It’s Not an Impossible Fish Tank, It’s Just Physics

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A Simple Click Really Helps

If you like fish tanks, you might like this neat trick that creates space for the fish to swim above the water level. Beyond being pretty cool, it provides a chance to examine to interesting physics concepts.

How Can You Have Water Above the Water Level?

The space I’m referring to is called a vacuum suspended fish tank, and it uses a glass jar or other vessel to create what amounts to a tank above the water level. The first question you probably have is why the water doesn’t simply fall out of that vessel. Let’s start with a diagram.

To understand this physics trick, first consider the idea of pressure. Imagine that the air and the water are tiny balls (which isn’t a crazy thing to imagine). These balls are moving at varying speeds and directions and, most importantly, colliding with other “balls” and the walls of the container. These collisions cause the momentum of the ball to change, which means these collisions exert forces on other balls and on the container walls. This ball collision can be described by a pressure, and that pressure depends on:

  • The momentum of the balls, which depends on their speed and the mass.
  • The number of balls. More balls means more collisions and a greater pressure.

The pressure does not depend upon the size of the wall. Sure, bigger walls lead to more collisions, but the pressure is essentially the number of collisions per unit area. Another important point—these gas-balls move in different directions. This means that no matter which way you look, these collisions can produce a force and the force of those collisions is the same magnitude.

Now look at the points A and B. Point A is on the water’s surface. (OK, this point has some size and is not, technically, a “point.”) The water at point A is essentially stationary, and the net force on it must be zero. (OK, technically I should say the zero vector because I can’t stop myself.) This means that the force from the pressure of water below Point A must be equal to the sum of the gravitational force pulling down on it and the force from the pressure of the air. But in all, everything is awesome.

What about point B? The net force there must also be zero (yes, zero vector). However, there is only water above point B. Still, the force pushing down at point B must be the same as the force pushing down at point A (assuming equally sized points). For point A, this downward force is due to the air above it, and point B it is due to the water above it. If these downward forces at A and B varied, the water at A and B would move up or down.

You’ll notice point C. Here there must be even more downward force because there is more water above it pushing down. The only way this can work is if the water pressure at point C is greater than the water pressure at A and B. Yes, pressure increases with depth. This is the only way to make the sum of the forces zero (vector).

So why does the water get sucked up into the glass? Simple—it doesn’t. Instead, the air pressure above the water within the glass is reduced by removing some of the air. This means the force pushing down at A is greater than at B and the water is gets pushed up by the atmosphere. Since the atmospheric pressure is equivalent to the pressure at a water depth of 10 meters, this glass water column could rise 10 meters—but no higher. If you built a container taller than 10 meters above the surface, you would just have water vapor above that height (at least that’s what I think would happen).

Incidentally, this is exactly how a straw works. You don’t suck water up a straw; you decrease the air pressure at the top of the straw (using your mouth) and atmospheric pressure pushes the water up. This is why you can’t suck liquid through a straw that is longer than 10 meters. Not even Superman could do it.

How Do You Get the Water in the Glass?

For the glass in the aquarium (as seen in the video), the air in the container is drawn out with a tube. But the video provides another very cool way of doing it—with fire. Place a small candle on a plate with a bit of water. Light the candle and place a glass over the flame, like this:

Candlesuck 2

You should be able to do this yourself. Just make sure you have enough water on the plate to cover the rim of the inverted glass. (If you try this with a plastic cup you will melt the cup—don’t do that. ) The candle will burn out, but not before the water moves up the glass. I added food coloring to make it easier to see. I also used a match in my candle because the wick was messed up.

How does it work? It’s all about the chemical reaction of wood with oxygen in the air. The wood is made of a bunch of stuff, but the most important is cellulose. Cellulose has a bunch of carbon and a bunch of hydrogen (this is how physicists talk about chemistry). When you add enough energy in the presence of oxygen, you get a chemical reaction that produces more energy along with carbon dioxide (CO2) and water vapor (H2O). It turns out that the number of carbon dioxide molecules is less than the starting number of oxygen molecules. The water vapor can easily condense into liquid so you are left with a gas that has one carbon dioxide for every two oxygens (or something like that). Overall, there are now fewer gas-balls in the glass, so there is a lower pressure. Under lower pressure, the atmosphere pushes the water up the glass.

Eventually the oxygen depletes to the point that the flame goes out. After this, more water will condense from the gas and the gas will cool (which also decreases pressure). Both of these things cause water to continue rising.

One final point: Don’t forget that burning wood doesn’t produce energy by breaking bonds. Actually, it requires energy to break chemical bonds. But you do get energy when you form new bonds (for carbon dioxide and water) and the energy you get from the new bonds is greater than the energy need to break the bonds. OK, I feel better now.

Homework

I have but one homework question: Imagine you are a fish in this tank—or maybe a diver. What would it feel like if you started at the surface and swam into the glass container and above the water level? I think I have an answer, but I don’t feel 100 percent confident in it.

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