by Walter Lewin
Just think, without even trying very hard, by brushing your hair or taking off that polyester shirt, you have created, at the ends of your hair, and on the surface of your shirt, electric fields of about 3 million volts per meter. So, if you reach for a doorknob and feel a spark at, say, 3 millimeters, then the potential difference between you and the knob was of the order of 10,000 volts.
That may sound like a lot, but most static electricity isn’t dangerous at all, mainly because even with very high voltage, the current (the number of charges going through you in a given period of time) is tiny. If you don’t mind little jolts, you can experiment with shocks and have some fun—and demonstrate physics at the same time. However, never stick any metal in the electric outlets in your house. That can be very very dangerous—even life threatening!
Try charging yourself up by rubbing your skin with polyester (while wearing rubber-soled shoes or flip-flops, so the charge doesn’t leak to the floor). Turn off the light and then slowly move your finger closer and closer to a metal lamp or doorknob. Before they touch you ought to see a spark jump across the air between the metal and your finger. The more you charge yourself up, the greater the voltage difference you’ll create between you and the doorknob, so the stronger the spark will be, and the louder the noise.
One of my students was charging himself up all the time without meaning to. He reported that he had a polyester bathrobe that he only wore in the wintertime. This turned out to be an unfortunate choice, because every time he took the robe off, he charged himself up and then got a shock when he turned off his bedside lamp. It turns out that human skin is one of the most positive materials in the triboelectric series, and polyester is one of the most negative. This is why it’s best to wear a polyester shirt if you want to see the sparks flying in front of a mirror in a dark room, but not a polyester bathrobe.
To demonstrate in a rather dramatic (and very funny) way how people can get charged, I invite a student who is wearing a polyester jacket to sit on a plastic chair in front of the class (plastic is an excellent insulator). Then, while standing on a glass plate to insulate myself from the floor, I start beating the student with cat fur. Amid loud laughter of the students, the beating goes on for about half a minute. Because of the conservation of charge, the student and I will get oppositely charged, and an electric potential difference will build up between us. I show my class that I have one end of a neon flash tube in my hand. We then turn off the lights in the lecture hall, and in complete darkness I touch the student with the other end of the tube, and there is a light flash (we both feel an electric shock)! The potential difference between the student and me must have been at least 30,000 volts. The current flowing through the neon flash tube and through us discharged both of us. The demonstration is hilarious and very effective.
“Professor Beats Student” on YouTube shows the beating part of my lecture: www.organic-chemistry.com/videos-professor-beats-student-%5BP4XZ-hMHNuc%5D.cfm.
To further explore the mysteries of electric potential I use a wonderful device known as the Van de Graaff generator, which appears to be a simple metal sphere mounted on a cylindrical column. In fact, it’s an ingenious device for producing enormous electric potentials. The one in my classroom generally tops out at about 300,000 volts—but they can go much higher. If you look at the first six lectures on the web in my electricity and magnetism course (8.02), you will see some of the hilarious demonstrations I can do with the Van de Graaff. You’ll see me create electric field breakdown—huge sparks between the large dome of the Van de Graaff and a smaller grounded ball (thus connected with the Earth). You’ll see the power of an invisible electric field to light a fluorescent tube, and you’ll see that when the tube turns perpendicular to the field it turns “off.” You’ll even see that in complete darkness I (briefly) touch one end of the tube, making a circuit with the ground, and the light glows even more strongly. I cry out a little bit, because the shock is actually pretty substantial, even though it’s not in the least bit dangerous. And if you want a real surprise (along with my students), see what happens at the end of lecture 6, as I demonstrate Napoleon’s truly shocking method of testing for swamp gas. The URL is: http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/.
Fortunately, high voltage alone won’t kill or even injure you. What counts is the current that goes through your body. Current is the amount of charge per unit of time, and as said before, we measure it in amperes. It’s current that can really hurt or kill you, especially if it’s continuous. Why is current dangerous? Most simply, because charges moving through your body cause your muscles to contract. At extremely low levels, electric currents make it possible for your muscles to contract, or “fire,” which is vital to getting around in life. But at high levels, it causes your muscles and nerves to contract so much that they twitch uncontrollably, and painfully. At even higher levels, it causes your heart to stop beating.
It is for these reasons that one of the darker sides of the history of electricity and the human body is the use of electricity for torture—since it can cause unbearable pain—and death, of course, in the case of the electric chair. If you’ve seen the movie Slumdog Millionaire, you may remember the horrible torture scenes in the police station, in which the brutish police attach electrodes to the young Jamal, causing his body to twitch wildly.
At lower levels, current can actually be healthy. If you’ve ever had physical therapy for your back or shoulder, you may have had the experience of what the therapists call “electrical stimulation”—stim for short. They put conducting pads connected to an electrical power source on the affected muscle and gradually increase the current. You have the odd sensation of feeling your muscles contract and release without your doing anything at all.
Electricity is also used in more dramatic healing efforts. You’ve all seen the TV shows where someone uses the electric pads, known as defibrillators, to try to regularize the heartbeat of a patient in cardiac distress. At one point in my own heart surgery last year, when I went into cardiac arrest, the doctors used defibrillators to get my heart beating again—and it worked! Without defibrillators, For the Love of Physics would never have seen the light of day.
People disagree about the exact amount of current that’s lethal, for obvious reasons: there’s not too much experimenting with dangerous levels. And there’s a big difference as to whether the current passes through one of your hands, for instance, or whether it goes through your brain or heart. Your hand might just burn. But pretty much everyone agrees that anything more than a tenth of an ampere, even for less than a second, can be fatal if it goes through your heart. Electric chairs used varied amounts, apparently; around 2,000 volts and from 5 to 12 amperes.
Remember when you were told as a kid not to put a fork or knife into a toaster in order to pull a piece of toast out, because you might electrocute yourself? Is that really true? Well, I just looked at the ratings of three appliances in my house: a radio (0.5 amp), my toaster (7 amps), and my espresso machine (7 amps). You can find these on a label on the bottom of most appliances. Some don’t have the amperage, but you can always calculate it by dividing the wattage, the appliance’s power, by the voltage, usually 120 in the United States. Most of the circuit breakers in my home are rated at between 15 and 20 amps. Whether your 120-volt appliances draw 1 or 10 amps is not really what matters. What matters is that you have to stay away from accidentally causing a short circuit and, above all, from accidentally touching with a metal object the 120 volts; if you did this shortly after you had taken a shower, it could kill you. So what does all this information add up to? Just this: when your mother told you not to put a knife into a toaster while it was plugged in, she was right. If you ever want to repair any of your electric appliances, make sure you unplug them first. Never forget that current can be very dangerous.
Divine Sparks
Of course, one of the most dangerous kinds of current is lightning, which is also one of the most rem
arkable of all electrical phenomena. It’s powerful, not completely predictable, much misunderstood, and mysterious, all at once. In mythologies from the Greek to the Mayan, lightning bolts have been either symbols of divine beings or weapons wielded by them. And no wonder. On average, there are about 16 million thunderstorms on Earth every year, more than 43,000 every day, roughly 1,800 every hour of the day, producing about 100 lightning flashes every second, or more than 8 million lightning flashes every day, scattered around our planet.
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 (1012) 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 O2. 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 O2 molecules, making O3—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, elect
rocuted 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!