Saturday, February 7, 2015

What you need to know about electricity and magnetism

Most of us give little thought to the act of flipping on a light switch, much less the enormous infrastructure that must go into the vast process of bringing electricity to nearly every corner of our beautiful modern world. And what about the remarkable scientific advancements that brought us from candle light to electrification over the past 100 years? How did that happen? It’s a small dot in the history of humankind, but a time of unparalleled change that revolutionized how we live, think, communicate, treat disease, and interact with the world. Electromagnetism is probably the single-most important scientific advancement in the history of humankind, and we’ll try to understand a bit about it here, at least enough to understand why electricity and magnetism are central to our ability to end our addiction to fossil fuels.

Electric cars, the solution to polluted cities, in the 1830’s!

Walking through London in the 1830’s, you would have had to use a torch to see where you were going — in the middle of the day. The pollution was so bad that darkness descended on industrial cities, at certain times of the year, as it often does today in some parts of China. But at the time, people were thrilled about a new exciting discovery — the electric motor. Trains, cars and even boats would soon be powered by clean electricity rather than coal. It never happened, of course, because people soon realized that batteries didn’t last forever and were tremendously difficult to keep replacing. Despite this spectacular flop, I’m here to tell you that — this time — it’s going to be different!

Why don’t your hands pass through each other?

Rub your hands and you can feel the texture of your skin, the soft muscle tissue wrapped over a hard skeleton. The nuclei of the atoms in your hand contain most of the mass — the heavy part — but they are spaced a surprisingly long distance apart. Consider this: if the nucleus was as big as you are, the next nucleus would be a mile away! There’s a tremendous gap of wide open space between every nucleus in your hand, occupied by essentially nothing, except for the occasional electron. The standard picture of atoms packed tightly together, with electrons spinning around the nucleus, is totally wrong. The real picture is one of mostly empty space. Why then can’t one object pass right through the other? Why doesn’t light pass right through the desk in front of you?

The answer is: the electrons. Electrons float around in the empty space, sometimes tied to a nucleus, sometimes allowed to roam free. It is the repulsive force between the electrons in your hand, and the electrons in whatever you touch, that gives the sensation of “touch.” You are physically feeling electric repulsion — the collision between electrons that don’t want to mix. You are physically seeing the collisions between light and the electrons in the table. These electric forces are strong, they dominate how we interact with the world, and they can generate spectacular effects like the Aurora and lightning. You might wonder why electrons don’t want to mix. Unfortunately, I can’t answer that one. No scientist can — we just don’t know why.

Turning on the light

Electrons move extremely slowly. Getting through a wire is tough work. There’s a lot of bumping into the lattice of the nuclei. Electrons move at about one meter per hour — unbelievably slow. How is it, then, that you can Skype across the world in real time? How is it that you can flip the light switch and immediately the lights flick on?

Think of a long high speed train packed with people. It’s so tight that no one can move. Now try to push someone in at one end of the back car. The only way to let that person in is if someone else gets off at the other end of the train. The only movement that must occur is for everyone to shift down a bit. The actual movement of the people is slow, yet the response of everyone shifting down a bit can propagate along the train at a much higher speed. Pushing electrons down a wire is how we move energy, almost instantaneously, through wires. Plug your cell phone into the wall, and you can’t start charging until someone starts shoving electrons into the wires at a power plant miles away. It’s amazing.

Magnets are magical

You marveled as a little kid at the amazing forces of attraction and repulsion between two magnets, depending on their orientation. It’s a magical force, invisible yet so strong. For all of the 19th century, while the technology of electricity and magnetism was developing, scientists had no idea what was actually going on. It turns out that magnetism is, in part, also a property of electrons. Unlike electric repulsion, however, magnet repulsion has an orientation. The direction in which the electron is pointing determines if the magnetic force is attractive or repulsive. Amazingly, all the electrons in magnetic materials like iron are naturally aligned to point in the same direction. You can think of it as a conspiracy. What is their ultimately goal? No one knows, but I think they are trying to tell us to solve the climate crisis by using the powers of electromagnetism.

Turning wires into magnets

There’s another way to obtain a magnet other than pointing all of the electrons in the same direction. It turns out that running electrons through a wire can also produce a magnet. As the electrons move through the wire, from their perspective, the nuclei in the wire actually contract a little bit, leaving an electric imbalance between the electrons and the nuclei. This contraction is due to the very not-obvious fact that everything that moves actually get squished a tiny amount. You might have heard of Einstein. He figured this out.

The strange thing is that this electric imbalance depends on the direction in which the electrons are moving, similar to how the magnetic force depends on the direction the electrons are pointing. In fact, the direction of the force is the same for these two cases, and everything else about the two forces happens to be the same, so we go ahead and call them both magnetic forces. It’s wild, but you can create a magnet by moving electrons through a wire.

Using magnets to create electricity

I realize that things are starting to get pretty weird, but it gets even worse. Not only can you create a magnet by moving electrons through a wire, but you can also create electricity by moving a magnet nearby a wire.

Take a wire, and take a magnet. Move the magnet. The electrons in the wire will get pushed through the wire by the magnetic force. Essentially, the electrons are responding to the motion of the magnet by turning themselves into a magnet. You can sit there all day and wiggle the magnet back and forth and you’ll get current wiggling back and forth. And that is basically how electricity is generated.

Using electricity to move magnets around

We can also do this in the opposite direction. Take a wire. Run electrons through it. The wire will produce a magnetic force. Place a magnet nearby. The wire-magnet will feel an attraction or repulsion from the magnetic force of the magnet, just like a normal magnet would.

What’s amazingly cool is that you can wrap the wire around a shaft and, together with the nearby magnet and electricity, cause the shaft to turn! And that is basically how an electric motor works.