I love physical analysis of science fiction, and so I would argue that the cartoon Merrie Melodies “Compression Hare“It takes place in the distant future when animals rule the world. I mean, Bugs Bunny and Wile E. Coyote walk on two legs, talk, and make things. How is that not science fiction?
Let me set the scene—and I don’t think we have to worry about spoiler warnings since this episode is 60 years old. The basic idea, of course, is that Wile E. Coyote decided to eat the rabbit. After several failed attempts to catch Bugs, he devised a new plan. First, he will drop a carrot-shaped piece of iron into Bugs’ rabbit hole. After the carrots are consumed (and I don’t know how that will happen), Wile E. Coyote will turn on giant electromagnet and immediately pull the rabbit towards you. It’s a great and simple plan, it just needs to work, right?
But wait! Here’s the part that I really liked: While Wile E. Coyote was assembling his device, we found it came in a giant box labeled “A set of 10,000,000,000 Volt Do It electromagnets.” Yourself.”
In the end, you can guess what will happen: Bugs don’t actually eat the iron carrot, so once the coyote turns on the magnet, it will zoom towards him and into his burrow. And of course many other things are also attracted to it — including lampposts, bulldozers, giant cruise ships and rockets.
Okay, let’s analyze the physics of this giant electromagnet and see if this would work if Bugs had fallen in love with it.
What is an electromagnet?
There are basically two ways to create a constant magnetic field. The first is with a permanent magnet, like those cling to your fridge door. They are made of some kind of ferromagnetic material such as iron, nickel, alnico or neodymium. A ferromagnetic material essentially contains regions that act like individual magnets, each with a north and south pole. If all these magnetic domains were aligned, the material would behave like a magnet. (There’s some very complicated stuff going on at the atomic level, but let’s not worry about that for now.)
However, in this case Wile E. Coyote has an electromagnet, which generates a magnetic field with an electric current. (Note: We measure current in amperes, not to be confused with potential difference, which is measured in volts.) All currents produce magnetic fields. Normally, to make an electromagnet, you would take some wire and wrap it around a ferromagnetic material, like iron, and turn on an electric current. Its magnetic field strength depends on the amperage and the number of turns the wire creates around the core. It is possible to make an electromagnet without an iron core, but it will not be as strong.
As the current generates a magnetic field, this field then interacts with the magnetic domains in the iron. Now iron also acts like a magnet – the electromagnet and the induction magnet attract each other as a result.
What about 10 billion Volts?
I don’t know how the script of this episode came about, but in my mind they had a group of writers working together. Maybe someone came up with the idea of an electromagnet and an iron carrot and everyone agreed to put it in. Surely someone raised their hand and said, “You know, we can’t just make an electromagnet. It has to be too big.” Another writer must have replied: “Put a number there. What about 1 million volts?” Another interjected: “Sure, 1 million volts is great—but about 10 billion volts? “
What does 10 billion volts even mean for an electromagnet? Remember, the most important thing about an electromagnet is the amperage (in amps), not the voltage (in volts). To make a relationship between voltage and current, we need to know the resistance. Resistance is a property that tells you how difficult it is to move charges through a conductor and it is measured in ohms. If we know the resistance of the electromagnet wire, we can use Ohm’s law to find the amperage. As an equation, it looks like this:
