CHAPTER 5

Violent weather is frequently accompanied by thunder and lightning. Everyone knows that a bolt of lightning can do considerable harm. Consequently we all take precautions during a thunder and lightning storm to avoid exposing ourselves to the lightning. Lightning has both frightened and fascinated people for many millennia.

What causes lightning? Why does it strike only during bad weather? What is the source of the "lightning flash"? What is the source of the sound we call "thunder"? When out in a thunder storm, how can we protect ourselves? If your automobile were struck by lightning, what should you do? If you are hiking on a high mountain trail and weather appears threatening, what should you do?

Historical Perspective

In 1746 Benjamin Franklin conducted some important and very dangerous scientific experiments with lightning. He was the first to convincingly demonstrate that lightning was associated with electrical phenomena, and because he published the results and conclusions of his experiments, his results became widely known. Franklin’s experiment with lightning was actually quite clever.

The question he posed was: What causes lightning? Franklin reasoned that if lightning were electrical, and he could think of some kind of experiment to do, then the clouds during a thunder storm should produce some kind of electrical phenomenon that he would recognize. He may have thought for a long time about predicting an "electrical effect" that lightning could cause. Possibly, an idea for his experiment simply occurred to him one day "out of the blue", as so often happens when people create new ideas. Somehow Franklin came up with the prediction that the electrical effect of lightning might be transferable to another object and cause an effect that could be recognized as an electrical phenomenon. No doubt Franklin had experienced the same "electric shock" that most of us have felt, when reaching for a metal door knob after walking across a carpeted room.

Franklin eventually published two separate versions of an experiment to test his hypothesis that lightning is an electrical phenomenon associated with thunder clouds. He thought that one version of his experiment was safer than the other. In fact both versions of Franklin’s experiment were quite dangerous, and in retrospect foolhardy. The first version involved mounting an iron rod 20-30 ft. high on top of a church steeple, so that the sharply pointed end of the rod protruded upward toward the clouds overhead. The experimenter was then supposed to touch both the iron rod and the ground during a thunder and lightning storm. Franklin’s prediction was that the cloud would discharge a bolt of lightning that would travel through the air, the iron rod, the church steeple and the experimenter into the ground. In the more cautious, second version of Franklin’s experiment, he described holding a wire insulated by a wax coating so that the wire--not the experimenter’s body-- served as the conduit for the electrical discharge from the rod to the ground.

A Swedish scientist attempted to perform Franklin’s experiment using an iron rod atop a church steeple in 1753, but suffered the bad luck of having lightning directly strike the iron rod while he was still setting up the experiment! Unfortunately the scientist became part of the experiment, and was killed instantly.

There is no record that Franklin himself actually performed either version of this experiment. Instead Franklin tested his hypothesis that lightning is an electrical phenomenon in another experiment using a kite suspended from a long, thin metal wire that he flew during a thunder storm. Franklin reasoned that the kite could probe the higher clouds, and be the conduit for electricity from the cloud to the kite, and then down the wire to an iron key attached near his hand at the end of the kite wire. He predicted that he would see a spark jump from the key to his hand if his hypothesis were correct. The effect Franklin predicted is much like the spark you experience when you walk across a carpet on a dry day and reach for a door handle. However, unlike the small discharge you experience just before you touch the door handle, the electric discharge in Franklin’s kite experiment could have been lethal.

Franklin was careful to hold the key with a cotton string, and not to touch the key directly. After flying the kite high in sky for a while, as he extended the kite at the end of the wire toward thunder clouds, Franklin moved his free hand toward the iron key. He noticed a spark jump from the key to his knuckle, which immediately confirmed his hypothesis that thunder clouds which produce lightning are associated with electrical phenomena. Franklin was lucky to have survived this famous experiment with the kite, because several other experimenters performed this same kite experiment and were electrocuted.

Others survived their kite experiments, as judged by several reports of successes confirming Franklin’s results during the mid-eighteenth century. All of the experiments were consistent with Franklin’s hypothesis that lightning is an electrical phenomenon, and that clouds contain can store electrical energy.

Personal Lab 6: What Causes the Tape to Move?

Fundamental Concepts and Applications

1) Two Kinds of Electric Charge in Nature

You may have discovered during your experiments that "prepared" or charged objects, such as the glass rod after being rubbed with the silk cloth, cause opposite reactions (i.e. repulsion or attraction) in the two oppositely charged tape strips. Also your experiments should have lead you to the conclusion that there are only two kinds of electric charges. Be sure that you can justify this conclusion with your experimental data. Benjamin Franklin was the first to realize that there are only two kinds of charges in nature, and called the two types, positive and negative electric charges. The tape strips or the rods, Styrofoam plates, etc. take on net positive or a net negative electric charge when rubbed or ripped rapidly from a surface because there are more electrons (negatively charged particles) than at the surface of these objects. When the Magic tape is ripped from a smooth surface, depending on the nature of the surface, electrons or positively charged molecules are physically lifted from the surface, so that the tape takes on an excess of positive or negative charges. Therefore the term "charge" refers to the excess type of electric charge an object has. We call this a static electric charge. The source of the net positive charge is found in positively charged atoms or molecules (those with missing electrons) The number of protons(+) in the nucleus exceed the number of electrons (-) in the atom or molecule, making a net positive molecular ion. For the net negative charge on a tape strip, negatively charged electrons or molecules (molecules with an excess of electrons over protons, or negative ions) dominate.

2) What Causes Lightning?

All objects seek to be in a neutral electrically charged state (equal numbers of positive and negative charges). The reason we see the "spark" and feel the "shock" when we touch a door knob after we shuffle our leather-soled shoes across a carpet is that the human body serves as a very efficient conduit for electric charges to the ground (as scientists doing Benjamin Franklin’s experiments sadly discovered). In fact Nature’s propensity to equalize the numbers of negative and positive charges is sufficiently strong that the excess (negative or positive) charges jump to (or from) your hand-- using air molecules as "stepping stones"--across the gap between your hand and the metal door knob. These excess charges literally ride the air molecules toward (or away) from our bodies which do the rest of the work in passing the charges (and their effects) to the ground thereby removing the excess charges between you and the door knob. In a darkened room, if the static charge on an object is sufficiently large, you can actually see a spark in the air gap between your hand and, say, a charged Styrofoam plate, as the plate discharges through your body to the ground. The spark is actually a trail of air molecules that become momentarily ionized by the electric charges as they are transferred across the air gap to your body.

Can you think of a way to explain lightning based on this model? What objects in the Earth’s atmosphere serve to store the electric charge? What eventually causes the discharge we see as lightning? What serves as the conduit to the ground? How do you think a lighting rod works?

3) Induced Polarization

What causes the paper bits to rise toward the comb?


Your experiments also included tests with neutrally charged objects such as bits of paper responding to a charged object such as a comb, or your finger (neutrally charged) causing the tapes on the electroscope to react.

From your experiments, did you find any patterns in the interactions between charged and neutrally charged objects, such as a finger tip and one of the charged Magic tape strips? Presumably you noticed that your finger always attracted both of the oppositely charged tape strips. A general statement to describe this interaction is: An object without a net electric charge (i.e. neutrally charged) is always attracted to any charged object.

What causes the water to move toward the comb?

Charge a plastic comb by running it through your hair 20-30 times. Turn on the faucet in your kitchen sink so that it produces a thin, steady stream of water. Slowly move the comb toward the stream of water. What do you observe? Which object, the water or the comb, had the electric charge, and which was neutrally charged? What do you think happens to the water molecules when the negatively charged comb is moved toward the water?

What causes neutral objects to always be attracted to charged objects?

Rub the bottom of a Styrofoam plate (8 inch diameter) with a wool cloth for 2-3 minutes. Slowly move the charged Styrofoam plate in a horizontal direction toward a 2 x 4 inch board (roughly six feet or so in length) which has been carefully balanced on a "frictionless" pivot (such as a watch glass) at the board’s "balance point". Hold the charged Styrofoam plate about an inch from the end of the board, and be sure not to make contact between the board and the plate. You may have to wait a few seconds for the board to respond. Have several students hold charged combs near the 2 x 4" board and observe what happens.

What do you observe?

What do you think caused the motion you observed?

The model to explain the results of this experiment involves a very interesting aspect of the electric interaction. The board--ordinary wood-- like most objects has no net electric charge (we call it neutrally charged). Another example of a neutrally charged object is a finger tip moved toward the charged tape strips. You should have noticed by now that the same electric effect is always observed for neutral objects when brought close to a charged object (like the charged balloon adhering to the neutrally charged wall).

A conceptual model for neutral objects always attracting charged objects is based on the fact that atoms and electrons are fairly free to move around even in solid objects such as your finger or the 2 x 4" board. We of course cannot see the atoms moving, but we can hypothesize that charges in the neutral object (the board or the finger) rearrange themselves in response to the charged object (negative charges in the electroscope pull positive charges in your finger closer to the skin surface). We could further extend our model by hypothesizing that after rearrangement of the charges in your finger tip, the negatively charged electroscope tape responds by being attracted toward your finger, the surface of which has a net positive charge. This effect of neutrally charged objects interacting with charged objects is called induced polarization.

Use the same model above to explain why the 2 x 4" board is always attracted to the charged Styrofoam plate? Describe a model to explain why the stream of water is always attracted to the electrically charged comb.

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