Thermal Physics Common Temperature Scales Temperatures are reported in degrees Celsius or degrees Fahrenheit. Temperatures changed, on the other hand, are reported in Celsius degrees or Fahrenheit degrees: Converting from a Fahrenheit to a Celsius Temperature A healthy person has an oral temperature of 98.6oF. What would this reading be on the Celsius scale? degrees above ice point ice point Converting from a Celsius to a Fahrenheit Temperature A time and temperature sign on a bank indicates that the outdoor temperature is -20.0oC. Find the corresponding temperature on the Fahrenheit scale. degrees below ice point ice point Kelvin Temperature Scale Thermometers Thermometers make use of the change in some physical property with temperature. A property that changes with temperature is called a thermometric property. DEFINITION OF HEAT Heat is energy that flows from a higher- temperature object to a lower-temperature object because of a difference in temperatures. SI Unit of Heat: joule (J) The heat that flows from OTHER UNITS hot to cold originates in the internal 1 kcal = 4186 joules energy of 1 cal = 4.186 joules the hot substance. 1 BTU = 1055 J It is not correct to say that a substance contains heat. Heat and Temperature Change: Specific Heat Capacity The heat that must be supplied or removed to change the temperature of a substance is specific heat capacity Common Unit for Specific Heat Capacity: J/(kg·Co) In a half-hour, a 65-kg jogger can generate 8.0x105J of heat. This heat is removed from the body by a variety of means, including the body’s own temperature-regulating mechanisms. If the heat were not removed, how much would the body temperature increase? Heat and Phase Change: Latent Heat Heat = transfer of energy From someplace hot to someplace cold Convection Conduction Radiation Convection Convection is the process in which heat is carried from one place to another by the bulk movement of a fluid. Concepts in action Hot water baseboard heating units are mounted on the wall next to the floor. The cooling coil in a refrigerator is mounted near the top of the refrigerator. Each location is designed to maximize the production of convection currents. Forced Convection CONDUCTION Conduction is the process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer. One mechanism for conduction occurs when the atoms or molecules in a hotter part of the material vibrate or move with greater energy than those in a cooler part. By means of collisions, the more energetic molecules pass on some of their energy to their less energetic neighbors. Another mechanism is free electrons within a metal conduct heat. Materials that conduct heat well are called thermal conductors, and those that conduct heat poorly are called thermal insulators. Conduction The amount of heat during a time t through a bar of bar depends on The heat Q conducted Q that is conducted through the length a number of factors: L and cross-sectional area A is 1. The time during which conduction takes place. 2. The temperature difference between the ends of the bar. 3. The cross sectional area of the bar. 4. The length of the bar. SI Units of Thermal Conductivity: J/(s·m·Co) Thermal Conductivity Insulation Materials with dead air spaces are usually excellent thermal insulators. Example: Thermal Conduction One wall of a house consists of plywood backed by insulation. The thermal conductivities of the insulation and plywood are, respectively, 0.030 and 0.080 J/(s·m·Co), and the area of the wall is 35 m2. Find the amount of heat conducted through the wall in one hour. Example: Thermal Conduction But first we must solve for the interface temperature. Example: Thermal Conduction Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. A material that is a good absorber is also a good emitter. A material that absorbs completely is called a perfect blackbody. The emissivity e is a dimensionless number between zero and one. It is the ratio of what an object radiates to what the object would radiate if it were a perfect emitter. THE STEFAN-BOLTZMANN LAW OF RADIATION The radiant energy Q, emitted in a time t by an object that has a Kelvin temperature T, a surface area A, and an emissivity e, is given by Stefan-Boltzmann constant Example: Radiation The supergiant star Betelgeuse has a surface temperature of about 2900 K and emits a power of approximately 4 x1030 W. Assuming that Betelgeuse is a perfect emitter and spherical, find its radius. Application: Thermos Bottle A thermos bottle minimizes heat transfer via conduction, convection, and radiation.