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    Home / Science lessons / The Pressure & Gas Laws
    • The Pressure & Gas Laws

      Pressure (P) can be described mathematically as 'force (F) divided by the area over which that force is applied,' or P = F / A. Pressure is measured in pascals (Pa), named in honor of the 17thcentury scientist and philosopher Blaise Pascal, who showed that confined liquids transmit pressure equally in all directions. Pressure can also be measured in pounds per square inch.

      You can use this formula to find the pressure exerted on your feet by your body; divide your weight in pounds by the number of square inches that your feet cover. If you weigh 100 pounds and the surface area of your feet is approximately 50 inches, then the pressure is around two lbs/in2. (To find total surface area of your foot, multiply average length by average width for each foot and then add those together.) The larger the area, the less pressure per square inch. There are advantages sometimes to having big feet!

      There are several scientific laws to describe the behavior of gases. Boyle's Law says that under constant temperature, the product of pressure (P) times volume (V) must remain constant, or PV = constant. This means that if one increases, the other must decrease (if P ↑ then V↓). Charles' Law says that under constant pressure, the quotient of volume (V) divided by temperature (T) is constant, or V/T=constant. This means that if one increases, so must the other, and vice versa (if T ↑, V must also). These laws are summed up in the Combined Gas Law: PV/T = constant. In other words, P1V1/T1= P2V2/T2, or the total of PV/T at one point will be the same as the total of PV/T at another point.

      If you have a medicine dropper or a basting tube with a rubber bulb at one end, you can demonstrate Boyle's Law. Partially fill a glass with water, then set the open end of the dropper or tube in the glass. Next, squeeze the rubber bulb at the end; as you exert pressure, the volume inside the tube decreases. What happens when you release the bulb? The immediate decrease in pressure causes the volume to rapidly fill up, pulling the water into the tube. To empty the tube, squeeze the bulb again. This increases the pressure and causes the volume to decrease, causing the water to squirt out.

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