8.2 Thermal energy transfer 💥
The Solar Constant
Black Body Radiation
Albedo and emissivity
Albedo
Any object with a temperature above absolute zero posseses internal energy due to the motion of its molecules. Energy will spontanepously transfer from a region at a high temperature to a region at a low temperature. There are 3 principal methods by which energy can 'flow' due to differences in temperature:
Emissivity
Albedo is the measure of the ratio between energy reflected by a given surface in a given time to total energy incident on the surface in the same time
Convection 
The Greenhouse Effect
Thermal Radiation
Conduction
Emissivity is a measure of the ratio between power emitted by a radiating object and the power emitted by a black body (of the same dimensions at the same temperature).
Reasons the solar constant varies
Equations including Albedo and/or Emissivity
Greenhouse Gases
P=eσAT^4
Absorb infrared radiation because its molecules have natural frequencies in the infrared region and absorb infrared radiation due to resonance.
albedo + emissivity = 1
Absorption of radiation depends on the surface characteristics of the absorbing object
(radiative energy transfer depends on the absorber)
nature of the emitted radiation depends on:
The most abundant greenhouse gases are: Water Vapour, Carbon Dioxide, Methane, Nitrous Oxide, Ozone, and Chlorofluorocarbons/Hydrofluorocarbons
Black Body is an object or a system that absorbs all electromagnetic radiation incident on it and re-radiates energy that is characteristic only of itself, independent of its incident radiation
Definition
absolute temperature (T) in K
surface area (A)
Perfect black body's have an emissivity of 1.
surface characteristics, represented by a constant 'e' (emissivity)
Cool facts:
The mechanism of the Greenhouse Effect is as follows: 1. Incoming radiation from the sun takes the form of ultraviolet and visible radiation. 2. Some of this radiation is absorbed by the Earth's atmosphere and some of the radiation is reflected back into space by the Earth's surface. 3. The radiation which is reflected back in to space takes the form of infrared radiation. 4. The greenhouse gases in the atmosphere absorb infrared radiation and reflect it back to the Earth's surface. 5. Thus, heat energy becomes trapped inside Earth's atmosphere and accumulates, leading to the greenhouse effect and an increase in average mean temperatures on Earth.
The solar constant: 1.36 kWm^-2
Precise definition: The mean solar radiant power per unit area (intensity), perpendicular to the solar rays received at the top of the atmosphere of the earth.
Includes all electromagnetic radiation.
In conduction, energy flows through the bulk of the material without any large-sclae relative movement of the atoms which make up the material.
At the hot end of a metal, for example, the ions vibrate at a large amplitude and with a large average speed. At the cold end the amplitude is lower and the speed at which the ions vibrate is lower. At the position where the metal bar is heated the ions vibrate with increasing amplitude and collide with their neighbouring ions. These collisions cause internal energy to be transferred to the neighbouring ions. This process continues until the metal bar reaches thermal equilibrium. Each region of the bar will now be at the same uniform temperature.
Conduction can occur in gases and liquids as well as solids, but, becasue the inter-atomic connections are wekaer and the atoms are furhter apart in fluids, less energy is transferred through conduction. Metals are good thermal conductors, whereas glass and some plastics don't conduct heat as well.
Thermal Energy Transfer
if e=1, object is:
The output of the Sun varies by around 0.1% during its 11-year sunspot cycle
A chain reaction may occur:
Exam Question Practice
in an equilibrium with its surroundings (emission=absorbtion)
The Earth's orbit is elliptical; the Earth is closer to the sun in January than in July - this accounts for a 7% difference in the solar constant
radiating energy that is characteristic of its temperature
a black body
Global warming reduces ice+snow cover. This causes a decrease in Earth's albedo, and the rate of heat absorption by the Earth increases.
The solubility of carbon dioxide in the sea decreases with increasing temperature, leading to an increase in the concentration of carbon dioxide in the atmosphere.
Other longer period cycles are believed to occur in the Sun
Effects:
It varies daily and seasonally, depends on the cloud cover (thick, thin); it depends on the terrain and the material of the surface
Rise in sea level (due to melting of ice)
depending on temperature the power emitted as radiation by an object with thermal energy is not distributed uniformly across all wavelengths
Extreme weather more likely to occur (e.g. heat waves / heavy flooding)
Earth's Albedo is 0.30, which means 30% of the Sun’s rays that reach the ground are reflected back into the atmosphere.
The ocean has an albedo as low as 0.06 and snow has one of 0.90
at smaller object temperatures the overall radiative power emitted is smaller and its wavelength distribution is shifted towards increasingly larger wavelengths.
The amount of energy that is incident at the top of Earth's atmosphere
black body radiation
always includes wavelengths larger than those of visible light
has an asymmetric intensity variation with wavelength that peaks toward smaller wavelengths
has an intensity peak at a wavelength that can be predicted by the object temperature
occupies a continuous spectrum of wavelengths with a sharp limit to short wavelengths (usually in the ultraviolet region)
Energy Balance in the Earth Surface-Atmosphere System
As the radiation from the Sun enters and travels through the Earth's atmosphere, it is subject to losses before reach the surface.
graph: the instensity of emitted radiation at different wavelengths for a black body at various temperature
Convection is the movement of groups of atoms or molecules within fluids (liquids and gases) that arises through variations in density.
In the diagram above, a bunsen burner heats one of the sides of a beaker filled with water. Potassium permanganate crystals are added so that the movement of the water molecules are more visible. The water which is at the bottom right of the beaker is being heated, causing its molecules to move further apart, decreasing the water density. Water which is less dense will rise, and hence it moves towards the top of the beaker. Here, it cools as it is no longer being directly heated by the bunsen burner. When water cools, the distance between the molecules decreases, causing the density of the water to increase. As a result, the water moves towards the bottom of the beaker. This movement of water is called a convection current.
This is the mechanism by which all tthe water heated in a saucepan on a stove eventually reaches uniform temperature.
There are many examples of convection in nature, such as sea breezes and convection in the Earth's mantle.
Wien's displacement law states that for a black body, the wavelength of peak radiation emission λmax is related to temperature (T) of a black body by:
Radiation is absorbed and scattered by the atmosphere.
(λmax/m)= (2.9010^(-3) mK) / (T/K)
black body glows red at λmax = 0.7 μm at 4500 K
black body glows blue at λmax = 0.4 μm at 7000 K
The degree to which the scattering and absorption occurs, depends on the position of the Sun in the sky at a particular place.
When the sun is lower in the sky, its radiation has to pass through a greater thickness of atmosphere, so more scattering and absorption takes place. This leads to change in colour in the sky.
Even when the energy arrives at the surface, it does not necessarily remain there - The surface of the Earth is not a black body so it reflects some energy back to the atmosphere. The extent to which a particular surface can reflect energy is known as its albedo.
emitted power of a black body
P(black body)=σA(blackbody)*T^4(black body)
σ=5.6710^(-8) Wm^(-2)*K^(-4)
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Thermal radiation is the transfer of energy by means of electromagnetic radiation. Electromagnetic radiation is unique type of thermal transfer, as it does not need a medium un order to propagate.
Atoms contain charged particles and when these charges are accelerated they emit photons. It is these photons that are the thermal radiation.
Black surfaces are very good at radiating and absorbing energy. The opposite is true for white or shiny surfaces; they reflect energy rather than absorb it and are poor at radiating energy.