Under Pressure Chapter 3

What is Pressure? In science, the concept of pressure involves both force and the surface area on which that force is exerted. Pressure is the result of a force applied in a perpendicular fashion to a surface.

Variables Force Surface Area

What is Force? Causing an object to Accelerate Slow down Change course Stop Change its shape Requires a certain amount of force to be applied. Force can be produced through an action or an attraction.

What is Surface Area? Surface Area is the facial dimensions of an object.

Force and Pressure The Greater the mass (means greater force) will end up exerting a greater pressure. Variation in Force Result If the force increases Pressure increases If the force decreases Pressure decreases

Surface Area and Pressure When force is distributed over a greater surface area then less pressure is applied. Variation in Surface Area Result If the surface area exposed to a force increases Pressure decreases If the surface area exposed to a force decreases Pressure increases

Calculating Pressure P = F A Pressure is measured in pascals (Pa) Pressure corresponds to the force applied per unit of surface area. The formula is: P = F A P= Pressure (Pa) F= Force (N) A= Exposed Surface Area (m2)

Pressure Exerted by Fluids Particles in fluids are in constant motion and therefore pressure is exerted equally in all directions.

Pressure exerted by Incompressible Fluids Incompressible fluids such as water, the force exerted comes from the mass of the fluid that is situated above the object and not the total quantity of fluid. U-Shaped manometer Depth gauge

A depth of 1m has the same pressure whether you’re in a pool or a lake. So, the more fluid above an object, the greater the force exerted by the fluid and the greater the pressure.

In Summary The pressure exerted on an object by an incompressible fluid depends on: The depth of the object in the fluid The density of the fluid

Pressure exerted by Compressible Fluids Particles in a compressible fluid (gas) are constantly moving. When the particles meet an obstacle (another particle or say the edge of the container) they change direction. During each collision , the fluid particles exert a force on the obstacle.

Pressure and Compressible Fluids Pressure depends on the number of collisions involving fluid particles. The more collisions = the greater the pressure. There is a direct relationship between pressure and the number of collisions.

Factors that Affect the number of Collisions Number of particles: the more particles, the more they collide. Temperature: According to the particle model, the speed of the particles is directly related to the temperature. The faster the particles move, the more they collide. Volume of fluid

Instruments Pressure Gauge U-Shaped manometer Dial-Faced Gauge

Volume and Compressible Fluids When the volume of a gas decreases the number of collisions goes up. This is because the particles have less space to move around and the frequency of collisions increases. The opposite is true for an increase in volume. Variation in volume Result If volume increases Pressure decreases If volume decreases Pressure increases

Boyle’s Law In 1661, he formulated the relationship between the volume and the pressure of the gas. *At a constant temperature; the volume of a compressible fluid is inversely proportional to pressure. If the pressure increases, volume decreases, and vice versa.*

Relating Boyle’s Law to the human ear When the external pressure decreases, for example when we fly high in an airplane, the eardrum swells. The eardrum returns to normal when the pressure is the same on both sides of the drum.

Atmospheric Pressure

Atmospheric Pressure Atmosphere is the layer of air that surrounds the earth. This air is a mixture of different gases and it exerts pressure on us. Although unnoticed, the pressure in the atmosphere is considerable.

Atmospheric pressure varies with altitude Atmospheric pressure varies with altitude. As altitude increases, atmospheric pressure decreases because there are less particles in the air and so less collisions. Instrument used to measure atmospheric pressure is called a barometer.

Principles For the variations in the pressure of fluids. Fluids naturally move from a zone of high pressure toward a zone of low pressure. See pg 81 figure 3.35

Pressure applied to the surface of a fluid inside a closed container gets uniformly distributed to every part of the fluid. See pg 82, figure 3.36

A transfer of pressure in a fluid can increase the force involved. See pg 82, figure 3.37 (hydraulics)

Natural Mechanisms

Blood Pressure See page 83 figure 3.38 The heart acts like a pump and provides the necessary changes to help the blood circulate through the body. Blood pressure in the arteries is measured with a blood pressure cuff (sphygmomanometer)

Breathing See pg 83 figure 3.39 When we breath in, our thorax swells and the volume of the lungs increases. This increase in volume lowers the pressure in the lungs relative to that of the air and vice versa. When breathing in and out, the volume of the lungs changes, letting air in and out of our body.

See pg 84 figure 3.40 Wind This pressure changes with altitude and with weather conditions. Wind is the movement of air from a high pressure zone toward a low pressure zone of depression.