Characteristics of Fluids

The states of matter are solid, liquid, gas and plasma. A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. This is because they have common properties that are distinct from solids. A fluid does not have a specific shape as does a solid. Instead, fluids take the shape of their containers. They also will flow or pour when under the influence of a force such as gravity.

Questions you may have include:

• What is the natural shape of fluids?

• How do fluids take the shape of their containers?

• How do fluids flow?

Natural shape
Solids have specific shapes because the molecular forces holding particles in place are stronger than the kinetic energy of the molecules. Usually, the molecules just vibrate in place, with little or no other movement.

On the other hand, fluids exist at higher temperatures and thus their particles have greater kinetic energy. The shape of a fluid adapts to its environment or container.

Liquids
A liquid in space will form the natural shape of a sphere. This is because the attraction between its atoms or molecules is greater than the forces from their kinetic energy moving outward. A sphere is a shape with the smallest surface area for a given volume of material.

A liquid sphere or drop of liquid—such as water—that is falling toward the Earth through the atmosphere will be a slightly flattened sphere, due to the air resistance.

If you spill some water on the floor, it will splash and spread out on the floor. Liquids like thin oil will spread out even more than water on the floor.

Gases
The molecules in a gas have more energy than when the material is in the liquid state, such that they overcome the molecular forces. A gas in space or in the atmosphere will continually spread in a shapeless form.

A gas that is heavier than air may gravitate toward the floor, where it then spreads out.

The rate that the gas expands is a function of its temperature or kinetic energy of its particles.

Plasmas
A plasma is an ionized gas, usually at extremely high temperatures. That means some of its electrons have been stripped off. Plasmas have most of the same properties as gases.

Shape in container
Under the influence of gravity, a fluid will take the shape of its container, provided the volume of the container is greater than or equal to the volume of the fluid.

Liquids
If you pour a liquid into a container, it will take the shape of the container, provided none overflows. Under the influence of gravity, a liquid will stay in an open container, such as a cup.

If the container is filled to the top, the volume of the liquid will equal the volume of the container. This fact has been used to measure the volume of an irregularly shaped container or flask.

Gases
A gas will take the shape of its container too. If the container is open and the gas is heavier than air, it will stay in the container for a while.

For example, Chlorine gas (Cl2) is one of the few gases that is colored. If you pour it into a container, you will see the light green gas take the shape of the container. But the high energy of the gas molecules will result in it slowly dissipating into the air.

Usually, gases are put in closed containers. Since gases tend to spread, and since the rate of spreading is proportional to the temperature of the gas or the kinetic energy of its particles, there is a constant pressure on the walls of the container in all directions. This pressure increases with increased temperature or reduced volume of the container.

Plasmas
Because of their high temperature, plasmas are seldom placed where they could take the shape of the container.

Flowing
The major feature of a fluid is that it flows when acted upon by some force. This makes a fluid different than a solid, which may be distorted by a force but will not start to flow. Typically, the force is that of gravity, but other forces can also apply.

Gravity
Fluids under the influence of gravity will flow or can be poured. You certainly have poured liquids from one container to another. Gases also can be poured. Since plasmas are typically very hot, they are seldom poured.

Although, you cannot see carbon dioxide (CO2), you can demonstrate pouring it from one jar to another. This is shown by using dry ice to fill a jar with CO2 and then pouring it into a jar containing a burning candle. The candle flame will be snuffed out as the invisible CO2 is poured into the jar.

Other forces
The forces caused by the acceleration, deceleration or change in direction of a moving container can cause the fluid to flow or change its shape.

The force of wind on a body of water will cause the water to flow, as well as to create surface waves.

Summary
A fluid is a subset of the states of matter, consisting of liquids, gases and plasmas. They have common properties that are distinct from solids. Fluids do not have a specific shape as do solids. Instead, fluids take the shape of their containers. They also will pour when under the influence of a force such as gravity.

Pressure in Fluids

Pressure is a measurement of the force per unit area. Fluid pressure can be caused by gravity, acceleration, or forces in a closed container. Since a fluid has no definite shape, its pressure applies in all directions. Fluid pressure can also be amplified through hydraulic mechanisms and changes with the velocity of the fluid.

Questions you may have include:

• 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 acceleration
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 is defined as force divided by the area on which the force is pushing. (See the lesson on Pressure for details.) You can write this as an equation, if you wanted to make some calculations:

P = F/A

where

P = pressure
F = force
A = area
F/A = F divided by A

Pressure due to gravity
Since the weight of an object or material is equal to the force it excerts due to gravity,

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 on your head.

The reason it does not affect you is that your internal body pressure increases to neutralize most of the water pressure. But at greater depths, the water pressure can become so great that it can harm the diver.

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
When weather report indicates high pressure, that means the column of air reaches up higher than it does for a low pressure reader. 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 that equalizes the air pressure.

Balloons
The air pressure inside a balloon pushes outward in all directions. When the pressure increases, the size of the balloon increases, until it finally bursts. The internal air pressure is much greater than the external air pressure.

Different altitudes
The normal air pressure in Denver, Colorado is less than in This is because the higher altitude of Denver means its column of air is not as high as in Milwaukee.

Since many snacks are sealed in pressurized bags, a bag sealed in Milwaukee requires higher internal pressure than one made in Denver. Thus a Milwaukee bag of snacks will expand when brought to the lower air pressure of Denver and could even explode.


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.

Summary
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

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How Objects Float in Fluids

At any depth in a fluid there is an upward force due to the effect of gravity on the fluid. This results in a pressure applied over an area. If the density of an object in the fluid is greater than the density of the fluid, the object will sink. If the density is less than that of the fluid, the object will float upward due to the buoyancy from the fluid. An object of lower density will float to the top and only be submerged by an amount according to the ratio of the densities.

Questions you may have include:

• What is the role of pressure at a depth in a fluid?

• How does the density of the object determine whether it will float?

• How do things float on the surface?

Pressure at a depth
The fluid pressure due to gravity at a given depth is caused by the weight of the column of fluid—such as water—above that depth level. It is the same in all directions.




An object submerged in a fluid displaces its volume of the fluid. The upward force, or buoyancy, depends on the difference in densities between the fluid and object. Density equals the mass divided by the volume.

Container of water
The upward and downward forces acting on a container of water that is submerged in water are equal.



Smaller density
On the other hand, if an object that was less dense than the fluid is submerged, the total weight of the column above a given depth will be less than that of the fluid.

For example, a piece of wood is typically less dense than water. Submerging the wood in the water results in an upward force—or buoyancy—that is greater than the downward force. The piece of wood tends to float upward.



Another example concerns a balloon filled with helium. Since the density of helium is less than that of air, the buoyant force pushes the balloon upward. That is, the upward pressure on the bottom of the balloon is greater than the downward pressure due to the weight of the balloon and column of air above it.

Floating on surface
An object with less density than the fluid will float upward until it reaches the surface of the fluid. At that position, only part of the object is submerged.




How much is submerged depends on the density of the object, as compared to the fluid. The equation is

ρfVs = ρoVo

where:

ρf is the density of the fluid
Vs is the volume submerged
ρo is the density of the object
Vo is the volume of the whole object
ρV is ρ times V


Floating wood
For example, consider a block of pine wood floating in water. Pine has a density of 0.53 gm/cm3. Pure water has a density of 1.0 gm/cm3. Thus 1.0*Vs = 0.53* Vo. The volume submerged would be 0.53 times the total volume of the object.

If the block of pine had a volume of 100 cm3, then slightly over 1/2 of it would be below the waterline. You can find the percentage by multiplying by 100%. Thus, 53% would be below the waterline and 47% would be above the waterline (100% - 53% = 47%).

Metal ship
Although the density of steel is 7.86, which is much greater than the density of water, a ship made of steel will float. The reason is because the boat is hollow and not made of solid steel. Measuring the weight of the steel and dividing by the total volume of the ship will result in a density less than 1.0, the density of water. In fact, often the density of a ship is about 0.3 gm/cm3 before it is loaded with goods.



Summary
There is an upward force due to the effect of gravity at any depth in a fluid. If the density of an object in the fluid is greater than the density of the fluid, the object will sink. If the density is less than the fluid, the object will float. An object will float to the top and only be submerged by an amount according to the ratio of the densities.

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Applications of Fluid Principles

There are several major applications of the special properties of fluids. The pressure of fluids can be amplified through the use of hydraulic mechanisms. Changes in pressure with the velocity of the fluid allow airplanes to fly. Fluids are also used to reduce friction.

Questions you may have include:

• How can pressure by amplified through hydraulic mechanisms?

• How do airplanes fly?

• How is friction reduced?

Hydraulics
Hydraulics is the scientific field that concerns fluids--usually liquids--that are in partially enclosed containers, such that you can apply pressure in one area. An example is a cylinder with a piston.

Pressure from single piston
If you have a cylinder filled with a liquid and apply a force to a piston on one end of the cylinder, the pressure (P) on the walls of the cylinder equal the force (F) divided by the area (A) of the piston in the cylinder.

P = F/A

Pressure on second piston
Now, if the first cylinder was connected to a second cylinder of larger diameter, the pressure inside that cylinder would be the same P, but the force F2 applied to the larger piston would now be:

P = F2/A2

Pressure is the same
The pressure for both is the same. Thus,

F2/A2 = F/A

or

F2 = (FA2)/A

For example, if F = 100 pounds and A = 5 square inches, then P = 20 pounds/square inch. P is the same on both pistons.

Force greater on large piston
If the larger piston had an area of A2 = 25 square inches, and the pressure remained at P = 20 pounds/square inch, then the resulting force on that piston would be:

F2 = (FA2)/A = (100 x 25) / 5 = 500 pounds.

This is a mechanical advantage, similar to that seen with levers.

Used in brakes
Hydraulic mechanisms are used in the brakes in your car. The force applied on the brake pedal is multiplied on the brake drums. Another use is to jack up a heavy item, like a truck.

Velocity reduces pressure
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.

Used by airplanes
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. This shape is called the airfoil. When the air moves over the curved top portion of the wing or airfoil, 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.

The principle is so simple, but not very obvious.


Flying upside-down
But if the airfoil gives lift, how can an airplane fly upside down?

If the airplane is going fast enough, other factors influence the lift. When the plane is upside-down, it is really flying at a slight angle, so it is going slightly upward to compensate for the loss of lift.

Some airplanes--such as an airliner--can have great difficulty flying upside down. Usually only smaller stunt planes and military craft can do this maneuver.

Friction reduced
Solids can have rough surfaces. Even microscopic roughness can result in a substantial resistive force of friction when two solids are rubbed together, as well as wear on the parts.

Fluids offer little resistance
On the other hand, a fluid does not have a rough surface and rubbing a solid along a fluid results in little resistive force. Instead of friction, the resistance is due to the thickness or viscosity of the liquid, which affects its ability to move and change its shape.

Used as lubricants
The reduction of friction of two solids can then be achieved by separating them by a layer of a fluid, so the solid surfaces are not in direct contact. This is called lubrication. Water can be used as a lubricant, but it also evaporates quickly. Oils are typically used to lubricate parts and prevent friction, as well as excessive wear from the friction. In some small, high-speed parts, such as the hard-drive of your computer, air is used as a lubricant.

Summary
Hydraulics use fluid pressure to create the same mechanical advantage as a lever. The Bernoulli Principle allows airplanes to fly from the lift created by reduced air pressure on the top of their wings. Fluids also can be used to reduce friction.



regards

faryal shah

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