Friday is here again, and with it the weekly "A Taste of Physics" column — number 45.
This week: adiabatic compression, the ideal gas law, and their connection to weather and black holes.
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The video shows what happens when ordinary air is rapidly compressed inside a piston — it ignites in a sudden burst of flame!
Humanity has been familiar with this phenomenon for hundreds of years, and if you're planning a trip to a deserted island, look up "Fire Piston" on Google beforehand.
Why does this happen?
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The ideal gas law defines the constant relationship between the volume, temperature, and pressure of a gas.
(The gas in the equation is "ideal" because it ignores variables such as intermolecular interactions, in order to simplify the calculation.)
The equation tells us that as the volume of a gas decreases, its pressure and temperature rise.
As the volume increases, the pressure and temperature fall.
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Adiabatic compression is compression that involves no addition of external heat.
Adiabatic expansion is expansion into a space that loses no heat to the surroundings.
The simplest example is blowing on the palm of your hand.
If you blow with your hand held close to your lips, the air will feel warm.
If you move it a little farther away, the exhaled air will feel cool.
This is because the exhaled air expands adiabatically into the surrounding space, and cools as its volume increases.
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Gasoline engines use an electric spark plug to ignite the fuel inside the engine's cylinders.
Diesel engines, by contrast, rely on compressing the air in the cylinder at a very high compression ratio of 1:15 to 1:22 of its original volume, raising its temperature enough to spontaneously ignite and combust the fuel droplets injected into it.
Diesel engines are significantly more fuel-efficient than gasoline engines due to their high compression ratio, but because the pistons must withstand such high pressures, they are also bulkier, heavier, and noisier.
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This phenomenon can also be found in some fascinating natural occurrences.
Temperature changes arise in part from the movement of wind across the ground.
Different terrain features can cause wind to compress or expand as it travels, thereby producing temperature changes.
One example is the Foehn wind — a wind that climbs the Alps, cooling adiabatically as it rises, and then compresses and warms as it descends on the other side.
Cosmic events such as the collapse of stars into black holes involve the compression of enormous amounts of matter in a very short time, dramatically raising the temperature and generating the shock waves produced in the process.
Shabbat Shalom 😊
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