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Pressure Thermodynamics Professor Lee Carkner Lecture 2

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Pressure Force per unit area SI unit: Pascal Pascals are really small so we often use kilopascals Atmospheric pressure is about 100 kPa More precisely: 1atm = 101325 Pa

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Measuring Pressure But, we mostly are dealing with things experiencing atmospheric air pressure Pressures less than air pressure are vacuum pressures We usually measure gage pressure but want absolute pressure

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Pressure with Depth The pressure of a column of fluid depends only on depth This assumes constant density For non-constant density the relationship is We would need a expression for density as a function of depth (z) to solve

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Multiple Fluids Different fluids mixed together with different densities tend to settle out The pressure at any point in the mixture is just the sum of all the gh above that point

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Barometer Since P = gh, we can measure pressure with a column of fluid of known density Since air pressure is very large, we need a dense fluid to keep the device from being too large Densest readily available fluid is mercury One mm mercury = 1 torr = 133.3 Pa

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Pascal’s Principle Not only is pressure independent of the shape of a container, any pressure applied to a fluid is transmitted equally to all walls of the container Known as Pascal’s Principle

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Ideal Mechanical Advantage Consider a curved pipe with ends of unequal width If we press down with force F 1 on the narrow end, the force on the wide end is: A 2 /A 1 is called the ideal mechanical advantage, and determines the amount by which our input force is multiplied

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Manometer Can evacuate the part of the tube above the fluid to measure absolute pressure, or keep it open to the air to measure gage pressure

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Pressure Drops The manometer will now measure the difference in pressure between the two points The end with the greater pressure will push the manometer fluid down further The difference in the height of the manometer fluid (h) is proportional to the pressure difference

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Pressure Difference The manometer fluid has pressure 2 gh and the flow system fluid has pressure 1 gh The pressure difference is: Can monitor fluid level to see if there is a problem with the flow

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Pressure Measuring Devices Manometers are often not practical Want a pressure sensor that is easy to use and read When fluid flows through the tube it tries to straighten out and moves the needle Most convenient are electronic sensors

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Next Time Read: 2.2-2.5 Homework: Chapter 2: 7, 28, 30

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