2 The clockwork Universe

3 The irreversible Universe

3.1 Thermodynamics and entropy 1/3

3.1 Thermodynamics and entropy 2/3

» 3.1 Thermodynamics and entropy 3/3

3.2 Equilibrium and irreversibility 1/2

3.2 Equilibrium and irreversibility 2/2

3.3 Statistical mechanics 1/2

3.3 Statistical mechanics 2/2

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

Part 1 of 3 | Part 2 | Part 3

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When heat flows between a steak and a plate there is no violation of energy conservation; the energy lost by the steak is gained by the plate. However, conservation of energy does not explain why the heat always flows from the hot steak to the cold plate; this is where the second law of thermodynamics comes in. Suppose the steak is at temperature T, the plate is at a slightly lower temperature 0.95T, and that a small amount of heat Q is transferred from the steak to the plate.

you will never see heat flowing spontaneously from a cold body to a hotter one

Then the entropy of the steak decreases by Q/T while the entropy of the plate increases by Q/0.95T. It is easy to see that the entropy lost by the steak is smaller than the entropy gained by the plate, so the total entropy of the Universe has increased; this process is therefore consistent with the second law of thermodynamics. If, on the other hand, heat Q had flowed from the cold plate to the hot steak, the entropy lost by the plate (Q/0.95T) would have been greater than the entropy gained by the steak (Q/T), and the total entropy of the Universe would have decreased. This violates the second law of thermodynamics, so we can be sure that the process is impossible. Heat flow is said to be an irreversible process - you will never see heat flowing spontaneously from a cold body to a hotter one.

Whenever energy is transferred or transformed, the final entropy of the Universe must be at least as high as the initial entropy. This usually means that heat flows are required to ensure that the total entropy does not decrease. Inventors should again take note. In most engines, heat is an unwanted by-product: the real aim is to transfer energy as work, perhaps to propel a vehicle or lift a weight. Since part of the energy initially stored in the fuel is inevitably wasted as heat, only a fraction is left to do useful work. Thus, thermodynamics imposes fundamental limits on the efficiency of engines. Fortunately, it also suggests ways of increasing efficiency, explaining for example, why a diesel engine is likely to be more efficient than a petrol engine, a topic we will return to in Classical physics of matter, book 4.

Question 1.3 Answer
When a room-temperature object is placed in a refrigerator, heat flows out of the object and its entropy decreases. Indeed, the refrigerator may be said to be a device for sucking entropy out of warm objects. How can such a decrease in entropy be consistent with the second law of thermodynamics?
Continue on to 3.2 Equilibrium and irreversibility, part 1 of 2

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