Entropy and Sun

From Weekly I/O#67

What does the Earth get from the Sun? While the total energy remains constant, we get low entropy energy that fuels life and natural processes before radiating it back into space as high entropy energy.

Video: The Most Misunderstood Concept in Physics - YouTube

What does the Earth get from the Sun? If your answer is energy, how much energy does the Earth radiate out? According to the first law of thermodynamics, the energy of a closed system (the universe) is constant. Therefore, the Earth will radiate 100% energy back because, otherwise, our Earth will get heated up.

But if the Earth emitted all the energy back, then what does the Earth actually get? The answer is: A steady stream of low-entropy energy. We can think of energy with low entropy as a more easy-to-use, concentrated power source, such as the flame from a lighter. On the other hand, energy with high entropy is a less useful, spread-out form of energy, like the heated air circulating around the lighter's flame. While the amount of energy transferred from the flame to the air remains constant, the flame's energy is more useful because it's concentrated (lower entropy).

The concept of entropy increase can be illustrated by comparing the quantity of photons that Earth receives from the Sun to the quantity it emits back into space. For every single photon that arrives from the Sun, Earth emits 20 photons back.

The energy we get from the Sun is more useful than the energy we give back. Ultimately, all of the solar energy that Earth absorbs is transformed into thermal energy and released back into space. And everything that occurs on Earth is part of this process, where fewer high-energy photons are converted into 20 times as many lower-energy photons.

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