Properties and Pressure of Fluids

Properties of Fluids

• Fluids are liquids and gases.
• Individual particles (atoms, molecules, ions) of fluids can change positions. This allows them to flow.

Pressure

• Pressure is force exerted per unit area.
• Pressure = Force / Area
• Smaller area = greater pressure for the same force.
• Larger area = smaller pressure for the same force.

Example with Shoes

• Shoes with wide soles distribute weight over a large area.
• Shoes with narrow soles (high heels) exert more pressure on the surface.
• A 100-pound person on an average shoe applies 5 pounds per square inch.
• A 100-pound person on a high heel applies over 400 pounds per square inch.

Consider a thumb tack

• A thumb tack exerts high pressure due to its small contact area.
• Pressure is inversely proportional to the area, given a constant force.

Atmospheric Pressure

• Atmospheric pressure is the force per unit area exerted by the weight of the air above a surface.
• At sea level, air pressure is 14.7 pounds per square inch.
• Air pressure decreases at higher altitudes.
• Air pressure is less in higher elevation; hence, it is harder to breathe in the mountains.

Air Pressure Varies with Elevation

• Air pressure varies with altitude (elevation).
• Sea level has higher air pressure compared to higher elevations, such as the highest mountain in the US (Mt. McKinley) or Mt. Everest.

Pressure in Fluids

• Pressure in fluids increases with depth.
• Deeper the fluid, higher the pressure.
• Water exerts greater pressure at the bottom of the tin can compared to the top.

Why does pressure increase with depth?

• Gravity pushes particles down.
• Deeper depths have more fluid above the object.
• This pushes down with a greater force causing a greater pressure.

The Bends

• The bends occur when divers experience rapid decompression.
• Nitrogen in the blood creates bubbles that can cause pain and death.

Some Common Units of Pressure

• Pascal (Pa) = 1 N/m² (SI unit)
• Pounds per square inch (psi) = 6,891 Pa
• Bar = 105 N/m² ≈ 1 atm
• Torr = 1 mm Hg ≈ 1 kPa
• 1 atm = 101.3 kPa = 760 mm Hg = 29.92 in Hg = 14.70 psi

Pressure in Confined Fluids (Pascal's Principle)

• In confined fluids (neglecting gravity) pressure is the same throughout the container.
• Pressure equals force divided by area; a small force applied to a small area can produce a large force on a larger area.
• Pascal's principle is fundamental to hydraulic systems using fluids.

Hydraulic Systems

• Hydraulic systems use Pascal's Principle to multiply forces.
• A small force on a small area produces a large force on a large area.
• Hydraulic brakes, pneumatic jacks and a car's braking systems are application types of hydraulic systems.

Hydraulic Brakes

• Hydraulic brakes transmit force through hydraulic fluids in a system.
• This uses pistons to transmit pressure from the master cylinder through to the brake shoes.

Pressure-Force Problems

• Pascal's principle states that pressure applied to a confined fluid is transmitted equally throughout the fluid.
• Force exerted on one part of a hydraulic system can be calculated if pressure is constant and the area of another part is known.
• The pressure applied to one side of a hydraulic system is transmitted proportionally to the other side given equal cross-sectional areas.

Practice Problems

• Problems involved calculating force required to raise an object of a particular mass.
• Calculating force in a hydraulic system given dimensions by using Pascal's principle.

One for the Road

• The exerted force on a smaller piston to lift a car in a hydraulic lift.
• Calculating force in a hydraulic system given dimensions by using Pascal's principle.

Fluid Behavior and Buoyancy

• Pressure from a fluid increases with depth.
• The net upward force on an object submerged in a fluid is buoyant force.
• The buoyant force is equal to the weight of the displaced fluid.

Buoyancy

• The ability of a fluid (liquid or gas) to exert an upward force on objects immersed in it.
• Positive, Negative or Neutral Buoyancy conditions. Floating, Hovering, and Sinking of objects.

Archimedes' Principle

• Immersed partially or totally in a fluid.
• The fluid exerts an upward force on the object, which is equal to weight of the displaced fluid.

Sink or Float Problems

• Whether a substance sinks or floats depends on density and buoyancy.
• Density of object compared to density of the fluid. This condition determines whether object floats or sinks.

Why objects sink or float in fluids.

• Density and buoyancy are two factors that influence whether an object sinks or floats.

Viscosity

• A fluid's resistance to flow.
• Internal friction between fluid particles.
• High viscosity fluids such as honey flow slowly.
• Viscosity decreases with increasing temperatures and vice versus.

Bernoulli's Principle

• As the velocity of a fluid increases, the pressure exerted by the fluid decreases.
• Higher velocity equates to lower pressure in a fluid.

Fluids in Motion - The Bernoulli Effect - Applications

• The Bernoulli effect is applied to a variety of situations, including vacuum cleaners and airplane wings.
• Air moving faster over the top of an object creates a lower pressure area beneath the object.

Explore the fundamental properties of fluids, including definitions of pressure and its relationship to area. Through examples like shoes and thumb tacks, understand how pressure varies with surface area. This quiz delves into atmospheric pressure and practical applications of these concepts.