Class Assignment from November 14

Emissivity:

As we know, heat can be transferred through convection, conduction, and radiation. As long as objects have a higher temperature than absolute zero (-273 Celsius), its molecules vibrate as a result they are constantly absorbing and emitting electro-magnetic radiation. For example, if an object is hotter than its surroundings, it will emit energy and if is colder, it will absorb energy from its surroundings. As a result, the net flow of electro-magnetic energy between two objects depends on the temperature difference between the two.

 

So what is emissivity?

Emissivity is the property that each object has to either emit or absorb electro-magnetic energy. The emissivity of an object depends two things: Its material and surface area.

 

The scale 0-1 describes the objects efficiency as an emitter and absorber.

è   A black object is a perfect emitter/absorber.

è   A shiny object is a poor emitter/absorber.

 

One must take into account that the objects efficiency rate as an emitter is equal to its efficiency as an absorber!

 

Since emissivity describes the electro-magnetic energy exchange between two objects, the emissivity for visible light (low wavelength) is different for that of infrared (high wavelength).

 

Question 3:

In what sense is radiation the only heat-transfer process affecting the planet’s energy-balance? In what sense do other processes play a role?

 

The earth receives all its energy from the sun through radiation and most of it is reflected back to space. The term energy balance refers to an equal amount entering and leaving a system. On earth, the energy balance is achieved through electro-magnetic radiation. Energy from the sun enters the earth in two forms, through light (short wave energy) and through infrared radiation (long wave energy). The short wave energy enters and leaves the earth without inhibition since none of the atmospheric gases absorb visible light. Yet molecules in the atmosphere such as carbon dioxide, nitrogen, water, and methane absorb the infrared radiation, or long wave energy. The short wave absorption by those gases is the key element in the establishment of the planet’s energy balance because some energy is then prevented from leaving the earth. As a result of the atmospheric gases the earth’s average temperature is raised from 254 Kelvin to 287 Kelvin. Our planet’s energy balance consists of an equal rate of short wave energy that enters and leaves the planet, yet the entrance and exit of long wave energy is not the same.

Other processes like clouds, oceans or geographical variations influence the planet’s energy balance. However, they are rather ignored in the big picture because their influence on the energy balance is minimal in comparison to the atmospheric gases. Yet the increase of fossil fuel consumption, which has significantly increased the presence of atmospheric gases, especially CO2, has contributed in altering the earth’s energy balance. The reason is that now more short wave energy is absorbed. As result, more energy from the sun is retained in the atmosphere than it naturally should!