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Q. 3.13

An Introduction to Thermal Physics
Found in: Page 97
An Introduction to Thermal Physics

An Introduction to Thermal Physics

Book edition 1st
Author(s) Daniel V. Schroeder
Pages 356 pages
ISBN 9780201380279

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Short Answer

When the sun is high in the sky, it delivers approximately 1000 watts of power to each square meter of earth's surface. The temperature of the surface of the sun is about 6000 K, while that of the earth is about 300 K.

(a) Estimate the entropy created in one year by the flow of solar heat onto a square meter of the earth.

(b) Suppose you plant grass on this square meter of earth. Some people might argue that the growth of the grass (or of any other living thing) violates the second law of thermodynamics, because disorderly nutrients are converted into an orderly life form. How would you respond?

(a) The change in entropy can be estimated to be 4.97×107 JK-1.

(b) The growth of grass doesn't violate the second law of thermodynamics as the entropy produced to transmit the energy from the sun to earth (disordered) would be much higher when compared to the entropy produced by the biochemical reactions (ordered) in the grass.

See the step by step solution

Step by Step Solution

Part (a) Step 1: Given

Power delivered per square meter=Ps=1000 W

Surface temperature of sun =Tsun=6000 K

Surface temperature of earth Tearth=300 K

Time period (day time) role="math" localid="1647243842287" =t=1 year=12×3600×365 s

Part (a) Step 2: Calculation

Amount of energy received by the earth per square meter can be calculated as:


By substituting the values, we get,

Q=1000×(12×3600×365)Q=1.57×1010 J

Now the changes in entropies can be calculated as:

For earth:

role="math" localid="1647244335644" ΔSearth=QTearth

By substituting the values, we get,

ΔSearth =1.57×1010300ΔSearth =5.23×107 JK-1

For sun:

role="math" localid="1647244543097" ΔSsun=-QTsun

By substituting the values, we get,

ΔSsun =-1.57×10106000ΔSsun =-2.62×106 JK-1

We know that the net entropy change can be calculated as:

ΔStotal =ΔSsun +ΔSearth

By substituting the calculated values, we get,

ΔStotal =5.23×107-2.62×106ΔStotal =4.97×107JK-1

Part (a) Step 3: Final answer

Hence, the total entropy change can be calculated as 4.97×107 JK-1.

Part (b) Step 1: Concept Introduction

Entropy is a measure of a system's randomness or disorder in general. The total entropy can't be reduced, but it can stay the same if the processes are reversible, according to the law of thermodynamics.

Part (b) Step 2: Explanation

The second rule of thermodynamics is not violated by growing grass. This is due to its ability to harness solar energy and transform disordered molecules in the soil into an organized life. However, in the event of a biological reaction, entropy would be substantially lower, and hence far lower than the entropy caused by transporting energy from the Sun to the Earth.

Part (b) Step 3: Final answer

The entropy produced by transmitting energy from the sun to earth (disordered) would be far more than the entropy produced by biological reactions (ordered) in the grass, hence the development of grass does not break the second law of thermodynamics.

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