Boyles Law: Pressure, Volume & Units Explained

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In summary, the conversation discusses the use of various units in Boyle's law, including kN/m^2 and mm Hg, and their relation to standard units like kPa. The concept of gauge pressure is also mentioned, along with the use of a manometer. The discussion also touches on the significance of volumetric units and how they cancel out in the equation.
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optrix
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Hi, I'm doing a question about Boyles law, and I don't understand some of the units they are giving.

Initial pressure = 20kN/m^2 (gauge) = 20kPa (gauge)
Initial volume = 47 volumetric units

Final pressure = 120kN/m^2 = 120kPa
Final volume = ?

(The barometric pressure was 762mm Hg)

I don't understand the units 762 mm Hg, and why they say (gauge) in brackets. I think it has something to do with a manometer, but I'm only used to working with Pa so far.

I'm guessing volumetric units could be anything, because the units probably cancel out when you put them into the equation, so only the ratio between the initial and final volumes is significant?

Thanks
 
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  • #2
I don't know about gauge, but 1 mm Hg indicates pressure exerted by pure and dry liquid mercury of 1mm high in a tube at 273Kelvin temperature. That tube is to be placed on 45 degree latitude of sea level.

101.325 kPa = 760 mm Hg pressure. It means that mercury column of 760 mm high will exert that much pressure under those above stated conditions. It's just describing pressure in term of the height of mercury column in the tube, nothing else.
 
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for your question! Boyles Law is a fundamental gas law that describes the relationship between pressure and volume in a gas at a constant temperature. In order to fully understand the units involved, it is important to have a basic understanding of the concepts of pressure and volume.

Pressure is a measure of the force exerted by a gas on its container. The standard unit for pressure is the Pascal (Pa), which is defined as 1 Newton per square meter. However, in some cases, it is more convenient to use other units such as kilonewtons per square meter (kN/m^2) or kilopascals (kPa). These units are equivalent to the Pascal, just on a larger scale.

When we talk about pressure, we also need to consider whether it is an absolute pressure or a gauge pressure. Absolute pressure is the total pressure exerted by a gas, including the atmospheric pressure. Gauge pressure, on the other hand, is the pressure relative to the atmospheric pressure. In your example, the initial pressure is given as 20kN/m^2 (gauge), which means it is 20kN/m^2 above the atmospheric pressure.

Moving on to volume, it is important to note that the units used for volume can vary depending on the system of measurement being used. In your example, the initial volume is given in volumetric units, which could represent any unit of volume such as liters or cubic meters. As you correctly mentioned, the specific unit of volume is not important in this case because it will cancel out when calculating the ratio between initial and final volumes.

Now, let's address the last part of your question about the barometric pressure of 762mm Hg. This is a unit of pressure traditionally used to measure atmospheric pressure and is equivalent to 101.325 kPa. It is important to include this value in the calculation because it is the reference point for gauge pressure. In other words, the initial pressure of 20kN/m^2 (gauge) is 20kN/m^2 above the atmospheric pressure of 762mm Hg or 101.325 kPa.

In conclusion, when working with Boyle's Law, it is important to pay attention to the units being used for pressure and volume and whether they are absolute or gauge values. By understanding these concepts, you can accurately apply the formula and solve for unknown variables, such as the final volume in your example. I hope this explanation helps clarify
 

1. What is Boyle's Law?

Boyle's Law is an experimental gas law that describes the relationship between the pressure and volume of a gas at a constant temperature.

2. How does Boyle's Law work?

Boyle's Law states that the pressure of a gas is inversely proportional to its volume. This means that as the volume of a gas decreases, the pressure increases, and vice versa, as long as the temperature remains constant.

3. What are the units used in Boyle's Law?

The units used in Boyle's Law are pressure in Pascals (Pa) or atmospheres (atm) and volume in liters (L) or cubic meters (m3).

4. What is the significance of Boyle's Law?

Boyle's Law is significant because it helps us understand the behavior of gases and how they respond to changes in pressure and volume. It also allows us to predict the volume or pressure of a gas when one of these variables is known.

5. How is Boyle's Law applied in real-life situations?

Boyle's Law has many real-life applications, such as in scuba diving, where changes in pressure and volume of the gas in a diver's tank must be carefully monitored to ensure their safety. It is also used in the production of carbonated beverages, where the pressure and volume of carbon dioxide gas are manipulated to create the desired level of carbonation. Additionally, Boyle's Law is used in medical equipment, such as ventilators, to control and adjust the pressure and volume of gases in the body.

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