Physics 102 Part II Thermal Physics Moza M. Al-Rabban Professor of Physics Fluids

Text Book Physics, by James S. Walker Chapters 15, 16, and 17

Outline 15-1 Density 15-2Pressure 15-3Static Equilibrium in Fluids: Pressure and Depth 15-4Archimedes' Principle and Buoyancy 15-5Applications of Archimedes' Principle 15-6Fluid Flow and Continuity 15-7Bernoulli's Equation 15-8Applications of Bernoulli's Equation *15-9Viscosity and Surface Tension

States Of Matter Solids Liquids gases Solids: Solids tends to hold their shapes. Although the atoms vibrate around fixed equilibrium positions, they do not have enough energy to break the bound with their neighbors.

Fluids Liquids and Gases Liquids and Gases, do not hold their shapes. A liquid flows and takes the shape of its container and a gas expands to fill its container. The atoms or molecules in Fluid do not have fixed positions, so a fluid does not have a definite shape.

Density Definition of Density,  The denser a material, the more mass it has in any given volume.

Pressure Definition of Pressure, P

Example 1: Find the pressure exerted on the skin of a balloon if you press with a force of 2.1 N using (a) your finger or (b) a needle. Assume the area of your fingertip is, and the area of the needle tip is (c) Find the minimum force necessary to pop the balloon with the needle, given that the balloon pops with a pressure of Solve (a)Calculate the pressure exerted by the finger: (b) Calculate the pressure exerted by the needle: (c)

Atmospheric Pressure and Gauge Pressure Atmospheric Pressure, P at

Gauge pressure, P g The pressure in a fluid acts equally in all directions, and acts at right angles to any surface. In many cases we are interested in difference between a given pressure and atmospheric pressure.

Gauge Pressure Does the flat tire on your automobile have zero air pressure? If it is completely flat, it still has the atmospheric pressure air in it. To be sure, it has zero useful pressure in it, and your tire gauge would read zero Newton per square meter. Most gauges read the excess of pressure over atmospheric pressure and this excess is called "gauge pressure". When a system is at atmospheric pressure like the left image above, the gauge pressure is said to be zero. In this image, the system has been opened so that it is at equilibrium with the atmosphere. In the right image, the system has been closed and the plunger pushed down until the pressure reads about 15 lb/in2.

Static Equilibrium in Fluids: Pressure and Depth This expression holds for any liquid with constant density  and a pressure P at at its upper surface.

The relation can be applied to any two points in a fluid. Dependence of Pressure on depth

Example3: A cubical box cm on a side is completely immersed in a fluid. At the top of the box the pressure is kPa; at the bottom the pressure is kPa. What is the density of the fluid? Example3: A cubical box cm on a side is completely immersed in a fluid. At the top of the box the pressure is kPa; at the bottom the pressure is kPa. What is the density of the fluid? Solve:

A Simple Barometer An interesting application of the variation of pressure with depth is the barometer, which can be used to measure atmospheric pressure. The pressure in the tube at depth h below the vacuum is 0 +  gh =  gh At the level of fluid in the bowel we know that the pressure is one atmosphere, therefore, P at =  gh A fluid that is often used in such a barometer is mercury (Hg), with a density of The corresponding height for a column of mercury is

Fluid Seeks Its Own Level (a)When the levels are equal, the pressure is the same at the base of each arm of the U tube. As a result, the fluid in the horizontal section of the U is in equilibrium. (b)With unequal heights, the pressure are different. In this case, the pressure is greater at the base of the right arm, hence fluid will flow toward the left and the level will equalize.

Example 4: Oil and Water Don’t Mix A U-shaped tube is filled mostly with water, but a small amount of vegetable oil has been added to one side, as shown in the sketch. The density of the water is, and the density of the oil is If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side? Solution 1. Find the pressure at point A, where the depth of the water is h 1 2. Find the pressure at point B, where the depth of the water is h 2 = 5.00 cm 3. Set P A equal P B :

Example 4: Oil and Water Don’t Mix A U-shaped tube is filled mostly with water, but a small amount of vegetable oil has been added to one side, as shown in the sketch. The density of the water is, and the density of the oil is If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side? 4. Solve for the depth of the water,h 1

Pascal’s Principal An external pressure applied to an enclosed fluid is transmitted unchanged to every point within the fluid. A hydraulic lift

End of Lecture 13