Section 9­2.notebook
January 25, 2012
Pressure ­ the magnitude of the force on a surface per unit area
P = F
A
Pressure = Force
Area
The SI unit of pressure is pascal (Pa) 1N/m2
The pressure of the atmosphere at sea level is about 105 Pa
The amount of air pressure under normal conditions is the basis of another unit is the atmosphere (atm)
105 Pa is the same as 1atm
1
Section 9­2.notebook
January 25, 2012
Pascal's Principle
Pressure applied to a fluid in a closed container is transmitted equally to every point of the fluid and to the walls of the container. A hydraulic lift makes use of Pascal's Principle. A small force F1 applied to a small piston of area A1 causes a pressure increase in a fluid, such as oil. According to Pascal's law, this increase in pressure, Pinc, is transmitted to A2 and the fluid exerts a force F2 on this piston.
Pinc = F1 = F2
A1 A2
2
Section 9­2.notebook
January 25, 2012
The small piston of a hydraulic lift has an area of 0.20 m2. A car weighing 1.20 x 104 N sits on a rack mounted on the large piston. The large piston has an area of 0.90 m2. How large a force must be applied to the small piston to support the car?
Pinc = F1 = F2
A1 A2
3
Section 9­2.notebook
January 25, 2012
Pressure varies with depth in a fluid. P = F = mg = ρVg = ρAhg = ρhg
A A A A This is known as the gauge pressure, not the total pressure it would need to also include the atmosphere itself on the surface.
Fluid Pressure as a Function of Depth
P = Po + ρgh
absolute pressure = atmospheric pressure + (density x free­fall acceleration x depth)
There is pressure on the top of the box and the bottom (buoyancy). Pnet = Pbottom + Ptop = (Po + ρgh2) ­ (Po + ρgh1) = ρg(h2 ­ h1) = ρgL
Fnet = PnetA = ρgLA = ρgV = mfg
4
Section 9­2.notebook
January 25, 2012
Kinetic Theory of Gases ­ refers to the rate of transfer of momentum of the gas particles to a container wall and being equal to the force exerted by the gas on the container wall. (Gas Pressure)
Temperature ­ a measure of the average kinetic energy of the particles in a substance.
Kinetic Theory predicts that temperature is proportional to the average kinetic energy of the particles of gas. The higher the temperature; the faster the particles move. The increase of speed will increase amount of collision, which due to the Kinetic Theory of Gases will increase the pressure.
Conversion from Celsius to Kelvin: add 273
5
Section 9­2.notebook
January 25, 2012
Calculate the absolute pressure at an ocean depth of 1.00 x 103 m. Assume that the density of water is 1.025 x 103 kg/m3 and that Po = 1.01 x 105 Pa.
P = Po + ρgh
6
Section 9­2.notebook
January 25, 2012
7