Chapter 3.4 Notes Thermal Rates
The amount of heat that is transferred per unit time is the heat flow rate Equation for heat flow rate = heat / time Heat = Q Hfr = Q / t
If an air conditioner has a heat flow rate of 15,000 Btu per hour and runs for 3 hours, what amount of heat is transferred? If an air conditioner has a heat flow rate of 15,000 Btu per hour and runs for 3 hours, what amount of heat is transferred? Hfr = Q / t 15,000 = Q / 3 15,000 = Q / 3 Q = 15,000 x 3 Q = 15,000 x 3 Q = 45,000 Q = 45,000
When heat is transferred to a colder object, the atoms vibrate faster and faster as the temperature increases. The transfer of thermal energy arising from a temperature difference between adjacent parts of an object is called heat conduction.
For heat conduction to take place, their must be a temperature difference within the object. The thermal conductivity (k) of a material is a measure of its ability to conduct heat.
Metal have a large thermal conductivity because they are good heat conductors. Heat conduction rate = (thermal conductivity x area x temperature difference) / thickness Q rate = (k x A x T)/x
A wall of a house has 3 inch thick fiberglass insulation. The thermal conduction =.32. The wall measures 10 ft by 10ft. The inside temperature is 68 degrees and the outside temperature is 32 degrees. What is the heat flow rate? A wall of a house has 3 inch thick fiberglass insulation. The thermal conduction =.32. The wall measures 10 ft by 10ft. The inside temperature is 68 degrees and the outside temperature is 32 degrees. What is the heat flow rate? Q rate = (k x A x T)/ x Q rate =.32 x (10x10) x (68-32) 3 Q rate =.32 x 100 x 36 / 3 = 384 J
Convection is a transfer of heat by movement of fluid. Convection The circulation of fluid by the transfer of energy due to temperature differences is called natural convection. Examples – Earth’s Atmosphere and cold spots in lakes
When natural convection takes place in the Earth’s atmosphere it causes wind. When convection is forced by a fan or pump it is called forced convection. Examples – Body’s circulation system, air conditioners
All objects radiate energy in the form of electromagnetic radiation. The rate of energy radiated by an object depends on the object’s temperature, area, and type of material.
Radiation transfers energy from one body to another through empty space. Radiation A hot object radiates energy, some of which is absorbed by a cooler object. The cooler object also radiates energy but not as much as the hot object and so it absorbs more energy that it gives off. Energy is transferred from the sun to all planets in the solar system by radiation.
Radiometer
Black Balloon Demo