Temperature is that property of matter that gives rise to the sensations of hot and cold.
A thermometer is a device that measures temperature.
A thermostat makes use of the different rates of thermal expansion in the metals of a bimetallic strip to switch heating and cooling systems on and off.
Two temperature scales are used in the United States:
The fahrenheit scale on which water freezes at 32°F and
boils at 212°F at sea level.
The celsius scale on which water freezes at 0°C and boils at
100°C at sea level.
4-2. Heat
The heat in a body of matter is the sum of the kinetic energies of all the separate particles that make up the body; this heat content is also called internal energy.
The SI unit of heat is the joule.
The specific heat capacity (or specific heat) of a substance is the amount of heat that must be added or removed from 1 kg of the substance in order to change its temperature by 1°C.
Heat can be transferred in three ways:
Conduction, in which heat is transferred from one place to
another by molecular collisions.
Convection, in which heat is carried by the motion of a volume of hot fluid.
Radiation, in which heat is transferred by electromagnetic waves.
4-3. Metabolic Energy
The complex of biochemical reactions that make food energy available for use by living organisms is called metabolism.
A kilocalorie is the amount of heat needed to change the temperature of 1 kg of water by 1°C; it is equal to one dietary "calorie."
The conversion of metabolic energy into biological work is relatively inefficient; much of the energy is lost as heat.
4-4. Density
The density of a substance is its mass per unit volume:
Because the pascal is a small unit, the kilopascal is often used (1 kPa = 1000 Pa).
Hydraulic machines are those in which forces are transmitted by liquids; pneumatic machines use compressed air.
Pressure in a fluid increases with depth.
Atmospheric pressure at sea level averages 101 kPa (equals approximately 15 lb/in2).
Instruments called barometers measure atmospheric pressures.
4-6. Buoyancy
The upward force exerted on an object immersed in a fluid is called buoyant force
Fb = dVg
where Fb = buoyant force, d = density of the fluid, V = volume of the fluid displaced by the solid object, and g = the acceleration of gravity (9.8 m/s2).
Buoyant force is always an upward force because the pressure underneath an immersed object is greater than the pressure above it.
Archimedes’ principle states: Buoyant force on an object in a fluid is equal to the weight of fluid displaced by the object.
4-7. Gas Laws
Boyle’s law states that the volume of a gas is inversely proportional to its pressure when the temperature is held constant:
where p = pressure, V = volume, and T = temperature (K).
4-8. Kinetic Theory of Gases
Gas molecules are small compared with the average distance between them; a gas is mostly empty space.
Gases are easily compressed.
Gases are easily mixed.
The mass of a certain volume of gas is much smaller than that of the same volume of a liquid or a solid.
Gas molecules collide without loss of kinetic energy.
Gas molecules exert almost no forces on one another, except when they collide.
4-9. Molecular Motion and Temperature
The absolute temperature of a gas is proportional to the average kinetic energy of its molecules.
Gas molecules, even at 0 K (-273°C), would still possess a small amount of kinetic energy.
Compression of a gas increases its temperature; expansion decreases its temperature.
4-10. Liquids and Solids
The intermolecular attractions between the molecules of a liquid are stronger than those in a gas but weaker than those in a solid.
Molecules of a solid do not move freely about but vibrate around fixed positions.
4-11. Evaporation and Boiling
Evaporation occurs when fast-moving molecules in a liquid leave the liquid’s surface and escape into the air.
The boiling point of a liquid is that temperature at which the molecules of the liquid attain enough kinetic energy to overcome their intermolecular attractions, resulting in a change of state from liquid to gas.
Evaporation differs from boiling in two ways:
Evaporation occurs only at a liquid surface; boiling occurs in the entire volume of liquid.
Evaporation occurs at all temperatures; boiling occurs only at the boiling point.
The heat of vaporization of a substance is the amount of heat energy needed to change 1 kg of the substance from liquid to gas at its boiling point.
4-12. Melting
The heat of fusion of a solid is the amount of heat needed to melt 1 kg of it. It is also equal to the amount of heat given off by 1 kg of a liquid when it hardens into a solid.
Sublimation is the direct conversion of a substance from the solid to the vapor state, or from the vapor state to the solid state, without it entering the liquid state.
4-13. Heat Engines
A heat engine is a device that converts heat into mechanical energy or work.
During operation a heat engine extracts energy from a flow of heat through it.
Some of the heat flow is converted into mechanical energy; the rest is lost as waste heat.
To produce a flow of heat energy, the heat must travel from a hot reservoir to a cold reservoir.
In an automobile, the ignited gases within the cylinder constitute the hot reservoir and the atmosphere is the cold reservoir.
A refrigerator operates as the reverse of a heat engine by using mechanical energy to force heat to flow from a cold reservoir (the refrigerator interior) to a hot reservoir (the atmosphere).
4-14. Thermodynamics
Thermodynamics is the science of heat transformation.
The first law of thermodynamics states: Energy cannot be created or destroyed, but it can be converted from one form to another.
The second law of thermodynamics states: It is impossible to take heat from a source and change all of it to mechanical energy or work; some heat must be wasted.
Maximum efficiency of a heat engine depends on the temperatures at which it takes in and ejects heat; the greater the ratio between the two temperatures, the more efficient the engine:
The heat energy of the universe increases at the expense of other forms of energy.
"Heat death" of the universe will occur when all particles of matter ultimately have the same average kinetic energy and exist in a state of maximum disorder.
4-16. Entropy
Entropy is a measure of the disorder of the particles that make up a body of matter.
In terms of entropy the second law of thermodynamics becomes: The entropy of a system isolated from the rest of the universe cannot decrease.
Living organisms require a constant energy input to overcome entropy and maintain the order of their biological systems.
The entropy of the universe is increasing with the passage of time.
