Fluids
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A fluid is a liquid or gas. Fluids are different than solids in that
they have a flexible shape. Pressure applied in a fluid has different
characteristics, compared with pressure applied by a solid. Also, some objects
can float in a fluid.
Questions you may have are:
·
What is the flexible shape of a fluid?
·
How is fluid pressure different than solid pressure?
·
How does an object float in a fluid?
This lesson will answer those questions.
Shape of
fluids
Fluids have a flexible shape and somewhat take the shape of their
containers.
Gases
A gas will often mix with other gases. If a gas is heavier or more dense than other gases, it will sink to the bottom and
take the shape of the container.
An example of that happened a few years ago in
A gas in a closed container will completely take the shape of the
container.
Liquids
Liquids settle and take the shape of the bottom of its container. The
top of the liquid is relatively flat, except for waves or molecular effects.
An interesting experiment is to fill a glass of water to the top. Then
carefully drop pennies into the water. See how many you can drop in the water
until it spills over the sides. You will be amazed how many coins you can drop
in, as the top surface of the water bulges. The reason is that the surface
tension or molecular attraction of the water and the surface of the glass
result in this unusual phenomenon.
Liquids usually don't mix well and the heavier or more
dense liquid will sink to the bottom. The shape of the upper surface is
determined by the surface tension of the two liquids with respect to the
container material.
Fluid
pressure
The pressure at any point in an open container filled with a fluid is a
result of the weight of all the fluid above that point. for
example, air pressure is determined by the weight of all the air above you. If
you are under water, the water pressure is the sum of all the water above you
(plus the air above the surface of the water).
The pressure of a fluid in a closed container can be increased by
causing the fluid to expand by heating it or a chemical reaction, or the
pressure can be increased by the use of a some
external pressure--like a piston in a cylinder.
Fluid pressure at a given point is the same in all directions.
Floating
The displacement of a fluid by an object helps to determine if it will
float or not.
Displaces
volume
When a solid object is placed in a fluid, it displaces its volume of the
fluid material. For example, if you placed 1 cubic meter (1 m3)
object in a full container of water, 1 cubic meter of water would flow over the
sides of the container. This is true for any fluid.
Weighing
object in fluid
If you tried to weigh the object, when it was submerged in the fluid, it
would weigh its weight in a vacuum minus the weight of the fluid that it
displaced.
The density of a material is its mass per unit volume. The ratio of the
density of an object compared with the density of water is defined as the
material's specific gravity.
Floating
If the specific gravity of the material is greater than 1, the object
will sink in water. If it is equal to or less than 1, it will float in water.
When an object floats in a fluid, the weight of the fluid displaced
equals the weight of the object. In other words, the weight of the water a
wooden block that floats 1/2 above the water surface equals the weigh of the
block. If you add a weight to the top of the block, an equal weight of water
will be displaced.
In
conclusion
A fluid has a flexible shape, usually of its container. Fluid pressure
is often due to gravity and the weight of the fluid above. The pressure at any
point is in all directions. Objects displace their volume in a fluid. The differences of density determines whether an object will
sink or float in the fluid.
Mini-quiz
to check your understanding
1. What is the shape of a falling water drop?
Top of Form 1
a) It has no shape
b) Somewhat spherical
c) The shape of its container
Bottom of Form 1
2. What usually causes the pressure in an
explosion?
Top of Form 2
a) A chemical reaction heats
and expands a gas
b) Gunpowder or dynamite
expands
c) Gravity
Bottom of Form 2
3. If you sunk a 2 cubic foot, 10 pound
concrete block in a bucket of water, how much water would it displace?
a) 2 cubic feet of water
b) 10 pounds of water
c) It depends on how much
overflows
Bottom
of Form 3
Pressure in Fluids
Pressure is a measurement of the force per unit area. Since a fluid is a
liquid or a gas, its pressure applies in all directions. Fluid pressure can be
in an enclosed container or due to gravity or motion. The pressure can also be
amplified through hydraulic mechanisms and changes with the velocity of the
fluid.
Questions you may have about pressure in fluids are:
·
How does gravity cause fluid pressure?
·
How does air and water pressure apply in all
directions?
·
What are other applications of fluid pressure?
Fluid
pressure from gravity or motion
The weight of a fluid can exert a pressure on anything underneath it. Also,
the relative movement of a liquid or gas can apply a pressure.
Pressure
Pressure (P) is defined as
force (F) divided by the area (A) on which the force is pushing. You
can write this as an equation, if you wanted to make some calculations:
P = F / A
An object can exert downward pressure due to its weight and the force of
gravity. The pressure you exert on the floor is your weight divided by the area
of the soles of your shoes. If the force is due to the weight (W) of the
object, the equation is then:
P = W / A
Water
pressure
The water pressure at the bottom of a lake is equal to the weight of the
column of water above divided by the area of that column.
Column on
top of head
If you were standing on the bottom of a swimming pool (assuming you
would not start floating), there would be a column of water the diameter of
your head all the way up to the water surface, pushing down on you. If you took
that column of water and weighed it, and then divided that weight by the area
of the top of your head, you would get the value for the water pressure.
Demonstration
with can
A demonstration of how water pressure increases with the depth of the
water can be done with a large tin can. Punch nail
holes in a vertical line up the side of the can every inch or several centimeters. Then fill the can with water. The water may
just dribble out the top holes, but the increased pressure with depth causes
the water to squirt out with more pressure at the bottom holes.
Air
pressure
Likewise, the air pressure on the top of your head is the weight of the
column of air (which is several miles high) divided by the area of the top of
your head. The average air pressure on your head is 16 pounds per square inch! That
is a lot of weight you are holding up.
Air
pressure in weather
A high pressure in the weather is caused by a higher column of air than
a low pressure. A barometer measures the air pressure or the weight of the
column of air.
Air pressure is due to the weight of all the air going several miles up
above you. It is approximately 16 pounds per square inch in all directions on
your body. Fortunately, our bodies have internal pressure which equalizes the
air pressure.
Balloons
The air pressure inside a balloon pushes outward in all directions. The greater the pressure, the bigger the balloon, until it finally
bursts. The internal air pressure is much greater than the external air
pressure.
Different
altitudes
The normal air pressure in Manzaneda, Ourense is less than in Noia, A Coruña. This is because the higher altitude of Manzaneda means its column of air is not as high
as in Noia.
Since many snacks are sealed in pressurized bags, a bag sealed in Noia requires higher internal pressure than
one made in
Direction
of fluid pressure
Now, what is different about pressure caused by a liquid, or gas is that
not only is there pressure pushing down at a given point, but there is also the
same pressure pushing up and to the sides.
All
directions
The pressure is the same in all directions in a fluid at a given point. This
is true because of the characteristic of liquids and gases to take the shape of
their container.
What this also means that any hollow container submersed in a liquid has
pressure on every square inch of its surface, top and bottom.
Swimming
under water
When you swim under water, the pressure of the water gets greater on
your body, the deeper you get. Now, the question is: "Why aren't you
crushed by all this weight?"
The reason is that your body compensates by creating an internal
pressure that is equal to the air or water pressure. You are somewhat like a
balloon filled with fluids under pressure. Now, when you go very deep under
water, the water pressure may get greater than your body can compensate for,
and you get uncomfortable.
Other
pressure effects
Other effects of fluid pressure are motion, heating and chemical
effects, as well as applications in the field of hydraulics and in aircraft.
Wind and
current
The movement of a fluid, such as with wind or the
current of a river can apply a pressure to an object in its way proportional to
the surface area perpendicular to the direction of motion.
Streamlining the object reduces this pressure.
Heating
and chemical effects
When you heat a fluid, it usually expands. If you heat a fluid that is
in an enclosed container, the expansion will result in greater internal
pressure. For example, heating a balloon will cause it to expand.
Likewise, chemical reactions that give off gases will increase the
pressure inside the container. For example, shaking a carbonated drink bottle
releases more gas and will result in greater internal pressure. This can be
experienced when you open the bottle and the drink squirts all over.
Hydraulics
When a fluid--especially a liquid--is in a partially closed container, a
force applied in one area can result in a greater force in another area. This
effect is used in hydraulics to create a mechanical advantage by having the
force applied to a small piston resulting in a greater force applied to large
piston.
Aircraft
The scientist Bernoulli discovered that the air pressure in a tube goes
down when the velocity of the air in the tube increases. This discovery became
known as Bernoulli's Principle.
The greatest application of this principle is used in airplanes. The
wing of an airplane is usually curved on top and flat on the bottom. When the
air moves over the curved top portion of the wing, it speeds up because of the
shape. This lowers the pressure with respect to the bottom part of the wing. Lower
pressure on the top results in the lift required to keep the airplane aloft.
In
conclusion
Fluid pressure from gravity is the weight of the fluid above divided by
the area it is pushing on. Fluid pressure applies in all directions. Internal
pressure of an object equals the external fluid pressure,
otherwise the object could be crushed. Wind and heating can also create
pressure.
Mini-quiz
to check your understanding
1. About how much pressure does your brain exert on your skull?
a) 16 pounds per square inch
b)There is no pressure
inside the skull
c) There is no brain inside my
skull
Bottom of Form 1
2. Why do your ears
sometimes pop when flying in an airplane?
a) Increased air pressure at
high altitudes cause the popping
b) Cheap headsets cause them
to pop
c) Your body is compensating
for the decreased air pressure
Bottom of Form 2
3. What would happen if you put a balloon in the refrigerator?
a) It would get smaller due to
increased internal pressure
b) It would get larger due to
reduced internal pressure
c) It would get smaller due to
reduced internal pressure
How Objects Float in Fluids
There are a number of factors related to whether an object will float or
sink in a fluid. Objects placed in a fluid displace their volume of the fluid. If
the density of the object is less than the density of the fluid, the object
will float. Density is also related to specific gravity of the object. The
weight of the object in a fluid is less than its weight outside the fluid.
Questions you may have about floating are:
·
What is displacement?
·
What is density and specific gravity?
· How do things float?
Displacement
When a solid object is placed in a fluid, it displaces its volume in the
fluid. In other words, it takes the place of a volume of the fluid equal to its
own volume.
For example, if you placed 1 cubic meter (1 m3) object in a full
container of water, 1 cubic meter of water would flow over the sides of the
container. Of course, if you put the object in a container that won't overflow,
the level of the fluid will rise, according the the
added volume.
The displacement principle is a factor in why certain objects float. It
is also handy in being able to find the volume of irregularly shaped objects.
The volume or amount of space an object takes up is related to its
density.
Density
and specific gravity
The density of a material is how closely packed its matter is and is
represented as its mass per unit volume. Specific gravity is a comparison of
the density of a material with that of water.
Density
Density of a material is a combination of the atomic weight of its atoms
and how tightly packed the atoms or molecules are.
Different
natural densities
Different elements and materials have different natural densities. Water
is more dense than machine oil. Oxygen is more dense than Helium. Although 100 pounds of feathers may
take up much more room than 100 pounds of steel, they both still weigh 100
pounds. The steel is heavier for its size, due to the fact that it is a denser
material.
Comparison of the density of a solid with the density of a fluid will
determine if the object will float in the fluid.
Measuring
density
The density (d) of a material is its mass (m) divided by its volume (V).
The equation for density is:
d = m / V.
Thus, a material such as feathers takes up much more room (volume) than
a denser material such as steel, for the same mass or weight.
The density of water in the metric system is 1 by the definition of the
units: 1 cubic centimeter of water weighs 1 gram. Thus
d = m/V = 1 gram / 1 cc = 1.0.
Density
same on the Moon
Since density is related to the mass of an object, the density of a
given volume of lead would be the same on both the Earth and the Moon, although
the lead would weigh less on the Moon because of the lower gravity there.
In English
system
In the English system of measurement, a pound is really a unit of weight
on Earth and not mass. But we can still use pounds to get an idea of density.
Used to
determine materials
If you know the natural density of a pure element, and you know the
weight and volume of an object, you can calculate its density and thus
determine if it is a pure element.
Story of
Archimedes
The ancient Greek philosopher
and scientist, Archimedes was asked by the king to determine if a gold statue
he had was 100% gold. Since it was an odd shape, Archimedes could not simply
measure the volume to determine the object's density and thus its composition.
Archimedes decided to take a
hot bath, to help him think about this problem. When he got in the bath tub, he
noticed the water rise. This clue led to the discovery that an object will
displace its volume when immersed in a liquid. When Archimedes realized that
objects displace their volume in water, he excitedly jumped out of the tub and
ran down the streets shouting, "Eureka! Eureka!" which means, "I
have found it!" Unfortunately, he didn't notice that he forgot to put his
clothes on!
When Archimedes put the statue
in a container full of water, he measured the volume of the overflow to
determine the volume of the statue. Then he measured the weight of the statue
and compared its density with the known density of pure gold. He discovered
that the statue was not made of pure gold, rather it
contained some other metal, like lead.
Specific
Gravity
Specific gravity is often used instead of density when comparing
materials with water.
The ratio of the density of an object compared with the density of water
is defined as the material's specific gravity.
Specific gravity is often used to compare liquids mixed with water. It
is used in medicine. It is also used to determine the amount of acid in your
car battery.
Floating
If you pushed a piece of wood under the water, a force will pull it up
to the top surface, where it will float. Likewise, when you hold a balloon
filled with Helium, you can feel a force pulling the balloon upward.
The force that pushes an immersed object upward is a result of the fluid
pressure from gravity and the difference in densities of the object and fluid.
Fluid
pressure from gravity
At any depth in a fluid, the pressure in all directions is proportional
to that depth. Thus, the water pressure at 10 meters is twice the pressure at 5
meters.
An object under the water will have a downward pressure on its top
proportional to the depth of its top, and it will have an upward pressure on
its bottom, proportional to the depth of the bottom.
Object
sinks
If an object weighed more than an equal volume of fluid--in other words,
its density was greater, then the downward pressure at
its bottom would be the weight of the fluid above plus the weight of the
object.
This force would be more than the upward force, due to only the weight
of the water. Since the downward force is greater than the upward force, the
object would sink.
Concrete
block in water
An example would be a concrete block dropped in the water. The block's
density or specific gravity is greater than that of the water, so the pressure
at the bottom of the block from its weight would be greater than the upward
pressure of the water at the point, and it would sink.
Object
floats
If the object weighed less than the fluid it displaces and was submerged
in the fluid, the upward pressure would be greater than the downward pull of
gravity and it would float.
Wood
floats
Wood is less dense than water. When it is placed in water, the water it
displaces equals its weight but not its volume. Thus the heavier water is
pulled to the bottom by gravity, pushing the wood to the top.
When an object floats in a fluid, the weight of the fluid displaced
equals the weight of the object. In other words, the weight of the water a
wooden block that floats 1/2 above the water surface equals the weigh of the
block. If you add a weight to the top of the block, an equal weight of water
will be displaced.
Why ships
float
Boats or ships made of steel are hollow. The total weight is less than
the water it displaces, and thus the ship will float.
Different liquids can have different densities. For example, since a pint cooking oil is lighter in weight than the same volume
of water, it is less dense than the water. Different gases can also have
different densities. For example, helium is lighter than air, because it is
less dense.
A ship will displace a volume of water that is equal to the weight of
the ship. That is why a loaded cargo ship will sit lower in the water than an
unloaded one. The difference would be equal to the weight of the load.
When
liquids and gases are buoyant
When you add liquids of different densities, the higher density liquid
would tend to settle at the bottom of the container. The denser liquid exerts
more pressure due to gravity, thus pushing the lighter material out of its way.
That is why oil will float on the top of water and why air bubbles also float
up to the top in water.
An interesting observation is that bubbles starting at the bottom of a
lake get bigger as the get closer to the surface. The reason is that the water
pressure is less as they go up. This would also be true for a balloon full of
air placed at different depths in water.
Just as a balloon full of air is much less dense than the water it
displaces, and the weight or pressure of the water pushes the balloon upward to
the top of the water, a balloon full of Helium is lighter than its volume of
air and thus is pushed upward into the atmosphere.
Weighing
object in fluid
If you put an object that did not float in a fluid and weighed that
object when it was submerged in that fluid, it would weigh its weight in air
minus the weight of the fluid that it displaced. (In reality, you should weigh
it in a vacuum, but the difference from weighing it in air is negligible.)
In other words, if you put a brick in a bucket of water that was filled
to the brim and weighed the brick in the water, the measured weight would equal
the weight of the brick in air minus the weight of the water that overflowed
from the bucket.
In
conclusion
Submerged objects displace their volume in the fluid. The density of an
object is its mass per unit volume. Objects float when their density is less
than the fluid. It is because the downward pressure of the object's weight is
less than the upward pressure of the fluid at that depth.
Mini-quiz
to check your understanding
1. What fluid must you
use to measure the volume of a solid object?
Bottom of Form 1
2. How could
Archimedes tell the statue wasn't pure gold?
Bottom of Form 2
3. Why would a brick weigh less when submerged in water?
a) The upward pressure on its
bottom surface would reduce the measured weight
b) You are also weighing the
water when you weigh the brick
c) Bricks float, so they weigh
less
Bottom
of Form 3