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Solids & Fluids
Transcript of Solids & Fluids
As you know, all matter is made up of tiny atoms and molecules. In a solid, the atoms and molecules are tightly packed and stay in place. This is why solids maintain their shape.
Different solids have different properties (e.g. some conduct heat better than others) as a result of the behavior of their atoms and molecules.
Properties of Solids
Different Characteristics of Matter
We use different characteristics to determine and distinguish different types of matter. For example, they melt and boil at different temperatures, they may have different colors or odors. Some can stretch without breaking while others shatter easily.
Characteristics that you can observe directly are called physical properties. It may help to think about these as properties you can observe with your senses: sight, smell, taste, and touch. Physical properties include color, texture, density, brittleness, and state.
Identifying Substances with
Gold is shiny, exists as a solid at room temperature and can be pounded into very thin sheets
Mercury is shiny, but exists as a liquid at room temperature.
A physical shape is any change in the size, shape, or phase of matter in which the identity of a substance does not change.
Chemical properties are properties that can only be seen when one substance changes into a different substance. A chemical property of iron is that it reacts with oxygen in the air to form iron oxide (rust).
Any change that transforms one substance into a different substance is called a chemical change.
Density is a physical property we learned about earlier this year. Density is the ratio of a material's mass to volume. The units we use for density is grams/cubic centimeter or g/cm.
Earlier in the year we measured volume by using the water displacement method. Remember, 1 milliliter (mL) of volume is the same as 1 cubic centimeter (cm ).
Density of Solids
Solids have wide ranges of density. One of the densest metals is platinum with a density of 21.5 g/cm. A ring of platinum has 3 times as much mass as a ring of the exact same size made of steel. Rock has a much lower density than metals, between 2.2 and 2.7 g/cm and wood is even less ranging from 0.4 to 0.6 g/cm.
Density of Liquids & Gases
The density of water is 1.0 g/cm and many common liquids have densities between 0.5 and 1.5 g/cm. The density of air and other gases is much lower. Gases have low densities because the molecules are far away from each other.
Why Density Varies
The density of a solid material depends on two things. One is the individual mass of each atom or molecule. The other is how closely the atoms or molecules are packed together.
How Tightly Packed are your
Diamond is made of carbon atoms and has a density of 3.50 g/cm. The carbon atoms in diamond are relatively close together. Paraffin wax is also mostly carbon but the density of paraffin is only 0.87 g/cm. The density of paraffin is low because the carbon atoms are mixed with hydrogen atoms in long molecules that take up a lot of space.
Crystalline & Amorphous Solids
Solids can be classified in two groups: Crystalline and Amorphous. The solids falls into each group according to their molecular and atomic arrangement.
Diamond & Silver
are Crystalline Solids
Wax & Glass
are Amorphous Solids
Most solids on Earth are crystalline. The particles of a crystalline solid are arranged in an orderly, repeating pattern. Salt is a great example of a crystalline solid. When seen under a microscope, salt has a cubic shape. This is because of the cubic arrangement of the sodium and chlorine atoms.
Other Crystalline Solids
Crystalline solids include salts, minerals, and metals. Metals don't look like crystals because solid metal is made from very tiny crystals fused together in a jumble of different orientations.
The word "amorphous" comes from the Greek for "without shape". Unlike crystalline, amorphous solids do not have a repetitive pattern in the arrangement of molecules or atoms. They are randomly arranged. While amorphous solids will hold their shape, they are often softer and more elastic than crystalline solids.
Rubber is an amorphous solid that can be made soft and elastic
Other Amorphous Solids
The reason amorphous solids are often softer and elastic than crystalline is because a molecule in an amorphous solid is not tightly connected to as many neighboring molecules as it is in a crystalline solid.
Glass is an amorphous solid, but is hard and brittle. This is because it is made from molten silica crystals that are cooled quickly. The rapid cooling leaves the silica molecules in random arrangement.
Mechanical Properties of Solids
When you apply a force to an object, the object may change its size, shape, or both. The concept of strength describes the ability of a solid object to maintain its shape even when force is applied.
The strength of an object can be determined based on two questions:
How much does the object bend or deform under an applied force?
How much force can the object withstand before it breaks?
Tensile strength is a measure of how much stress from pulling, or tension, a material can withstand before breaking. Strong materials like steel have high tensile strength, but materials like wax and rubber have low tensile strength. Also, any material that is brittle will also have low tensile strength.
High Tensile Strength
Low Tensile Strength
Hardness measures a solid's resistance to scratching. Diamond is the hardest natural substance found on Earth. Geologists sometimes classify rocks based on hardness.
Elasticity describes an solid's ability to be stretched and then return to its original size. This property also gives objects the ability to bounce and to withstand impact without breaking.
Brittleness is defined as the tendency of a solid to crack or break before stretching very much. Glass is a great example of a brittle material. You cannot stretch glass even one-tenth of a percent before it breaks.
If this is the case, how on Earth do they make glass into so many different shapes?
A ductile material can be bent a relatively large amount without breaking. For example, a steel fork can be bent in half and the steel does not break but a plastic fork will crack when it has been bent only a little.
One of the most useful properties of metals is their ductility. The ductility of metals, like copper, allows them to be formed into wire.
Malleability measures a solid's ability to be pounded into thin sheets. Aluminum is a highly malleable metal. Aluminum foil and beverage cans are two good examples of how manufacturers take advantage of the malleability of aluminum.
When the temperature increases, the energy and vibrations of the atoms & molecules increases as well. The increased vibrations makes each particle take up a little more space, causing thermal expansion. Almost all solids expand as the temperature increases. Some (like plastic) expand a lot while others (like glass) expand only a little.
All bridges longer than a certain size have special joints that allow the bridge surface to expand and contract with changes in temperature. Without these special joints, the bridge surface would crack.
Real Life Application of Tensile Strength
What are Fluids?
A fluid is defined as any matter that flows when force is applied. Liquids like water are one kind of fluid. Gases, like air, are also fluids. You may notice cool air flowing into a room when a window or door is open, or the smell of someone's cologne or perfume drifting your way. These examples are evidence that gases flow.
Density of Fluids
A piece of pure silver in the shape of a candle holder has the same density as a ring made of pure silver. Size and shape do not change a material's density. But what about its state of matter?
The density of a liquid is the ratio of mass to volume, just like a solid. When the silver ring is melted, its mass does not change. The volume, however, does change.
Why does an object's volume increase
when it becomes a liquid?
The particles in a solid, as you remember, are fixed in position. They are vibrating, but cannot switch places with other particles. The particles in a liquid are less rigidly organized and can slide over and around each other. Because of this ability, they take up a little more space and increase the volume.
Water is an exception to the rule!
Most material will be more dense in solid form than in liquid form for the reasons previously stated. Water, however, is an exception to this rule. Ice is actually less dense than liquid water. When water molecules freeze into ice crystals, they form a pattern that has an unusually large amount of empty space. They are more tightly packed in water's liquid form.
Have you ever tried to cool down a water bottle by putting it in the freezer? What happened if you forgot about it and it froze?
Forces in Fluids
When you push down on a solid object, like a bowling ball, the force is transmitted down in the same direction as the applied force. Think about what happens when you push down on a balloon filled with air. The downward force you apply creates forces that act sideways as well as down. Because fluids change shape, forces in fluids are more complicated than forces in solids.
A force applied to a fluid creates pressure. Pressure acts in all directions (like the arrows in the balloon picture) of the applied force. Pressure is defined as the amount of force exerted per unit area.
Units of Pressure
The units of pressure are force divided by area. Pounds per square inch (PSi) and Pascal (Pa) are the units for pressure.
If your bike tires are inflated to 35 PSi, then a force of 35 pounds acts on every square inch of area inside the tire.
What causes pressure?
On the atomic level, pressure comes from collisions between atoms and molecules. When water is inside a pitcher, the molecules of water move around and bounce off each other and off the walls of the pitcher. The bouncing force is applied
the inside surface of the pitcher.
According to Newton's 3rd Law, a equal and opposite reaction force is exerted
the pitcher. The reaction force is what creates the pressure acting on the inside surface of the pitcher.
Pressure is Potential Energy
Difference in pressure create potential energy in fluids just like difference in height create potential energy from gravity. A pressure difference of one Pascal is the equivalent to one joule per cubic meter. We get useful work when we allow a fluid under pressure to expand.
Everything obeys the law of conservation of energy, even fluids. In addition to kinetic and potential energy, the fluid also has pressure energy. If friction is neglected, the total energy stays constant for any particular sample of a fluid.
Bernoulli's Three Variables
Bernoulli's principle says the three variables of height, pressure, and speed are related by energy conservation. If one variable is increased, at least one of the other two must decrease. For example if the speed of flow increases, pressure goes down.
If the pressure is increased, the speed of flow will decrease.
Using Bernoulli's Principle to Fly!
An important application of Bernoulli's principle is the airfoil shape of wings on a plane. The shape of an airfoil causes air flowing along the top to move faster than air flowing along the bottom. When the speed is reduced, the pressure goes down. When a plane is moving, the top surface of the wing has less pressure and the bottom surface has more pressure. This creates the lift force that supports the plane in the air.
Hydraulics and Pascal's Principle
Hydraulic lifts and other hydraulic devices use pressure to multiply forces and do work. The word hydraulic refers to anything that is operated by a fluid under pressure. Hydraulic devices operate on the basis of Pascal's principle, named after Blaise Pascal.
Pascal's principle states that the pressure applied to an incompressible fluid in a closed container is transmitted equally in all parts of the fluid. An incompressible fluid does not decrease in volume when pressure is increased.
Calculating the Pressure
To show Pascal's Principle mathematically, we can use the following equation for calculating pressure.
When you rearrange the equation to solve for force, you begin to see Pascal's Principle displayed mathematically.
Force = Pressure x Area
The pressure stayed the same in the larger cylinder, but area increased, resulting in a larger output force exerted by the piston. The greater the differences in the areas of the cylinders, the greater the output force exerted by the piston at the larger cylinder.
What is viscosity?
Viscosity is the measure of a fluid's resistance to flow. High viscosity fluids take longer to pour from their containers than low-viscosity fluids.
Viscosity is determined in large part by the shape and size of the particles in a liquid. If the particles are large and have bumpy surfaces, a great deal of friction will be created as the slide past each other. What happens to viscosity when the fluid is heated?
How does a battleship float?
If you drop a steel hexagonal nut into a glass of water, it will sink to the bottom. The steel does not float because it has a higher density than the water. If this is true, how on Earth does a battleship like the USS Alabama ship float when a tiny steel nut sinks? The steel nut has a mass of only 15.5 grams while the battleship has a mass of thousands of tons! The answer has to do with gravity and weight.
Weight & Buoyancy
We all tend to use the terms weight and mass interchangeably, but the two terms mean two different things scientifically. Mass refers to how much matter an object has, while weight is a force caused by gravity. Your mass is constant throughout the universe, but you weight is not!
It is much easier to lift yourself (or a friend) in a swimming pool than to lift yourself on land. This is because the water in the pool exerts an upward force on you that acts in a direction opposite to your weight. We call this force Buoyancy. Buoyancy is a measure of the upward force that a fluid exerts on an object that is submerged.
Pushing a ball under water.
The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. Most of you have played with a beach ball or other inflatable object in a pool or lake. You probably noticed that the further under the water you pushed the ball, the more force you had to use. This is because the more ball (volume) you submerge the stronger the buoyancy force pushing the ball to the surface.
In the 3rd century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of the fluid displaced by an object. We call this relationship Archimedes' Principle.
For example, if a rock with a volume of 1,000cm is dropped into water, it will displace 1,000cm of water, which has a mass of 1 kilogram. The buoyant force on the rock is the weight of 1 kilogram of water or 9.8 N.
"Will it float or not float?
That is the question."
Buoyancy explains why some objects sink and others float. A submerged object floats to the surface if the buoyant force is greater than the object's weight. If the buoyant force is less than the object's weight, it will sink.
Denser objects float
lower in the water.
If you placed a foam block and a same-sized wooden block in water, they both float. However, the wooden block floats lower in the water. This is because the wooden block weighs more than the foam and the buoyant force needed to float the wood is greater. Since the buoyant force must be greater, more water must be displaced causing the wooden block to float lower in the water.
Density & Buoyancy
If you know an object's density, you can immediately predict whether it will sink or float- without measuring its weight. An object sinks if its density is greater than that of the liquid it is submerged into. It floats if its density is less than that of the liquid.
Why one floats & the other sinks
When they are completely underwater, both balls have the same buoyant force because they displace the same volume of water. However, the steel ball sinks because its weight is greater than the volume of water due to its high density. The wooden ball floats because its density is lower than water and its weight is less than the volume of water displaced.
Boats & Average Density
In the last slide, you saw that a steel ball will sink when placed in water due to its higher density (7.8 g/cm). But, right now there are thousands of ships made of steel that are floating around the world? What's the secret? The answer is that average density determines whether an object sinks or floats.
How to make steel float!!!
To make steel float, you have to reduce the average density somehow. Making the steel hollow does exactly that. Making a boat hollow expands its volume a tremendous amount without changing its mass. Steel is so strong that it is quite easy to reduce the average density of a boat to 10 percent of the density of water by making the shell of the boat relatively think.
What about cargo & passengers?
Now, we are not going to send an empty ship out to sea without someone to steer it and we will also need shipmates. So, the density of a new ship must be designed to be under 1.0 g/cm to allow for cargo. When objects are placed on a boat, its average density increases and the boat sinks a little deeper to displace more water and increase the buoyant force.
Floating in The Dead Sea