Properties of Fluids
SCI 8: Fluids Unit
Curriculum Outcomes Addressed:
- Describe the movement of objects in terms of balanced and unbalanced forces (309-2)
- Describe the science underlying hydraulic technologies (111-5)
- Explain quantitatively the relationship between force, area, and pressure
• We will study the following properties of fluids:
1. Density
2. Buoyancy
3. Displacement (Archimedes’ Principle)
4. Viscosity
5. Pressure (Pascal’s Principle)
Key Terms
• Pressure: A measure of the distribution of force
over a given area
P = _F_
A
• Force: Strength or energy as an attribute of physical
action or movement
• Surface Area: Total space or area covered by the
outside of an object
• Pascal’s Principle: An external force exerted on a
confined fluid is distributed evenly in all directions
inside the surface area of the container
Pressure - Pascal’s Principle
• Pascal’s Principle states that “any change in pressure
applied to a fluid in a confined space is transferred
unchanged throughout that object”.
• In other words, a fluid within a closed space exerts
an equal amount of force (pressure) into all
directions.
• A good example of Pascal’s Principle
can be observed in hydraulics in
everyday life and in Cartesian divers.
DEMONSTRATION
Pascal’s Principle Example #1: Cartesian Diver
Originally, the cartesian diver
(pen cap with clay) is positively
boyant (floating at the top).
When the bottle is squeezed,
the water exerts pressure in all
directions, and the cap
becomes negatively buoyant,
sinking to the bottom.
When the bottle is released,
the pen cap becomes neutrally
buoyant again, rising to the
middle of the bottle.
How the Cartesian Diver Works
• There is air inside the diver (pen cap), making it positively
buoyant so it floats at the top.
• When the bottle is squeezed, pressure is exerted on the
water. This pressure is exerted in all directions, and it
increases the pressure on the air bubble inside the diver
(pen cap). The air inside the diver (pen cap) compresses
and reduces in volume, allowing more water to enter the
diver (pen cap). The diver now becomes more dense and
negatively buoyancy, so it sinks.
• When the pressure on the bottle is released/let go, the air
inside the diver (pen cap) expands again, making it less
dense, and allowing it to become positively buoyant
therefore rise and float at the top once again.
Pascal’s Principle Example #2: Hydraulics
Hydraulics are technologies, tools, or machines, that are operated
by a liquid moving in a confined space under pressure
Hydraulics: Force, Pressure, and Area
• Force: Strength or energy that causes change
• Pressure: Force per unit Area; The continuous
physical force exerted on or against an object
• Area: The amount of space occupied by
something
Pressure = Force
Area
P = _F_
A
Force, Pressure, and Area cont’d
• As area increases, pressure decreases, because the
force is spread out over a bigger space/area
• For example…
In the first example, the pin is pushed into the wood. In the second
example, the pin is pushed into the finger. The reason for this is the
difference in pressure. Assuming the same force is applied, each
case would have a different pressure acting on the thumb. In the
first diagram the thumb pushes on a large area so the force is
spread out and the pressure is low. In the second diagram the force
is concentrated on a small area so the pressure is much higher.
Hydraulics: Force, Pressure, and Area cont’d
• Changing the amount of area (space) covered by
something will change the force exerted
• As area increases, pressure decreases, because the
force is spread out over a bigger space/area
• So, a bigger area will require less force to lift
something on the other side of a hydraulic (since
force is exerted in all directions equally in a fluid)
Pressure = Force
Area
P = _F_
A
Hydraulics: Force, Pressure, and Area cont’d
F = Force
• PHET SIMULATION of Pressure, Temperature, etc.
http://phet.colorado.edu/en/simulations/categor
y/physics/work-energy-and-power AND Spangler
Science Bed of Nails/ Balloon Video