Heat
Heat is the transfer or flow of energy, represented by Q, -heat is not the same as temperature; however if an object feels hot the heat you feel is the transfer of thermal energy -Heat spontaneously flows from hot to cold
calories (1 cal = 4.186 J, energy to warm 1 gram of water 1 degree C) Heat Units Heat is measured in calories (1 cal = 4.186 J, energy to warm 1 gram of water 1 degree C) Kcal or Calorie (1 kcal 4,186 J, energy to warm 1 kg of water 1 degree C) The SI unit for Heat is the Joule
Heat Transfer Conduction- Transfer of Heat through direct molecular contact. This occurs when molecules from a hotter region of an object with more energy collide with molecules with from a cooler region with less energy and transfer energy from hot to cold.
Thermal Conductors Typically metals have a large number of electrons that are free to move around within an electron cloud. Substances that can easily transfer energy from one molecule to another through conduction are considered Thermal Conductors. Thermal Insulators are typically nonmetals and have electrons that can not easily flow from one molecule to another.
Described at time rate of heat flow- ⧍Q/⧍t ⧍Q/⧍t - Is proportional to the Area of the substance, its change in temperature and inversely proportional to its thickness (d) ⧍Q/⧍t ∝ A⧍T/L (A= area; T= temp; L= thickness) ⧍Q/⧍t = КA⧍T/L (К=thermal conductivity Constant)
Practice Problem One way the body dispels of excess thermal energy is via conduction through body fat. Suppose that heat energy travels through 3 cm of fat in reaching the skin, which has a total surface area of 1.7 m2 and a temperature of 34OC. If the body maintained at 37oC, find the amount of energy that reaches the skin in half an hour. The thermal conductivity is 0.2 W/mC.
Convection The transfer of heat through fluid currents. Unlike in conduction the material itself moves from one place to another. Convection only occurs in fluids. Naturally convection occurs because of gravity, hot expanded fluids float above cool dense fluids and the process of hot rising and cold sinking causes convection currents.
Radiant Energy Transfer All objects emit energy through electromagnetic radiation. Objects emit thermal radiation while absorbing some emitted by other objects, and this rate may be less than, equal to or be greater than the rate of emission. A system that has absorption and emission perfectly balanced will maintain a constant temperature if isolated from other forms of heat transfer.
A good absorber of Radiant Energy is also a good emitter. A theoretical body that absorbs all the radiation incident upon it is referred to as a “Blackbody”. Theoretical Black bodies absorb all light throughout the EM Spectrum. In doing so they emit more radiation per area than any real object at a given temperature. This follows Stefan’s Radiation Law- This not an AP Equation, just somewhat interesting!!! Q/T= σAT4 (σ= sigma- stefan’s constant 5.670x10-8 W/(m2K4)) So if the absolute temperature of a substance is doubled the rate at which it radiates EM Energy x16!!!
Emissivity Real bodys are not perfect absorbers and therefore emit less than a blackbody. Emissivity is the ratio of the emitted power of the body to that of a blackbody at the same temperature, in this Stefan's Law becomes Q/T= еσAT4 (e= emissivity constant for a material, has a value greater than 0 and less than 1)
Homework- 391 -1-5 394 -56-59, 61, 63, 68, 74