Energy Transfer, Radiation, and The Greenhouse Effect
Notes for Lab 2
Recall. . . Temperature
• Measure of the energy of the air molecules, specifically the Kinetic energy • Substances with higher temperatures have molecules that move faster than those with colder temperatures
– KE = ½ mv2
• Kinetic energy related to molecular motion
Temperature and Density
• Warmer air is less dense
– Consider the ideal gas law: P=rRT – If we consider constant pressure, then: P/R = rT Or: Constant = rT – We can see then, if the temperature increases, density must decrease
• It follows: Colder air is more dense
Temperature Scales
• Fahrenheit
– Water boils at 212oF, and freezes at 32oF – Zero point is the lowest temperature obtained with a mixture of ice, water, and salt.
• Celsius
– Number 0 assigned to the temperature at which pure water freezes, and 100 to the temperature at which water boils
• Kelvin
– Absolute zero scale, minimum temperature possible – molecules here possess minimum energy and no thermal motion – Contains no negative numbers
Temperature Conversions
• Celsius to Fahrenheit F=(9/5)C + 32 • Celsius to Kelvin
K = C + 273.15
• Note: Change in temperature
– Each unit Kelvin is the same as each degree Celsius – Each degree Fahrenheit is 1.8 times larger than each degree Celsius
Energy Transfers
• Conduction: Energy transfer by molecular collisions
– Ex: The sun warms the ground, and this heats a thin layer of air above the surface – In general: inefficient, air poor conductor
• Convection: energy transfer by the motion of matter from one location to another
– Ex: parcel of air rising – Important in our atmosphere
• Radiation: transfer of energy not requiring contact between bodies or a fluid between them
– Ex: the sun warms the surface of the earth
Radiation
• Travels in the form of waves • Electromagnetic waves
• Can also be viewed as streams of particles • photons
Radiation
• Travels in the form of waves • Electromagnetic waves
• Can also be viewed as streams of particles • photons
* Radiation behaves with properties of both waves and particles
Radiation
• Radiation laws
– Stefan Boltzman law
• E = sT4
– Wien’s Law
• lmax = 2897/T
• Note: these laws apply to blackbodies
– A hypothetical object that absorbs all of the radiation that strikes it. It also emits radiation at a maximum rate for its given temperature – It has nothing to do with the objects color
Types of Radiation
What happens to Radiation?
• Radiation incident upon a medium is either:
– Absorbed – Reflected – Transmitted – Ei = Ea + Er + Et
• Kirchoff’s Law
– emissivity equals absorptivity at the same wavelength (monochromatic) – Good emitters are also good absorbers
Selective Absorption
A Note on Units
• Newton
– SI Unit of force, F = ma – 1 Newton = 1 kg ms-2
• Joule
– SI Unit of energy, KE = ½ mv2 – 1 Joule = 1 kg m2s-2
• Pascal
– SI Unit of pressure, P = F/A – 1 Pa = 1 kg m-1s-2