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Section summary

  • Entropy is the loss of energy available to do work.
  • Another form of the second law of thermodynamics states that the total entropy of a system either increases or remains constant; it never decreases.
  • Entropy is zero in a reversible process; it increases in an irreversible process.
  • The ultimate fate of the universe is likely to be thermodynamic equilibrium, where the universal temperature is constant and no energy is available to do work.
  • Entropy is also associated with the tendency toward disorder in a closed system.

Conceptual questions

A woman shuts her summer cottage up in September and returns in June. No one has entered the cottage in the meantime. Explain what she is likely to find, in terms of the second law of thermodynamics.

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Consider a system with a certain energy content, from which we wish to extract as much work as possible. Should the system’s entropy be high or low? Is this orderly or disorderly? Structured or uniform? Explain briefly.

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Does a gas become more orderly when it liquefies? Does its entropy change? If so, does the entropy increase or decrease? Explain your answer.

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Explain how water’s entropy can decrease when it freezes without violating the second law of thermodynamics. Specifically, explain what happens to the entropy of its surroundings.

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Is a uniform-temperature gas more or less orderly than one with several different temperatures? Which is more structured? In which can heat transfer result in work done without heat transfer from another system?

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Give an example of a spontaneous process in which a system becomes less ordered and energy becomes less available to do work. What happens to the system’s entropy in this process?

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What is the change in entropy in an adiabatic process? Does this imply that adiabatic processes are reversible? Can a process be precisely adiabatic for a macroscopic system?

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Does the entropy of a star increase or decrease as it radiates? Does the entropy of the space into which it radiates (which has a temperature of about 3 K) increase or decrease? What does this do to the entropy of the universe?

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Explain why a building made of bricks has smaller entropy than the same bricks in a disorganized pile. Do this by considering the number of ways that each could be formed (the number of microstates in each macrostate).

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Problem exercises

(a) On a winter day, a certain house loses 5 . 00 × 10 8 J size 12{5 "." "00"´"10" rSup { size 8{8} } " J"} {} of heat to the outside (about 500,000 Btu). What is the total change in entropy due to this heat transfer alone, assuming an average indoor temperature of 21.0º C size 12{"21" "." 0°C} {} and an average outdoor temperature of 5.00º C size 12{5 "." "00"°C} {} ? (b) This large change in entropy implies a large amount of energy has become unavailable to do work. Where do we find more energy when such energy is lost to us?

(a) 9.78 × 10 4 J/K size 12{9 "." "79" times "10" rSup { size 8{4} } " J/K"} {}

(b) In order to gain more energy, we must generate it from things within the house, like a heat pump, human bodies, and other appliances. As you know, we use a lot of energy to keep our houses warm in the winter because of the loss of heat to the outside.

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On a hot summer day, 4 . 00 × 10 6 J size 12{4 "." "00"´"10" rSup { size 8{6} } " J"} {} of heat transfer into a parked car takes place, increasing its temperature from 35.0º C size 12{"35" "." 0°C} {} to 45.0º C size 12{"45" "." 0°C} {} . What is the increase in entropy of the car due to this heat transfer alone?

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A hot rock ejected from a volcano’s lava fountain cools from 1100º C size 12{"1100"°C} {} to 40.0º C size 12{"40" "." 0°C} {} , and its entropy decreases by 950 J/K. How much heat transfer occurs from the rock?

8.01 × 10 5 J size 12{8 "." "01" times "10" rSup { size 8{5} } " J"} {}

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When 1 . 60 × 10 5 J size 12{1 "." "60"´"10" rSup { size 8{5} } " J"} {} of heat transfer occurs into a meat pie initially at 20.0º C size 12{"20" "." 0°C} {} , its entropy increases by 480 J/K. What is its final temperature?

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The Sun radiates energy at the rate of 3 . 80 × 10 26 W size 12{3 "." "80"´"10" rSup { size 8{"26"} } " W"} {} from its 5500º C size 12{"5500"°C} {} surface into dark empty space (a negligible fraction radiates onto Earth and the other planets). The effective temperature of deep space is 270º C size 12{-"270"°C} {} . (a) What is the increase in entropy in one day due to this heat transfer? (b) How much work is made unavailable?

(a) 1 . 04 × 10 31 J/K size 12{1 "." "04" times "10" rSup { size 8{"31"} } " J/K"} {}

(b) 3 . 28 × 10 31 J size 12{3 "." "28" times "10" rSup { size 8{"31"} } " J"} {}

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(a) In reaching equilibrium, how much heat transfer occurs from 1.00 kg of water at 40.0º C size 12{"40" "." 0°C} {} when it is placed in contact with 1.00 kg of 20.0º C size 12{"20" "." 0°C} {} water in reaching equilibrium? (b) What is the change in entropy due to this heat transfer? (c) How much work is made unavailable, taking the lowest temperature to be 20.0º C size 12{"20" "." 0°C} {} ? Explicitly show how you follow the steps in the Problem-Solving Strategies for Entropy .

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What is the decrease in entropy of 25.0 g of water that condenses on a bathroom mirror at a temperature of 35.0º C size 12{"35" "." 0°C} {} , assuming no change in temperature and given the latent heat of vaporization to be 2450 kJ/kg?

199 J/K

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Find the increase in entropy of 1.00 kg of liquid nitrogen that starts at its boiling temperature, boils, and warms to 20.0º C size 12{"20" "." 0°C} {} at constant pressure.

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A large electrical power station generates 1000 MW of electricity with an efficiency of 35.0%. (a) Calculate the heat transfer to the power station, Q h size 12{Q rSub { size 8{h} } } {} , in one day. (b) How much heat transfer Q c size 12{Q rSub { size 8{c} } } {} occurs to the environment in one day? (c) If the heat transfer in the cooling towers is from 35.0º C size 12{"35" "." 0°C} {} water into the local air mass, which increases in temperature from 18.0º C size 12{"18" "." 0°C} {} to 20.0º C size 12{"20" "." 0°C} {} , what is the total increase in entropy due to this heat transfer? (d) How much energy becomes unavailable to do work because of this increase in entropy, assuming an 18.0º C size 12{"18" "." 0°C} {} lowest temperature? (Part of Q c size 12{Q rSub { size 8{c} } } {} could be utilized to operate heat engines or for simply heating the surroundings, but it rarely is.)

(a) 2 . 47 × 10 14 J size 12{2 "." "47" times "10" rSup { size 8{"14"} } " J"} {}

(b) 1 . 60 × 10 14 J size 12{1 "." "60" times "10" rSup { size 8{"14"} } " J"} {}

(c) 2.85 × 10 10 J/K size 12{2 "." "86" times "10" rSup { size 8{"10"} } " J/K"} {}

(d) 8.29 × 10 12 J size 12{8 "." "31" times "10" rSup { size 8{"12"} } " J"} {}

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(a) How much heat transfer occurs from 20.0 kg of 90.0º C size 12{"90" "." 0°C} {} water placed in contact with 20.0 kg of 10.0º C size 12{"10" "." 0°C} {} water, producing a final temperature of 50.0º C size 12{"50" "." 0°C} {} ? (b) How much work could a Carnot engine do with this heat transfer, assuming it operates between two reservoirs at constant temperatures of 90.0º C size 12{"90" "." 0°C} {} and 10.0º C size 12{"10" "." 0°C} {} ? (c) What increase in entropy is produced by mixing 20.0 kg of 90.0º C size 12{"90" "." 0°C} {} water with 20.0 kg of 10.0º C size 12{"10" "." 0°C} {} water? (d) Calculate the amount of work made unavailable by this mixing using a low temperature of 10.0º C size 12{"10" "." 0°C} {} , and compare it with the work done by the Carnot engine. Explicitly show how you follow the steps in the Problem-Solving Strategies for Entropy . (e) Discuss how everyday processes make increasingly more energy unavailable to do work, as implied by this problem.

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Questions & Answers

What is the difference between a principle and a law
the law is universally proved. The principal depends on certain conditions.
Dr
state Faraday first law
aliyu Reply
what does the speedometer of a car measure ?
Jyoti Reply
Car speedometer measures the rate of change of distance per unit time.
Moses
describe how a Michelson interferometer can be used to measure the index of refraction of a gas (including air)
WILLIAM Reply
using the law of reflection explain how powder takes the shine off a person's nose. what is the name of the optical effect?
WILLIAM
is higher resolution of microscope using red or blue light?.explain
WILLIAM
can sound wave in air be polarized?
WILLIAM Reply
Unlike transverse waves such as electromagnetic waves, longitudinal waves such as sound waves cannot be polarized. ... Since sound waves vibrate along their direction of propagation, they cannot be polarized
Astronomy
A proton moves at 7.50×107m/s perpendicular to a magnetic field. The field causes the proton to travel in a circular path of radius 0.800 m. What is the field strength?
Celedonio Reply
derived dimenionsal formula
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what is the difference between mass and weight
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assume that a boy was born when his father was eighteen years.if the boy is thirteen years old now, how is his father in
Isru
what is head-on collision
Javaid Reply
what is airflow
Godswill Reply
derivative of first differential equation
Haruna Reply
why static friction is greater than Kinetic friction
Ali Reply
draw magnetic field pattern for two wire carrying current in the same direction
Ven Reply
An American traveler in New Zealand carries a transformer to convert New Zealand’s standard 240 V to 120 V so that she can use some small appliances on her trip.
nkombo Reply
What is the ratio of turns in the primary and secondary coils of her transformer?
nkombo
what is energy
Yusuf
How electric lines and equipotential surface are mutually perpendicular?
Abid Reply
The potential difference between any two points on the surface is zero that implies È.Ŕ=0, Where R is the distance between two different points &E= Electric field intensity. From which we have cos þ =0, where þ is the angle between the directions of field and distance line, as E andR are zero. Thus
MAHADEV
sorry..E and R are non zero...
MAHADEV
Practice Key Terms 3

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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