Chapter 4 Physics of Matter
Matter: Phases, Forms & Forces • We classify matter into four categories: • Solid: rigid; retain its shape unless distorted by a force. • Liquid: flows readily; conforms to the shape of a container; has a well-defined boundary; has higher densities than gases. • Gas: flows readily; conforms to the shape of a container; does not have a well-defined surface; can be compressed readily. • Plasma: has gaseous properties but also conducts electricity; interacts strongly with magnetic fields; commonly exists at higher temperatures.
Matter: Phases, Forms & Forces, cont’d • The chemical elements represent the simplest and purest forms of everyday matter. • There are currently 114 different elements. • 110 of them have accepted names. • Each element is composed of incredibly small objects called atoms. • There are 114 different atoms, one for each of the known elements. • Only about 90% of the elements exist naturally on Earth. • The others are artificially produced in laboratories.
Matter: Phases, Forms & Forces, cont’d • The atom is not indivisible. • It has its own internal structure. • Every atoms has a very dense, compact core called the nucleus. • The nucleus is composed of two kinds of particles: • Protons: have a positive electric charge. • Neutrons: have no electric charge. • The nucleus is surrounded by one or more particles called electrons. • Electrons have the same electric charge as protons but are negatively charged.
Matter: Phases, Forms & Forces, cont’d • Every atom associated with a particular element has a fixed number of protons. • The number of protons distinguishes the element. • Atoms with two protons are helium atoms. • The atomic number of an element specifies the number of protons. • The atomic number of helium (He) is 2 because it has two protons. • Each element is given its own chemical symbol. • This is a one- or two- letter abbreviation.
Matter: Phases, Forms & Forces, cont’d • Atoms can have various numbers of neutrons. • Atoms with different numbers of neutrons for a certain elements are called isotopes. • More on this in chapter 11.
Matter: Phases, Forms & Forces, cont’d • Chemical compounds are the next simplest form of everyday matter. • Examples: water, salt, sugar, etc. • Compounds are made from building blocks called molecules. • Every molecule of a particular compound consists of the same unique combination of two or more atoms. • Each water molecule consists of two hydrogen atoms and one oxygen atom.
Matter: Phases, Forms & Forces, cont’d • Each compound can be represented by a chemical formula. • water is H2O; • salt is NaCl; • carbon dioxide is CO2; • sugar is C12H22O11; • ethyl alcohol is C2H5OH.
Matter: Phases, Forms & Forces, cont’d • Many substances are composed of two or more different compounds that are physically mixed together called mixtures and solutions. • Air is a mixture of several gases. • The actual composition varies widely from day to day and place to place.
Behavior of atoms and molecules • The constituent particles of atoms and molecules exert electrical forces on each other. • “Static cling” is an example of an electrical force. • The forces depend upon the configuration of the atoms in each atom.
Behavior of atoms and molecules, cont’d • Solids:Attractive forces between particles are very strong; the atoms or molecules are rigidly bound to their neighbors and can only vibrate. • Liquids:The particles are bound together, though not rigidly; each atom or molecules move about relative to the others but is always in contact with other atoms or molecules. • Gases:Attractive forces between particles are too weak to bind them together; atoms or molecules move freely with high speed and are widely separated; particles are in contact only when they collide.
Behavior of atoms and molecules, cont’d • Atoms or molecules in a solid arranged in a regular geometric pattern are called crystals. • Solids that do not have a regular crystal structure are called amorphous solids.
Behavior of atoms and molecules, cont’d • Carbon has two common crystalline forms. • Graphite forms crystalline sheets with little bonding between sheets. • Diamond forms very strong bonds between adjacent carbons — it’s the hardest known natural substance.
Behavior of atoms and molecules, cont’d • In liquids, the inter-atomic forces are insufficient to bind the atoms rigidly. • The atoms are free to move and vibrate. • In gases, the inter-atomic forces are virtually negligible unless the atoms are very close. • Gaseous atoms have rather high speeds: ~1,000 mph.
Behavior of atoms and molecules, cont’d • Whenever a high-speed gas atom impacts a large force on a container. • It is this force that produces a pressure on the container. • As the air molecules strike the inside of a tire, they produce the pressure that inflates the tire. • If the molecules are stationary, there is no pressure.
Behavior of atoms and molecules, cont’d • Gases are easily compressed because the majority of their volume is the space between the atoms. • If you compress it enough, you force the atoms close enough together that you form a liquid.
Pressure • Pressure is the force per unit area for a force acting perpendicular to a surface. • Since we use the perpendicular component of the force, pressure is a scalar.
Pressure, cont’d • The units of pressure are: • Metric: • pascal (Pa; 1 Pa = 1 N/m2) — SI unit; • millimeters of mercury (mm Hg). • English: • pound per square foot (lb/ft2); • pound per square inch (lb/in2 or psi); • inches of mercury (in. Hg).
Pressure, cont’d • Some pressure conversions: • 1 psi = 6,890 Pa. • We also use an atmosphere (atm) as a pressure unit: • One atmosphere is the average pressure exerted by air at sea level: • 1 atm = 1.01×105 Pa; • 1 atm = 14.7 psi.
ExampleExample 4.1 A 160-pound person stands on the floor. The area of each show that is in contact with the floor is 20 square inches. What is the pressure on the floor? Assume the person’s weight is shared equally between the two shows.
ExampleExample 4.1 ANSWER: The problem gives us: Since the shoes support the weight equally, each show must support a weight of 80 lb. The pressure is
ExampleExample 4.1 DISCUSSION: If the person stands on only one foot: If the person wore high heels and stood on only one heel (0.5 in by 0.5 in):
ExampleExample 4.2 In the late 1980s, there were several spectacular aircraft mishaps involving rapid loss of air pressure in the passenger cabins. The cabin pressure of a passenger jet cruising at high altitude (25,000 ft) is about 6 psi (0.41 atm) greater than the pressure outside. What is the outward force on a window measuring 1 foot by 1 foot and on a door measuring 1 meter by 2 meters?
ExampleExample 4.2 ANSWER: The force on the window is: The force on the door is
ExampleExample 4.2 DISCUSSION: The force on a window is approximately the weight of four adults. The force on the door is nearly the weight of five pickups.
Pressure, cont’d • Pressure is a relative quantity. • When you measure a pressure, you are measuring it relative to some other pressure. • When you measure your tire pressure, you are measuring the pressure in the tire above the atmospheric pressure.
Pressure, cont’d • The gauge pressure is the pressure relative to the current atmospheric pressure. • The absolute pressure is the gauge pressure plus the atmospheric pressure.
Pressure, cont’d • The standard pen-shaped tire gauge compares the tire pressure against a spring and the atmospheric pressure.
Pressure, cont’d • An important statement about gases. • Consider a gas held at constant temperature. • Increasing the temperature increases the motion of the gas atoms and therefore the pressure. • Under these conditions:
Pressure, cont’d • This means: • Decreasing the volume increases the pressure. • Imagine squeezing part of an inflated balloon. • Decreasing the pressure increases the volume. • Imagine letting the air out of an inflated balloon.
Density • Mass density is the mass per unit volume of a substance. • It is the ratio of the mass to the volume of the substance.
Density, cont’d • Units of mass density: • Metric: • kilogram per cubic meter (kg/m3); • gram per cubic centimeter (g/cm3). • English: • slug per cubic foot (slug/ft3).
Density, cont’d • Measure density by finding the mass of a sample and dividing by the volume of the sample. • The actual size of the sample is irrelevant. • If you use a sample with twice the volume you will twice the mass.
Density, cont’d • You can measure the freezing point of your car’s coolant by measuring the density. • The density of water and antifreeze are different. • Water: 1,000 kg/m3. • Antifreeze: 1,100 kg/m3. • The overall density depends on the ratio of the amount of water and amount of antifreeze.
ExampleExample 4.3 The dimensions of a rectangular aquarium are 0.5 meters by 1 meter by 0.5 meters. The mass of the aquarium is 250 kilograms larger when it is full of water than when it is empty. What is the density of the water?
ExampleExample 4.3 ANSWER: The problem gives us: The volume is:
ExampleExample 4.3 ANSWER: The mass density is then
ExampleExample 4.3 DISCUSSION: If the width of the tank was doubled, the amount of water would be doubled — the density would remain the same. If filled with gasoline, the mass is 170 kg. The density of gasoline is
Density, cont’d • If you know the volume and what type of substance you have, you can find the mass:
ExampleExample 4.4 The mass of water needed to fill a swimming pool can be computed by measuring the volume of the pool. Let’s say a pool is going to be guilt that will be 10 meters wide, 20 meters long, and 3 meters deep. How much water will it hold?
ExampleExample 4.4 ANSWER: The problem gives us: The volume is:
ExampleExample 4.4 ANSWER: Since the density of water is 1,000 kg/m3, the amount of water is:
ExampleExample 4.4 DISCUSSION: Since a common bathtub is about 0.25 m3 (let’s say 1 m by 0.5 m by 0.5 m), this swimming pool is about 2400 bathtubs of water.
Density, cont’d • Weight density is the weight per unit volume of a substance. • It is the ratio of the object’s weight and its volume.
Density, cont’d • Units or weight density: • Metric: • newton per cubic meter (N/m3). • English: • pound per cubic foot (lb/ft3); • pound per cubic inch (lb/in3).
ExampleExample 4.5 A college dormitory room measures 12 feet wide by 16 feet long by 8 feet high. What is the weight of the air in it under normal conditions?
ExampleExample 4.5 ANSWER: The problem gives us: The volume is:
ExampleExample 4.5 ANSWER: The weight of the air in the room is
ExampleExample 4.5 DISCUSSION: Notice that as the temperature increases, the density would decrease. So the air in the room would weigh less on hotter days. Because there is less air (fewer air molecules) in the room.