1. States of Matter

2. Fluid States In science, gases and liquids are fluids Fluid pressure is the force exerted by the fluid on an area of a surface. p = F/A The unit of pressure is the pascal (Pa).

3. Pascal’s Principle Any change in pressure applied to a confined fluid at any point is transmitted undiminished throughout the fluid. Also note, fluid pressure acts in all directions. This is the principle used in hydraulics.

4. Hydraulics If the pressure in a fluid is equal throughout the fluid (assuming the same depth), then P 1 = F 1 /A 1 & P 2 = F 2 /A 2, then F 1 /A 1 = F 2 /A 2

5. Example Calculation A 20.0-N force is exerted on a small piston of a hydraulic system. The cross-sectional area of the small piston is 0.0500 m 2. What is the magnitude of the weight that can be lifted by a larger piston which has an area of 0.100 m 2 ?

6. Example Calculation (cont’d) Known: F 1 = 20.0 N, A 1 = 0.0500 m 2 A 2 = 0.100 m 2, F 2 = ? Equation: F 1 /A 1 = F 2 /A 2 Solution: (F 1 /A 1 )*A 2 = F 2 –F 2 = (20.0 N/0.0500 m 2 )/0.100 m 2 = 40.0 N

7. Fluid Pressure Changes with Depth The pressure of a flat, horizontal object is the weight of the liquid above the object per unit area. Recall: F w = mg  (density) = m/V V = A*h

8. Fluid Pressure Changes with Depth Rearranging the density equation gives: m =  *V V = A*h F w = (  *V)*g =  *(A*h)*g Substitution gives: p = F w /A = (  *A*h*g)/A =  *h*g Note- pressure is proportional to the depth and the density. The shape has no effect.

9. Fluid Pressure Changes with Depth p = F w /A = (  *A*h*g)/A =  *h*g Consider the object now vertically in the fluid. The bottom of the object has some length ( l ) greater depth than the top. Therefore, p =  *(h + l )*g

10. Archimedes’ Principle The force on the bottom is greater than on the top --> F = p*A F top = (  *h*g) * A F bottom = (  *(h + l )*g) * A F bottom - F top = {(  *(h+ l )*g)*A} - {(  *h*g)*A} –= {  *h*g*A +  * l *g*A} - {(  *h*g)*A} –=  * l *g*A =  *A* l *g =  *V*g –= F w of fluid displaced

11. Archimedes’ Principle An object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.

12. Archimedes’ Principle Note, the buoyant force does NOT depend on the weight of the submerged object, only the weight of the displaced fluid. A solid cube of aluminum and a solid cube of iron of equal sizes have the same buoyant force.

13. Archimedes’ Principle This works for all densities. If the object has a greater density than the fluid, it will sink because its weight will be greater than the weight of the same volume of fluid; its weight will be more than the buoyant force. The net force will be down.

14. Bernoulli’s Principle As the velocity of a fluid increases, the pressure exerted by that fluid decreases. This is the principle that causes a difference in pressure around airfoils- used for airplane wings, race cars, propellers, etc.

15. Liquids & Gases Although liquids and gases are both fluids, they are not the same. Liquids have definite volumes. Gases fill the container. Liquids are incompressible. Gases are easily compressed. The particles of liquids are close together- the volume of the liquid is practically the volume of the particles, whereas the particles of a gas are very far apart and make up very little of the total volume of the gas.

16. Liquids & Gases Liquid particles exert cohesive forces on each other. These are attractive electromagnetic forces that occur because the particles are so close together (i.e. surface tension) Liquids with strong cohesive forces form spherical drops on flat surfaces; those without will flatten out.

17. Liquids & Gases Liquid particles demonstrate adhesion between particles of different substances (i.e. capillary action).

18. Liquids & Gases Evaporation: when the particles of a liquid have enough energy to escape the net downward cohesive force from the liquid. Evaporation is a cooling process for the liquid. When we sweat, heat from our body is used to increase the energy of the sweat turning it into a gas. This cools us down. Volatile: A liquid that evaporates quickly

19. Liquids & Gases Condensation: the process in which a gas loses enough energy to become a liquid.

20. Solids: Crystals Crystal lattice: the particles of a liquid have lost enough energy that the cohesive forces prevent the particles from moving past each other. Amorphous solid: substances that have no regular crystal structure but have a definite shape and a definite volume.

21. Solids: Elasticity Elasticity: the ability of an object to return to its original form when external forces are removed. It depends on the electromagnetic forces that hold the particles of a substance together.

22. Thermal Expansion Thermal expansion: the expansion of a substance when heated and the contraction of a substance when cooled. Most substances continue to contract when cooled. Water, however, expands as it freezes. This allows the density of water to decrease as it freezes. It floats. Water freezes from top down.

23. Thermal Expansion: Convection As air is heated it expands. This increases its volume and decreases its density. The hot air rises. As the hot air replaces the colder air at the top, the colder air falls. This is a convection current.

24. Thermal Expansion: Coefficients The change in length or volume of a solid is proportional to the change in temperature. L f = L i +  L i (T f -T i )  L =  L i (T f -T i ) Similarly,  V =  V i (T f -T i )