BAM - B Thermo and Fluids Help

Pressure and Ideal Gas Law

Do you feel that? On your shoulders? That weight? About 1200 lbs. Right now there is about half a ton of air sitting on your shoulders, pressing you down. Before you start thinking that you are a beast and can squat way more than you thought, you have to be aware that we can't really feel this weight, or at least we are used to it. That doesn't mean that its not there. In fact, that weight of air can be used to crush a 55 gallon steel drum.

Not impressed? What if I told you that air could even crush a train? Don't believe me... Go ahead, watch this.... I'll wait.

Air pressure, or the force exerted by a gas trying to expand, can be incredible. We are already pretty familiar with this idea with water pressure. Ever try to swim to the bottom of a pool? This is what is happening to your chest and ears.

The questions remains though: Why does this happen? Or, why don't things get crushed all the time? To answer that we'll need to understand pressure and the ideal gas law. This tells us that there is a relationship between temperature, pressure, and volume. But if that is the case, pressure should be able to do more than just crush things, right? Kaboom.

Here are some specifications for each scenario so we can do some investigation of our own. Assume that each example took place at 1 atmosphere of pressure: 1.01 x 10⁵ pa.

55 Gallon Drum: 23 in diameter, 35 in tall.Assume no interior pressure.

Train Car Dimensions: 6.1 m long, 2.6 m high. Assume no interior pressure.

Hot Water Heater Dimensions: 22 in diameter, 30 in height; Interior Pressure: 2.3 x 10⁶ pa.

Some Thoughts on Heat

We just started thinking about heat energy (Q) again so I thought I would share a shot from our diagram on the board and a few thoughts. When you add energy to something (any kind of energy) that something changes. If I add a Big Mac to you, you gain weight. If I add kinetic energy to you, you start moving faster. If I add heat to you, you get hotter.... sometimes. We really get two choices every time we add heat to something: 1) Change Temperature or 2) Change State. Take a look at the diagram below.

When we add heat to ice, it wants to get hotter. However, it gets to a point (Melting Point) where it can't get any hotter as a solid, at that point it begins to convert to a liquid, which also takes energy (Heat of Fusion). Once all of the solid has converter we can get hotter again, until we hit another boundary (Boiling Point). No we must use energy to convert to a gas (Heat of Vaporization). At that point we can keep going up (and eventually become plasma.

Although turning ice into steam can be energy intensive and seems tough, its a problem made up of lots of simple steps. Take the example in the graph above, converting 1 kg of ice at -30oC to steam at 120oC. Although it takes 5 different stages of adding energy, each step is pretty simple.

This is kind of unrelated, but I thought you'd enjoy. Here is a video of some experiments with dry ice and a microwave:

Light and Sound Review

As promised, here is a video review of ALL of our light and sound topics. I hope that it helps you prepare for our quiz!

Superposition and Sound

So I happen to be in the chorus room for parent teacher conferences and figured this would be a great opportunity to do a demo with sound that I have been meaning to get to. The video has to do with the superposition of waves using a guitar. This image represents a whole bunch of possible standing waves that can take place on a string of length L.

When you play a note on a guitar you are making these waves, but they all happen at the same time. Imagine adding all of those waves together, the resulting interference is referred to as superposition. Check out this video I made demonstrating the idea of superposition: multiple waves existing at the same place at the same time.