Got a Sock Stuck in Your Vacuum? It’s Time for Some Physics

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Imagine yourself running the vacuum cleaner over the carpet when all of sudden—a sock. Boom. It’s stuck in the hose, the whine of the vacuum getting higher and higher, louder and louder. It sounds like overload is imminent, like the motor is working way too hard. But is it? To know, let’s look at some cool physics principles.

Electric Motor

Vacuum cleaners don’t actually suck. They blow. No, really. They use a fan that blows air out of a hole. That air must come from somewhere, so as air gets blown out of the vacuum, it gets drawn into the vacuum, bringing dirt and, occasionally, a sock with it.

At the heart of that process lies an electric motor. In its most basic form, an electric motor consists of a rotating coil of wire and a magnet. (Yes, there are many types of motors, but this is your basic motor). Run a current through the coil and it creates a magnetic field. This magnetic field makes the coil interact with the magnet (which also makes a magnetic field) and the coil spins. That’s pretty much it.

You can build a motor yourself using some basic materials. It looks like this:


Electric Generator

To understand the effect on a plugged-up vacuum cleaner, you must understand how an electric generator works. A generator is, like a motor, essentially a coil of wire and a magnet. What happens when you place a changing magnetic field within coil? Glad you asked. Let me provide a quick demo (No, this isn’t actually a generator.) using a coil of wire, a magnet, and something to measure the current in the wire. Notice what happens as I move the magnet into and out of the coil.

Induction 1

Merely hold a magnet in a coil of wire and nothing happens. It’s not a magnetic field that generates a current, it’s a changing magnetic field. You must move the magnet in or out to induce a current. This is exactly what happens in an electric generator. The electric generator creates a changing magnetic field (in the loop) by rotating the loop.

So you can see that both an electric motor and an electric generator have a rotating loop of wire in a magnetic field. In fact, the only difference between a motor and a generator lies in their use. Run current through the motor and it will turn. Turn the motor, it will generate a current. (Yes, technically it will generate a change in potential and create a current only with a complete circuit).

A Spinning Electric Motor

Back to the vacuum cleaner. Plug the inlet hose and the motor-fan inside spins faster because it has less air resistance. But what happens to the electric current? The faster the motor spins, the more it acts like a generator. There is actually a change in electric potential across the coils that acts in the opposite direction as the voltage source. This makes the effective voltage lower and gives a lower current. So a spinning motor actually uses less current the faster it spins. That probably seems backward, but it’s true.

Think of it this way. What would take more energy? Pushing a bunch of air through the vacuum cleaner or pushing now air through the vacuum? Pushing less air takes less energy—so there is lower energy requirement and lower current. Sure, the motor sounds like it’s working harder, but it isn’t. Also, look at the other end. What happens when a motor stops spinning? In this case the current increases and the insulation keeping wires apart can actually melt. Engineers call this a “bad thing.”

Now for an experiment. Once upon a time, I built an anemometer—or a wind sensor using an electric motor. Now I can put it to good use! Here’s what I’m going to do. I will measure the speed of the air coming out of the vacuum and its power consumption for the device. Here’s what that looks like:

Photo Google Photos

Just to be clear, I am using a shop-vac and connected the hose to its output so it blows air. I’m not sure this will provide the best measurement of air speed, but I can at least get an estimate. To change the speed of air, I will cover part of the intake so that less gets in.

Now for a plot of power vs. air speed.

It sort of works—I must admit that my anemometer wasn’t doing so great—but I still got some data. This shows that as the speed of the air coming out of the vacuum cleaner increased (so not blocked) it used more power. When you get that sock stuck in your vacuum cleaner, it uses less power—even though it’s screaming like a banshee. So don’t worry about it. Still, try to avoid vacuuming up socks.

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