?? That is, only one thing apart from making your toast the size of a matchbox or serving breakfast from a very low coffee table.
?? The nice thing about tossing a coin is that it shows that the overall movement of the object and its spin can be independent. The coin would move in the same arc whether or not it was spinning. But if you flick it in the right way, you give it spin as well as upward speed. The spin and the movement of the center of mass don’t interfere with each other.
§§ The complete gravitational picture is slightly more complicated than this, but the basic idea is correct. Look up Milankovich cycles if you’d like to know more.
?? Although the Earth has been spinning continuously since its formation, it has slowed down ever so slightly because the tug of the Moon provides a very gentle brake. It’s only causing a tiny change, but every one hundred years a day on Earth gets longer by about 1.4 milliseconds. Every few years, a leap second is added to a year to take this into account.
CHAPTER 8
When Opposites Attract
A SELF-TIDYING BAG SOUNDS like a pipe dream. But it might not be as impossible as it sounds. One day last year, I’d popped into London’s Science Museum to buy some lovely spherical magnets (some for a friend and some for me—that’s how it should be with science toys, right?). I stopped off for a hot chocolate and a few minutes’ play with my new toys, and then I tucked the clump of magnets into the sweaters at the top of my travel bag and carried on. Two days later, in Cornwall, I remembered that I hadn’t seen the magnets for a while and dug around to see where they were. When I found them, they were right at the bottom of the bag and the magnet cluster had expanded to include seven coins, two paper clips, and a metal button. I was just congratulating myself on having found a new way of keeping my bag tidy when I noticed that there was plenty more loose change at the bottom of my bag that hadn’t joined in this new game. So I started sorting the coins to see which ones stuck and which ones didn’t. Some 10p pieces did, and some didn’t. Nothing with a value higher than 20p stuck. Most 1p and 2p coins did, but not the ones with dates earlier than 1992.
The thing about magnets is that they’re very selective. They have no attraction at all for most materials—plastics, pottery, water, wood, or living things. But for iron or nickel or cobalt it’s a different story. These will leap toward a magnet if they are free to do so. It’s a weird thought, but if iron weren’t one of the most common materials in our world, we’d probably never come across magnetism in our everyday lives. Just this one element makes up 35 percent of the Earth’s mass, and steel (which is mostly iron with a few other things mixed in) is an essential part of our modern infrastructure. If refrigerator doors weren’t reliably made of steel, fridge magnets would never have happened. But steel is everywhere, so magnetism is common.
The magnets in my bag were sorting the coins according to their composition. Modern 1p and 2p coins have a steel core with a thin layer of copper on the outside. Before 1992, they were 97 percent copper. The old and new pennies look almost identical to me, but the magnets are responding to their hidden innards.* The silvery 20p coin doesn’t stick to magnets because, oddly, it’s mostly copper. So are the older 10p coins, but any minted since 2012 are nickel-plated steel. Everything sticking to the magnet was mostly iron, even the “coppers.”
A magnet is surrounded by a magnetic field, something you might call a “force field.” That means there’s a region around it that can push and pull on other objects, even if the magnet itself isn’t touching them. That’s a bit of a weird idea, but it’s the way the world is. The problem with magnetic fields is that we can’t see them and can’t usually feel them, so they’re hard to imagine. But we do see the effect they have, and that can help our imagination along. And the most important thing about all magnets is that they all have two distinct ends, a north pole and a south pole.
Magnetic north of one magnet will attract magnetic south of another magnet, but two north poles will repel each other. My coins weren’t magnetic to start with, but the magnets played a clever trick to attract them. Inside each one of my new 1p coins, different regions of the iron have magnetic fields pointing in different directions. These regions are called domains, and the magnetic fields of the atoms inside each one are all lined up. Each domain has an overall magnetic field of its own, but because all those domains have their magnetic north pointing in randomly different directions, the whole thing cancels out. As I brought a coin close to one of my magnets, the strong magnetic field from the magnet was busily shoving on all the individual domains in the coin. The atoms didn’t move, but their magnetic field swung around so that the north end was as far from the north of my magnet as possible. That left all the south poles of the coin domains lined up so that they were closest to the magnet. And since opposite magnetic poles attract, the south pole of the coin was attracted to the north pole of the magnet and the coin stuck. And as soon as I took the coin away from the magnet, all its magnetic domains went back to being randomly oriented.
It’s a weird phenomenon, but we humans have learned to make use of it in ways that now permeate our lives. It starts with coins and paper clips and fridge magnets, but ultimately, magnets are essential to the way we generate power for our world. At the heart of every single device that feeds electricity into our power grid, there’s a magnet. However, magnets don’t do it alone, and magnetism is only half of the story. It’s linked in a very fundamental way to electricity, something so vital to modern society that we hardly notice it anymore.
It was the science-fiction writer Arthur C. Clarke who said that “any sufficiently advanced technology is indistinguishable from magic.” Electricity and magnetism together are responsible for more magically advanced technology than almost anything else. When you look really hard at the physics, you can see that these invisible forces are two sides of the same phenomenon: electromagnetism. They are bound together, each influencing the other. But before we look at the connection, let’s dig a little bit deeper into the side that we’re most familiar with: electricity. Unfortunately, the first time most of us experience electricity in a direct way, it hurts.