Energy to the Earth

  2. Energy production
  3. Global thermal energy transfer
  4. Energy to the Earth
    5. 20'

The Subject Guide suggests that this is the best topic in which to learn about Conduction and Convection. However, we have these covered in the heat transfer section.

This is where we take the energy that is incident on the Earth from the Sun and consider what happens to it.

Key Concepts

Spheres and discs

The Earth absorbs like it's a disc but emits as a sphere. A way of thinking about this is to consider looking at a ball from a large distance - as far as you're concerned, it could be a pancake.

Therefore:

  1. We can find the total power of the radiation incident on the Earth by multiplying the solar constant by the area of the silhouette of the Earth
  2. We can find the average intensity distributed across the Earth by dividing (1) by the total surface area of the Earth as a sphere

 

Emissivity and albedo

Emissivity is how well a body emits radiation, with a black body having e = 1. It is equal to the following ratio:

\(e={E_{emitted}\over E_{absorbed}}\)

Albedo is how well a body scatters radiation:

\(\alpha={E_{reflected}\over E_{incident}}\)

Global albedo is on average 0.3.

Notice that from this ratio we can determine the amount of energy absorbed by the body, for subtitution into the emissivity equation:

\(E_{absorbed}=(1-\alpha )E_{incident}\)

 

Energy flow

From conservation of energy (first law of thermodynamics), we recall that energy cannot be created or destroyed. Therefore:

  • the quantity of radiation incident on a body will equal the sum of the radiation reflected or absorbed
  • the radiation absorbed with then equal the sum of the radiation emitted or remaining

In summary, energy in = energy out.

 

 

Essentials

Electromagnetic radiation and the atmosphere

The atmosphere is made of many different gases that interact differently with electromagnetic radiation.

 

The greenhouse effect

Radiation entering the Earth's atmosphere increases in wavelength (tending towards infrared) when it reflects off the Earth's surface.

Greenhouse gases absorb this infrared radiation on its way back out to space:

  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Water (H2O)
  • Nitrous oxide (N2O)

However, gas molecules do not discriminate when re-emitting this radiation, and so a significant proportion returns to Earth. Statistically therefore, the Earth increases in thermal energy over time and global temperatures rise.

This is beneficial for life on Earth, but there is scientific evidence that rising temperatures due to human emission of greenhouse gases are contributing to long-term global warming.

 

Test Yourself

Use quizzes to practise application of theory.


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Exam-style Questions

Online tutorials to help you solve original problems

 
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