Flashcards in 8.6 Thermal Energy Transfer Deck (11):
What is intensity?
Power received per unit area I = P/A if a source emits equally in all direction than at a distance d from the source the intensity is
I = P/(4pid^2)
What is the solar constant?
The intensity of solar radiation at the Earths distance from the sun d earth. Since the total power radiated by the sun is P = 3.8 x 10^26 W the solar constant is
P/(4pid^2earth) = 1400Wm-2
What is a black body?
A theoretical body that absorbs all the energy that is incident on it reflecting none.
What is the Stefan Boltzmann law?
The power emitted per unit area of a black body at an absolute temperature of T is
P/A = OT^4 where O = 5.67x10-8 Wm-2K-4.
Thus P = OAT^4
What is the albedo?
a = (reflected power)/(total incident power).
The Earth as a whole has an average albedo of about 0.30 indicating that it reflect about 30% of the radiation incident on it. The baled is different for different parts of the Earth depending on soil type, water depth, forestation and cloud cover.
How can thermal energy be transferred?
Conduction, convection and radiation.
How does conduction work?
If there is metal in somewhere hot the other end will soon get hot, the heat causes atoms at the hot end to vibrate about their equilibrium positions more violently so there is an increase in temperature. Collisions between neighbouring atoms result in an increase in the amplitude of vibration of neighbouring atoms, that is their temperature increases too. The rate at which heat is transferred through a material is proportional to its cross sectional area and to the temperature difference across it and inversely proportional to the thickness.
How does convection work?
Applies mainly to fluids, part of a fluid that is warmer than its surroundings will expand and become less dense. Buoyant forces will then make this part of the fluid rise while colder fluid will take its place. This upward motion of the warm fluid creates connection currents.
How does radiation work?
Unlike conduction and convection radiation can travel through a vacuum. All bodies at a given absolute temperature radiate electromagnetic waves whose energy is distributed over an infinite range of wavelengths. Surfaces that are black and dull as opposed to polished and shiny are good approximations to a theoretical black body which radiates according to the Stefan Boltzmann law P = OAT^4. Everything else radiates according to P = eOAT^4 where the constant e is the emissivity of the surface and may be defined as (power per unit area radiated at absolute temperature T)/(power per unit area raided by a black body at absolute temperature T). It measures how effectively a body radiates and varies between 0 and 1.
What does the area under a black body curve represent?
The total power radiated from the surface and is proportional to T^4.