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Chapter 12. Would you believe that someone has made a solid material that has about the same density as air?
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Chapter 12 • Would you believe that someone has made a solid material that has about the same density as air? • If someone put a chunk of it your hand, you might not even notice. Silica aerogel is a foam that’s like solidified smoke. Aerogel is mostly air and has remarkable thermal, optical and acoustical properties.
12.1 Properties of Solids • Different kinds of matter have different characteristics. • Characteristics that can you observe directly are called physical properties. • Physical properties include color, texture, density, brittleness, and state (solid, liquid, or gas). We also call state phase. Ex. Iron is solid at room temp.
Physical Properties First half-stop after physical changes 1:20
12.1 Properties of Solids • A physical change is any change in the size, shape, or phase of matter in which the identity of a substance does not change. • For example, when water is frozen, it changes from a liquid to a solid. But it is still water!
12.1 Properties of Solids • Properties that can only be observed when one substance changes into a different substance are called chemical properties. • Any change that transforms one substance into a different substance is called a chemical change. Ex. If you leave a nail outside, it rusts.
Chemical Properties • Chemical properties use at 1:20.
12.1 Properties of Solids The density of a solid material depends on two things: • the individual mass of each atom or molecule, • how closely the atoms or molecules are packed together. Carbon atoms in diamond are packed very tightly.
Density • Density is the mass per unit volume of a substance. • Density is a physical property • D= m/V • Density is an intrinsic property of matter
12.1 Properties of Solids • Paraffin wax is also mostly carbon, but its density is only 0.87 g/cm3. • Paraffin’s carbon atoms are mixed with hydrogen atoms in long molecules that take up more space. The density of paraffin is low compared to diamond.
12.1 Properties of Solids The atoms or molecules in a solid are arranged in two ways. • If the particles are arranged in an orderly, repeating pattern, the solid is crystalline. • If the particles are arranged in a random way, the solid is amorphous.
12.1 Properties of Solids • Metals don’t look like “crystals” because solid metal is made from very tiny crystals fused together in a jumble of different orientations.
12.1 Mechanical properties • “Strength” describes the ability of a solid object to maintain its shape even when force is applied.
12.1 Mechanical properties • Tensile strength is a measure of how much stress a material can withstand before breaking.
12.1 Mechanical properties • Hardness measures a solid’s resistance to scratching. How might you compare the hardness of these two metals?
12.1 Mechanical properties • Elasticitydescribes a solid’s ability to be stretched and then return to its original size. • Brittlenessis defined as the tendency of a solid to crack or break before stretching very much.
12.1 Mechanical properties • A ductile material can be bent a relatively large amount without breaking. • The ductility of many metals, like copper, allow then to be drawn into wire.
12.1 Mechanical properties • Malleability measures a solid’s ability to be pounded into thin sheets. • Aluminum is a highly malleable metal.
12.2 Properties of Fluids • A fluid is defined as any matter that flows when force is applied. • Liquids like water or silver are kinds of fluid. • Remember the air we breath is a fluid too.
12.1 clicker Quiz Properties of Gases
12.2 Pressure • A force applied to a fluid creates pressure. • Pressure acts in all directions, not just the direction of the applied force.
12.2 Forces in fluids • Forces in fluids are more complicated than forces in solids because fluids can change shape.
12.2 Units of pressure copy slide • The units of pressure are force divided by area. • One psi is one pound per square inch.
12.2 Units of pressure copy slide • The S.I. unit of force is the pascal. • One pascal (unit of force) is one newton of force per square meter of area (N/m2).
12.2 Pressure • If your car tires are inflated to 35 pounds per square inch (35 psi), then a force of 35 pounds acts on every square inch of area inside the tire. What might happen if you over-inflate a tire?
12.2 Pressure • On the microscopic level, pressure comes from collisions between atoms. • Every surface can experience a force from the constant impact of trillions of atoms. • This force is what we measure as pressure.
12.2 Pressure • In a car engine high pressure is created by an exploding gasoline-air mixture.
12.2 Energy conservation and Bernoulli’s Principle • Streamlinesare imaginary lines drawn to show the flow of fluid. • Bernoulli’s principletells us that the energy of any sample of fluid moving along a streamline is constant.
12.2 Bernoulli’s Principle • Bernoulli’s principle says the three variables of height, pressure, and speed are related by energy conservation.
12.2 Three Variables and Bernoulli’s Principle • If one variable increases along a streamline, at least one of the other two must decrease. • For example, if speed goes up, pressure goes down.
12.2 The air foil Nye flight 1 • One of the most important applications of Bernoulli’s principle is the airfoil shape of wings on a plane. • When a plane is moving, the pressure on the top surface of the wings is lower than the pressure beneath the wings.
Flight • Airfoil • Wings of Flight!
12.2 Viscosity • Viscosity is the property of fluids that causes friction. • Viscosity is determined in large part by the shape and size of the particles in a liquid.
12.2 Viscosity and temperature • As the temperature of a liquid increases, the viscosity of a liquid decreases. • Increasing the kinetic energy of the substance allows the particles to slide past one another more easily.
12.3 Buoyancy is a force • Buoyancyis a measure of the upward force a fluid exerts on an object that is submerged. 12.2 First 8 The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.
Volume and Pressure. • Gas laws ppt.
12.3 Volume and buoyancy • The strength of the buoyant force on an object in water depends on the volumeof the object that is underwater. As you keep pushing downward on the ball, the buoyant force gets stronger and stronger. Which ball has more volume underwater?
12.3 Weight and buoyancy • Weight is a force, like any other pushing or pulling force, and is caused by Earth’s gravity. • It is easy to confuse mass and weight, but they are not the same. • Weight is the downward force of gravity acting on mass. What is the rock’s weight? What is the rock’s mass?
12.3 Weight and buoyancy • In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object. • A simple experiment can be done to measure the buoyant force on a rock with a spring scale when it is immersed in water.
Buoyancy • The force with which a more dense fluid pushes a less dense substance upward. • In short its ability to float.
12.3 Weight and buoyancy • In air the buoyant force on the rock is 29.4 N. • When the rock was submerged, the scale read 19.6 N. • The difference is a force of 9.8 N, exactly the amount of force the displaced water exerts.
12.3 Weight and buoyancy These blocks are the same total volume. Which block has more buoyant force acting on it? Which block weighs more in air?
12.3 Weight and buoyancy • Buoyancy explains why some objects sink and others float. • Whether an object sinks or floats depends on how the buoyant force compares with the weight.
12.3 Density and buoyancy • If you know an object’s density you can quickly predict whether it will sink or float. Which ball will sink in water? Which ball will float in water?
12.3 Density and buoyancy Average densityhelps determine whether objects sink or float. • An object with an average density GREATER than the density of water will sink. • An object with an average density LESS than the density of water will float.
