the country, a kind of freshness, almost as if the storm had washed the air clean? It’s hard to smell it in the city, because there’s always so much exhaust from cars. But even if you have experienced that wonderful fragrance—and if you haven’t I recommend you try to make note of it the next time you’re outdoors right after a lightning storm—I’ll bet you didn’t know that it’s the smell of ozone, an oxygen molecule made up of three oxygen atoms. Normal odorless oxygen molecules are made up of two oxygen atoms, and we call these O
₂
. But the terrific heat of lightning discharges blows normal oxygen molecules apart—not all of them, but
enough to matter. And these individual oxygen atoms are unstable by themselves, so they attach themselves to normal O
₂
molecules, making O
₃
—ozone.

While ozone smells lovely in small amounts, at higher concentrations it’s less pleasant. You can often find it underneath high-voltage transmission lines. If you hear a buzzing sound from the lines, it generally means that there is some sparking, what we call corona discharge, and therefore some ozone is being created. If the air is calm, you should be able to smell it.

Now let’s consider again the idea that you could survive a lightning strike by wearing sneakers. A lightning bolt of 50,000 to 100,000 amperes, capable of heating air to more than three times the surface temperature of the Sun, would almost surely burn you to a crisp, convulse you with electric shock, or explode you by converting all the water in your body instantaneously to superhot steam, sneakers or not. That’s what happens to trees: the sap bursts and blows off the tree’s bark. One hundred million joules of energy—the equivalent of about fifty pounds of dynamite—that’s no small matter.

And what about whether you are safe inside a car when lightning strikes because of the rubber tires? You might be safe—no guarantees!—but for a very different reason. Electric current runs on the outside of a conductor, in a phenomenon called skin effect, and in a car you are effectively sitting inside a metal box, a good conductor. You might even touch the inside of your dashboard air duct and not get hurt. However, I strongly urge you not to try this; it is very dangerous as most cars nowadays have fiberglass parts, and fiberglass has no skin effect. In other words, if lightning strikes your car, you—and your car—could be in for an exceedingly unpleasant time. You might want to take a look at the short video of lightning striking a car and the photos of a van after having been hit by lightning at these sites:
www.weatherimagery.com/blog/rubber-tires-protect-lightning/and www.prazen.com/cori/van.html
. Clearly, this is not something to play around with!

Fortunately for all of us, the situation is very different with commercial
airplanes. They are struck by lightning on average more than once per year, but they happily survive because of the skin effect. Watch this video at
www.youtube.com/watch?v=036hpBvjoQw
.

Another thing not to try in regards to lightning is the experiment so famously attributed to Benjamin Franklin: flying a kite with a key attached to it during a thunderstorm. Supposedly, Franklin wanted to test the hypothesis that thunderclouds were creating electric fire. If lightning was truly a source of electricity, he reasoned, then once his kite string got wet from the rain, it should also become a good conductor of that electricity (though he didn’t use that word), which would travel down to the key tied at the base of the string. If he moved his knuckle close to the key, he should feel a spark. Now, as with Newton’s claim late in life to have been inspired by an apple falling to the ground out of a tree, there is no contemporary evidence that Franklin ever performed this experiment, only an account in a letter he sent to the Royal Society in England, and another one written fifteen years later by his friend Joseph Priestley, discoverer of oxygen.

Whether or not Franklin performed the experiment—which would have been fantastically dangerous, and very likely lethal—he did publish a description of another experiment designed to bring lightning down to earth, by placing a long iron rod at the top of a tower or steeple. A few years later, the Frenchman Thomas-François Dalibard, who had met Franklin and translated his proposal into French, undertook a slightly different version of the experiment, and lived to tell the tale. He mounted a 40-foot-long iron rod pointing up into the sky, and he was able to observe sparks at the base of the rod, which was not grounded.

Professor Georg Wilhelm Richmann, an eminent scientist born in Estonia then living in St. Petersburg, Russia, a member of the St. Petersburg Academy of Sciences who had studied electrical phenomena a good deal, was evidently inspired by Dalibard’s experiment, and determined to give it a try. According to Michael Brian Schiffer’s fascinating book
Draw the Lightning Down: Benjamin Franklin and Electrical Technology in the Age of Enlightenment
, he attached an iron rod to the roof of
his house, and ran a brass chain from the rod to an electrical measuring device in his laboratory on the first floor.

As luck—or fate—would have it, during a meeting of the Academy of Sciences in August 1753, a thunderstorm developed. Richmann rushed home, bringing along the artist who was going to illustrate Richmann’s new book. While Richmann was observing his equipment, lightning struck, traveled down the rod and chain, jumped about a foot to Rich-mann’s head, electrocuted him and threw him across the room, while also striking the artist unconscious. You can see several illustrations of the scene online, though it’s not clear whether they were the creations of the artist in question.

Franklin was to invent a similar contraption, but this one was grounded; we know it today as the lightning rod. It works well to ground lightning strikes, but not for the reason Franklin surmised. He thought that a lightning rod would induce a continuous discharge between a charged cloud and a building, thus keeping the potential difference low and eliminating the danger of lightning. So confident was he in his idea that he advised King George II to put these sharp points on the royal palace and on ammunition storage depots. Franklin’s opponents argued that the lightning rod would only attract lightning, and that the effect of the discharge, lowering the electric potential difference between a building and the thunderclouds, would be insignificant. The king, so the story goes, trusted Franklin and installed the lightning rods.

Not long thereafter a lightning bolt hit one of the ammunition depots, and there was very little damage. So the rod worked, but for completely the wrong reasons. Franklin’s critics were right: lightning rods do attract lightning, and the discharge of the rod is indeed insignificant compared to the enormous charge on the thundercloud. But the rod really works because, if it is thick enough to handle 10,000 to 100,000 amperes, then the current will stay confined to the rod, and the charge will be transferred to the earth. Franklin was not only brilliant—he was also lucky!

Isn’t it remarkable how by understanding the little crackle when we take off a sweater in winter, we can also come to some kind of understanding
of the massive lightning storms that can light up the entire night sky, as well as the origin of one of the loudest, most terrifying sounds in all of nature?

In some ways we’re still latter-day versions of Benjamin Franklin, trying to figure out things beyond our understanding. In the late 1980s scientists first photographed forms of lightning that occur way, way above the clouds. One kind is called red sprites and consists of reddish orange electrical discharges, 50 to 90 kilometers above the earth. And there are blue jets as well, much larger, sometimes as much as 70 kilometers long, shooting into the upper atmosphere. Since we’ve only known about them for a little more than twenty years, there is an awful lot we don’t yet know about what causes these remarkable phenomena. Even with all we know about electricity, there are genuine mysteries on top of every thunderstorm, about 45,000 times a day.

CHAPTER 8

The Mysteries of Magnetism

F
or most of us magnets are just fun, partly because they exert forces that we can feel and play with, and at the same time those forces are completely invisible. When we bring two magnets close together, they will either attract or repel each other, much as electrically charged objects do. Most of us have a sense that magnetism is deeply connected to electricity—nearly everyone interested in science knows the word
electromagnetic
, for instance—but by the same token we can’t exactly explain why or how they’re related. It’s a huge subject, and I spend an entire introductory course on it, so we’re just going to scratch the surface here. Even so, the physics of magnetism can lead us pretty quickly to some eye-popping effects and profound understandings.

Wonders of Magnetic Fields

If you take a magnet and put it in front of an older, pre-flat-screen television when it’s turned on, you’ll see some very cool patterns and colors across the screen. In the days before liquid crystal display (LCD) or plasma flat screens, beams of electrons shooting from the back of the TV
toward the screen activated the colors, effectively painting the image on the screen. When you take a strong magnet to one of these screens, as I do in class, it will make almost psychedelic patterns. These are so compelling that even four-and five-year-olds love them. (You can easily find images of these patterns online.)

In fact, children seem to discover this on their own all the time. Anxious parents are all over the web, pleading for help in restoring their TVs after their children have run refrigerator magnets across the screens. Fortunately, most TVs come with a degaussing device that demagnetizes screens, and usually the problem goes away after a few days or a few weeks. But if it doesn’t, you’ll need a technician to fix the problem. So I don’t recommend you put a magnet near your home TV screen (or computer monitor), unless it’s an ancient TV or monitor that you don’t care about. Then you might have some fun. The world-famous Korean artist Nam June Paik has created many works of art with video distortion in roughly the same way. In my class I turn on the TV and pick out a particularly awful program—commercials are great for this demonstration—and everyone loves the way the magnet completely distorts the picture.

Just as with electricity, magnetism’s history goes back to ancient times. More than two thousand years ago the Greeks, the Indians, and the Chinese apparently all knew that particular rocks—which became known as lodestones—attracted small pieces of iron (just as the Greeks had found that rubbed amber would collect bits of leaves). Nowadays we call that substance magnetite, a naturally occurring magnetic mineral, in fact the most magnetic naturally occurring material on Earth. Magnetite is a combination of iron and oxygen (Fe
₃
O
₄
) and so is known as an iron oxide.