Pressure Thermodynamics Professor Lee Carkner Lecture 2

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 = Pa 

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

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

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 

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 = Pa

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

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

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 

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

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

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 

Next Time  Read:  Homework: Chapter 2: 7, 28, 30