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Science
by Pat Murphy & Paul Doherty

ASKING QUESTIONS

 

For the past twenty years, this column has been a team effort. We collaborated well because I (Pat Murphy) am very good at asking questions, and Paul Doherty was very good at answering them.

This is the last column we will collaborate on. Paul passed away in August of 2017, following a battle with cancer.

In this last collaboration, I'd like to give our faithful readers a taste of what it was like to discuss science with Paul Doherty. Long before his illness, I conducted an interview with Paul on the subject of electricity.

I had taken physics in college and had studied electromagnetism. The way my professors explained electricity annoyed me. They offered analogies that didn't help at all and they never seemed to dig down to the nitty gritty questions about what was really going on. So I decided to pepper Paul with questions until I was quite satisfied. This column is an excerpt from that conversation.

Since our very first column, Paul and I have taken care to link each discussion of science to a work of science fiction. In the case of electricity, that link is easy. Back in 1818, Mary Shelley published Frankenstein, a novel that many celebrate as the first science fiction novel. Shelley's work was inspired in part by Luigi Galvani's discovery that the leg of a frog would jerk when hit with an electric spark. Back then, people were amazed and intrigued by electricity and the effects. These days, we all take electricity for granted. But if you think about it, electricity is truly weird and fantastic. It runs in wires in your house and makes the lights come on and makes motors run. It makes all your devices do whatever it is they do. But what exactly is going on in those wires? That's what I wanted to know.

 

*   *   *

 

PAT: The first question is a tough one. So Paul, what is electricity?

PAUL: Arrrrrgh! (laughter)

PAT: You can give me a simple answer…

PAUL: Yes!

PAT: …and then we'll expand on it and worry it to death.

PAUL: Okay. Electricity is the name we give to motions of and forces between things called charges. Just like there's mass, and mass is pulled by gravity, there's another thing entirely separate from mass called "electric charge." And just like all masses attract, the way the Earth attracts the Moon, all electric charges exert forces on each other. These forces and the resulting motions are called "electricity." With a little simplification, there it is.

PAT: That doesn't help much. Let me try a different question. You've got a light bulb and you turn on a switch. Something goes through the wires to make the light turn on. What's going through the wire?

PAUL: Electrons are going through the wire. The world is made of atoms. The atoms have protons, which we say are positively charged—whatever that means—and neutrons, which have zero charge. That's the nucleus of the atom. Around the nucleus there are negatively charged things called electrons. In some very special materials—metals, for example—the electrons can be shared by the atoms. In other words, an electron can move from one atom to the next atom. We call those conduction electrons.

The electrical wires in your house or your car are made of metal, probably copper. These copper atoms share electrons with each other. So if I push on the electrons somehow, they move through the wire.

PAT: Okay, now we are getting somewhere. So how do you push on an electron?

PAUL: With an electric force.

PAT: Fine. So how do you make an electric force?

PAUL: Most people have heard that like charges repel each other and unlike charges attract each other. The force of attraction or repulsion is the electric force. So to make an electric force, you actually bring some electrons over and pile 'em up on the end of the wire. They'll repel each other and all the electrons in the wire. They'll attract all the positive nuclei in the wire. But the positive charges in the nucleus are stuck; they don't move because the positive charges of the nuclei and their attached electrons are too big. But the conduction electrons are free to move, so they'll move away from the end of the wire where you've piled up electrons. Those conduction electrons get pushed away by the pile of charges and move slowly through the wire.

PAT: Okay. So current is the movement of electrons through the wire. Suppose you have a light bulb at the end of a six-foot electrical cord. How long does it take the electrons at the socket end of the wire to get from there to the light bulb?

PAUL: Let me change the question just a little. Let's look at the headlights of your car. The car has a twelve-volt battery. Chemical reactions in the twelve-volt battery essentially make a pile of electrons at the negative terminal of the battery. At the positive terminal of the battery, these reactions pull away the electrons, leaving positives behind. A wire runs from the negative terminal of the battery through your headlights, through a switch, back to the positive terminal of the battery, making a circuit. When the switch is open, the circuit is open and the lights are off. When you close the switch, the electrons in the wire start moving, and the light comes on.

An electron from your car battery will take several hours to actually reach the light bulb and several hours more to get back to the battery. So this is maybe a six-hour tour.

PAT: So it take hours for an electron to reach the bulb. But the light comes on right away. What gives?

PAUL: The thing is—the light bulb doesn't start glowing when the electron from the battery gets to the filament. The light glows when the electrons that are in the filament start to move. And that's something quite different. Electrons in the filament start to move only a few billionths of a second after you close the switch.

Here's why.

When the switch was open, the pile of negatives at the battery pushed the electrons away, all the way down the wire until they came to that open switch. The electrons piled up along the wire and at one side of the switch.

The battery pulled electrons away from the wire on the other side of the switch. So there was a pile of negative charges on one side of the switch and a pile of positives on the other side.

As soon as that switch was closed, the negatives on one side of the switch could move over to where the positives were, and then the negatives behind them could move over, and the negatives behind them. It's like a row of dominoes falling over.

Actually, there is a better analogy than that. Suppose you take twenty dominos and put 'em down on a table, all lying flat and all lined up with their long axes in the same direction. Separate the dominos by about a sixteenth of an inch. Now, take the domino at one end of the line and start moving it along the table at about a centimeter a second, in very slow motion. Run it into the line of dominos and watch what happens. The gaps between the dominos will close up rapidly—much faster than you're moving your domino.

The gap closings move maybe twenty times faster than the dominos do. The gap is only a twentieth of the length of a domino, so the closing up of the gaps will go zzzip. That's what happens in the wire. The electrons are drifting along at a millimeter a second, but when one electron starts to move, its electric push—we call it the electric field—reaches out and pushes on its neighboring electron, so its neighbor starts to move very shortly after the first electron started to move.

PAT: So the movement moves quickly.

PAUL: Yes, it moves very quickly. In a wire, it moves about one foot per nanosecond, or the length of this page in a billionth of a second, the speed of light.

PAT: That domino analogy works, but I have another one. I think the electrons are like people standing in a line at a ticket window. Even though the person at the back of the line hasn't reached the window yet, that doesn't mean nothing's happening up at the ticket window. As soon as that ticket window opens, people start moving past the window. There's a "current" flowing past the window.

PAUL: That's a good analogy. Electrons are like pushy people.

PAT: So how do you measure an electric current? If I wanted to measure how fast a line was moving, I'd count people served per minute.

PAUL: To measure a current, you count the number of electrical charges passing you in a given time—say for one second. But there's a little bit of trickiness here. You can't actually see the charges moving in a wire. You have to deduce their motion from experiments. An additional bit of difficulty comes about because positive charges moving by to the right have exactly the same effect as negatives moving by to the left.

PAT: Now hold on there! You told me earlier that positives don't actually move.

PAUL: In a wire, only the electrons move. In the rest of the universe—in gases and liquids, positives move and negatives move at the same time, and therein lies the problem with electrical current. If only positives were moving, then electrical current would be really easy; you'd just say, "Okay, how many positives move by this point per unit time?"

PAT: You did it again! We are talking about measuring electricity in a wire—where only the negatively charged electrons move. But you switched from talking about the negatives moving to talking about the positives moving. Why?

PAUL: It's easier this way.

PAT: That doesn't seem like a good enough reason.

PAUL: It's a lot easier. People don't think well in terms of negatives moving.

PAT: Isn't there an historical reason? I thought people started talking about current in terms of positive flow, and then found out later that it was the electrons that were actually flowing. But at that point it was too late to change.

PAUL: That's part of it. Before Ben Franklin, experimenters thought there were two kinds of electricity: vitreous, which you got by rubbing glass with cloth, and resinous, which you got by rubbing resin. Ben Franklin then created a model in which there was only one type of electricity. It could move from regions of excess, which he called plus, to other regions which were depleted, which he called minus. Ben called vitreous electricity plus.

It was an arbitrary choice. He could have called resinous electricity plus and made life simpler for us, because resinous electricity turned out to result from an excess of electrons. But he called the vitreous electricity plus. So electrons became minus, negatively charged. But it's electrons, the negatively charged stuff, that actually flows in wires. And that's the trouble. In the end, it's just human naming.

PAT: So I can blame Ben Franklin and his arbitrary names. Maybe he should have just called them "green" and "red."

PAUL: But he had a really good reason to call electric charges plus and minus. When he added an equal number of pluses to an equal number of minuses, he got nothing. [f you add an equal amount of green to an equal amount of red, you get yellow, which isn't nothing! So he used plus and minus. (As an aside, your idea of using colors isn't that far off. Quarks have an entirely new type of charge in addition to mass and electric charge. There are three different types of this new charge, so physicists named them red, green, and blue. They use the analogy of color addition: red, green, and blue add up to white.)

PAT: Let's get back to electrical current. You said that you had an electrical current if charges were moving past a certain point. Now, suppose you charged a comb up with static electricity by running it through your hair, and then you walked across the room carrying the comb. Would that be a electric current?

PAUL: Yes. But a very tiny one. The electric current is the number of charges that pass in a given time. That comb might have a billionth of a coulomb of charges on it. A coulomb is a unit of measure. It's just a number, like a dozen or a gross, except a coulomb happens to be about 6 x 1018.

PAT: That's a lot of billions.

PAUL: That's six billion billions. A comb might acquire a charge of a billionth of a coulomb, which means a mere six billion electrical charges. Now, scientists measure electrical current in amperes or amps. An ampere is one coulomb per second. So when you walk across the room with your charged comb, you have six billion electrons moving at the speed you're walking. That's still almost no electric current.

PAT: But it really is an electric current?

PAUL: Sure.

PAT: Okay, so I see that you don't need a wire for an electrical current.

PAUL: There are lots of places that currents flow without wires. In an inkjet printer, for instance, the inkjet is charged. They put charges on the balls of ink and then shoot 'em out and bend 'em to the side, using electric and magnetic forces. And so that flow of charged particles from the inkjet to the paper is an electrical current. It's not the kind of current we usually think of, but it's a current.

Most electric currents in the universe don't flow in wires. Electric currents are generated when the magnetic field of the Earth sweeps through the electric charges sprayed into space by the Sun. These currents produce the aurora borealis and the aurora australis.

In any case, for most effects, negative charges moving to the left produce the exact same effects as positive charges moving to the right. You could say that a current is ten negatives going to the left per second and five positives going to the right per second, but you'd have to keep track of exactly what's moving in each direction, which is very hard to do. So scientists keep track of the overall thing. Ten to the left of negatives and five to the right of positives is the same as fifteen positives moving to the right.

PAT: That's where it gets confusing.

PAUL: That's the mythical "positive electrical current" that engineers use in describing electrical current. They treat electrical current as if it goes from the positive terminal of the battery through the circuit, and then comes back to the negative terminal.

PAT: Which makes sense, intuitively.

PAUL: Oh, it's a very nice model. It gets rid of a negative sign so you have less chance of making an error in calculations.

In the sense that the positives don't actually move in wires, it's wrong, but it gives you the right answer. In all real calculations in electrical circuits, the negatives that flow through the solid wire from the negative terminal of the battery to the positive side are ignored. That's the true flow, and a mythical positive flow called "the electrical current" is made up that has exactly the same effects.

PAT: So the words "electric current" refer to the flow of positive…stuff.

PAUL: Mythical flow of positive stuff.

PAT: Right. And the positive stuff isn't really moving, but the flow of negative stuff amounts to the same thing.

PAUL: In a wire.

PAT: Damn that wire! Okay. In a wire.

PAUL: Here's a model I give my students. I have four students stand in a row; they're atoms of copper. I give each student a piece of paper representing a dollar and a piece of paper representing a debt: they owe me a dollar. So I've given them nothing. I gave them plus one dollar and minus one dollar, so they each are, as far as money is concerned, neutral. Zero. I say, "Let's assume that only pluses can move." I have the person on the end next to me pass their dollar bill to their right. So their dollar moves to the next person. Then I have that person pass their bill to the next person, and so on. The person next to me ends up with a debt, negative money. The person next to them has a dollar and a debt.

PAT: Still neutral.

PAUL: Still neutral. The person next to them has a dollar and a debt, still neutral. The person at the end of the line ends up with two dollars and a debt. A net positive. That's the flow of positive money.

PAT: I get that.

PAUL: Then I start all over. I start back again neutral, but now I let only the negatives move. Only debt can flow. I have the person nearest me receive a debt from the person next to him—the debt flows to the left. I have the debts move all the way through the line. In the end, the person on the left has two negatives and one positive, so he ends up with a debt, just as he did before. At the other end of the line, the last person has one positive and no negatives. So the result is absolutely the same if positives move to the right and debt moves to the left.

PAT: So if you wanted to put it in accounting terms, engineers only look at the bottom line. And physicists look at all the intricacies along the way.

You could make a lot of money and spend a lot of money and end up with ten dollars. Or you could make ten dollars and spend no money and still have ten dollars. It's the same result.

PAUL: Right.

PAT: Okay. I understand the accounting system and why you go along with the myth of the movement of positives.

Let me ask a new question. We've talked about wanting to "pile charges up" over here or over there. But how do you go about piling charges up? I mean, how do you generate electricity?

PAUL: You can generate electricity with chemical reactions in a battery. Or you can create a generator. If you take a straight piece of wire and move it down past the north pole of a magnet, keeping it perpendicular to the magnet, the magnet will exert a force on the charges in the wire. As we've said before, the positives in the wire are locked in place, but the negatives can move. So when you move the wire down past the magnet, the magnetic forces will push the mobile negative charges to one end of the wire, making a pile of them, and leaving positive charges at the other end. Now, if you make a loop of wire, then the push that's given to these electrons as they go by the magnet will push electrons next to them all the way around the loop and make an electrical current in the wire. And that's all that a generator is.

PAT: Okay. Now, here's a tough question. Why does a magnetic field push on these electrons?

PAUL: Magnetic fields and electric fields are closely related. In the old days, electricity and magnetism seemed to be different things with some similar properties. But Einstein, with his theory of relativity, showed that magnetism is nothing but electricity viewed from a moving frame of reference. If you look at charges moving past you, you see the electric field of the charge, plus a magnetic field. And that magnetic field that you see is just a relativistic transformation of the electric field because it's moving past you. If you move along with the charge, its magnetic field disappears. This was the first great unification of physics. Now we talk about electromagnetism, because both electricity and magnetism are really the same thing. Magnetism is nothing but electricity, electric fields, viewed in a moving frame.

PAT: Now wait. You're saying magnetism is nothing but electric fields viewed in a moving frame. It seems to me the magnet is sitting still and the electrons are running around.

PAUL: But think about the electrons in an iron magnet. The electrons are spinning as they move within the iron atoms. The electric fields of those electrons are turned into a magnetic field that we see.

PAT: But electrons are spinning in my body.

PAUL: Sure.

PAT: Electrons are spinning in this rug.

PAUL: Yep.

PAT: But it doesn't have a magnetic field.

PAUL: Oh, every electron has a magnetic field. But each electron in your body and each electron in the rug is spinning in a different direction. When you add up all these different magnetic fields, with arrows in all directions, they all add up to zero and you have no magnetism.

The neat thing about iron is that one iron atom which has a magnetic field winds up parallel to its neighbors, which line up parallel to their neighbors, over a region of space called the magnetic domain. They all line each other up. Now, most iron isn't magnetic. That's because the iron is full of magnetic domains that are all pointed in different directions. In a magnet, a lot of these iron atoms are lined up so they all have the same direction of magnetism: All their north poles are pointing in the same direction.

You can also use a strong magnet to turn an iron nail into a magnet. Just let the flat end of the nail snap onto the magnet The impact and the vibration and the strong magnetic field will cause the nail to become magnetized. Pull the nail straight away from the magnet. Now the nail will pick up a metal paper clip. If you drop the nail or throw it gently onto a cement floor, the vibration will demagnetize it.

PAT: So it's really a physical lining up of all these fields.

PAUL: It's a physical lining up of the individual magnets, which are the atoms.

PAT: Here's one last question—and it's really the reason I wanted to do this interview. Of all the topics that we cover here at the Exploratorium, electricity is the one that seems to make people shriek and tear their hair out the most. Electricity is clearly an area that a lot of people have an aversion to. Do you have any idea why?

PAUL: Maybe because you can't see electricity. You can't see electrical current flowing. You can see the effects. But the scientists and the engineers have their models of moving charges, and those are abstractions. I think the other problem is that a lot of people out there who are trying to explain electricity don't understand it themselves.

PAT: I know that one of the reasons I used to get very peeved with electricity—and with attempts to explain it to me—was that there seemed to be a lot of lying going on.

PAUL: A lot of lying?

PAT: People would tell me one thing, like: "Positive charges flow through the wire." And then I'd find out that it was really the negative charges that were moving and that they're not really flowing through the wire, they're sort of nudging each other along.

All along the way, as I've learned about electricity, it's been a process of learning one way and then having to unlearn that and learn a new way and unlearn that way and learn a new way. So I ended up distrusting the whole thing.

PAUL: But all science is like that. Any model we have for the universe—whether it's a model for light, or a model for electricity, or whatever—the model is not the thing. At some point, any model breaks down. The trick about science is picking the simplest model that gives the right answer. But of course, that's a very difficult thing to do. You have to really understand all the models and when they fail in order to pick the right one to use to answer any question.

 

*   *   *

 

That was always Paul's approach to answers: Always begin with the simplest possible model that answers a question or explains an experiment. And always remember (and remind your questioner) that every answer contains an asterisk,* an invisible footnote that says *"But it's more complicated than that."

I am sure that some of our faithful readers will miss Paul's insights about science. I miss Paul—and I know I'm going to keep missing him every time I think of something I'd like to share with him, something I'd like to ask him about, something it would be so fun to write about in collaboration.

I don't know what happens to us after our bodies pass away. But I do believe that part of us stays here on Earth in the form of all the people we have taught and touched and cared about.

In that sense, Paul is still here. When I think about science, I know that my thinking has been shaped by Paul's thinking. When I ask questions that lead to other questions, I know Paul is right there with me. When I tackle a new project, I know my enthusiasm and openness was influenced by his example. When I work to communicate something tricky and complex, I will always think about Paul's invisible footnote that says *"It's more complicated than that."
 

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