Thermodynamics
The First Law of Thermodynamics
The Second Law of Thermodynamics
The Third Law of Thermodynamics
Temperature
Absolute Temperature
Pressure-volume (constant temperature)
Volume-temperature (constant pressure)
Pressure-temperature (constant volume)
A complete ideal gas law
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pressure-volume (constant temperature)
What happens to the volume of a gas as the pressure on it changes. Let's try the following experiment using equipment that might be found in your kitchen.
Marshmallows are a mixture of sugar, air, and gelatin. The sugar makes them sweet, the air makes them fluffy, and the gelatin holds everything together. Marshmallows are a frozen foam and are mostly air by volume. When placed in a vacuum pump, they expand as the pressure decreases. Break the seal on their container and they shrink during the return to normal atmospheric pressure. Since the vacuum pump pulls on the marshmallows hard enough to burst some of the air bubbles, they are actually a bit smaller and more shriveled at the end of this experiment. This illustrates a fundamental, yet important, property of gases. The pressure of a gas is inversely proportional to its volume when temperature is constant. Symbolically .
P = 1/ V
This correlation was discovered independently by Robert Boyle (1627-1691) of Ireland in 1662 and Edme Mariotte (1620-1684) of France in 1676. In Great Britain , America , Australia , the West Indies and other remnants of the British Empire it is called Boyle's law , while in Continental Europe and other places it is called Mariotte's law .
Mariotte added the important provision that temperature remain constant. Boyle neglected to mention it, but the data he used to derive his law were most likely collected during a period in which the temperature did not experience any significant change. Since the gas needs to be in thermal equilibrium with its environment (or some other heat reservoir) to maintain an even temperature, the pressure-volume relationship normally applies only to "slow" processes. The marshmallow-vacuum experiment shown above is an example of a "slow" process. The pressure is reduced at a rate slow enough that heat from the environment is able to keep the jar and its contents at nearly room temperature. Such a transformation that takes place without a change in temperature is said to be isothermal .
Pumping a bicycle tire with a hand pump is an example of a "fast" process. The work done pushing the piston transforms into an increase in the internal energy (and thus an increase in the temperature) of the air molecules within the pump. People familiar with hand bicycle pumps will attest to the fact that they get hot after use. Likewise, when a gas is allowed to expanded into a region of reduced pressure it does work on its surroundings. The energy to do this work comes from the internal energy of the gas and so the temperature of the gas drops. You can experience this yourself without the aid of any apparatus other than your mouth. Purse your lips so that your mouth has only a tiny opening to the outside and blow hard. The air rushing from your mouth will be quite cool despite coming from the core of your body, which is normally quite hot (around 37 °C ). During a "fast" process like the ones just described, pressure and volume are changing so rapidly that heat doesn't have enough time to get into or out of the gas to keep the temperature constant. Such a transformation that takes place without any flow of heat is said to be adiabatic .
P 1 V 1 = P 2 V 2 = constant
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