Here are some basic ideas that you will need to know to understand what follows:
Temperature Kelvins
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Pressure KPa
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Here is an example of the last point. If you drag your pointing device on the image at the right, you will make the barrier move against the gas volume and change its pressure and temperature
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There are several ways to explain this effect. One way is to imagine a single gas molecule colliding with the descending barrier. In this case, the barrier's velocity is added to the velocity of the molecule as it bounces off, thus increasing its speed and its temperature. Conversely, as the barrier rises, a gas molecule that collides with it will recoil with less velocity, therefore its temperature will drop.
Another, more general, way to explain this effect is to realize that, when you compress the gas, you are making a fixed amount of energy occupy a smaller volume, "concentrating" it, so to speak. This equal amount of energy in a smaller volume raises the temperature of the gas.
1. This virtual engine disregards some second-order effects, so don't depend on its pressure / temperature readout if you're designing a rocket — or, for that matter, a toaster.