A stone rolls down a hill by itself and never climbs back up on its own. Heat behaves the same way: it runs from a warm body to a cooler one without help, and never the other way by itself. The first law keeps the energy accounts but says nothing about this one-way road. The second law does.
1. What the first law leaves out
The first law says heat can become work and work can become heat. It leaves two questions open. First, in which direction will heat flow, or work be done? Second, can a given amount of heat be turned completely into work?
Heat comes easily out of mechanical energy. But without a heat engine, work does not come out of heat. The heat made by water falling at a waterfall cannot be turned back into work without some other device.
Studying engines, Sadi Carnot concluded that heat energy can never be turned completely into work. That one idea is the basis of the second law. Clausius and Kelvin later, separately, gave Carnot's finding its general form. So the law can be stated in several ways, and every way carries the same core.
2. One idea, four statements
The core of every form: heat can never pass spontaneously from a body at a lower temperature to a body at a higher temperature. Shorter still, the law reads “Efficiency cannot be one”: the efficiency of nothing can be one.
Try to build a perfect machine: an engine that turns all its heat into work, or a fridge that runs on no work.
a perfect machinepush to 100%
Where the second law says no
Q₁ = 100 and Q₂ = 40 are model numbers.
Clausius's statement: without the help of outside energy, it is impossible for a self-acting machine to transfer heat from a body at a lower temperature to a body at a higher temperature. Heat passes by itself from warm to cool, just as an object rolls down a hill by itself. The other way needs outside work. No refrigerator that runs without a supply of energy has ever been made, and that shows the truth of Clausius's statement.
Kelvin's statement: it is impossible to obtain a continuous supply of energy by cooling a body below the coldest part of its surroundings. Planck's statement: it is impossible to make a heat engine that absorbs heat continually from a heat source and converts it completely into work. Carnot's statement: it is impossible to build a machine that converts a given amount of heat energy completely into mechanical energy.
cut from the noteTap a name
Who said what
NameClausius
StatementWithout outside energy heat does not go from cold to hot. Keyword: refrigerator.
NameKelvin
StatementNo continuous energy by cooling below the coldest surroundings. Keyword: surroundings.
NamePlanck
StatementNo engine turns all heat from a source into work. Keyword: heat engine.
NameCarnot
StatementA given heat cannot be fully turned into mechanical energy. Keyword: complete conversion.
3. First law against second law
The first law is a special form of the conservation of energy, and in a conversion heat and mechanical energy are equivalent. By the second law, though, when heat becomes work only a part of it does. For that you need a hot body and a cold body together. Tap each statement and see which law it belongs to.
overlaptap a statement
Two laws
First law only. The first law is a special form of the conservation of energy.
First law only. In a conversion, heat and mechanical energy are equivalent.
First law only. The first law points to a variable called internal energy.
Second law only. The first law has nothing to say about which way heat goes.
Second law only. When heat becomes work, only a part of it does.
Second law only. The second law points to a variable called entropy.
Second law only. For part of the heat to become work you need a hot body and a cold body together.
Both laws. The first says heat and work are equivalent, the second says not all of the heat becomes work.
4. What each law points to
The three laws each point to one variable. The zeroth law points to temperature, the first law to internal energy, and the second law to entropy. Now place three variables and three facts in the box of the right law.
sort themPlace each variable
Law and variable
Pick up a chip, then drop it in a box.
Right. Next one.
Not yet. Ask yourself: is this about temperature, about energy, or about direction?
All in place. The zeroth law points to temperature, the first to internal energy, the second to entropy.
5. Check yourself
Three questions. Start with the first.
check yourselfpick one answer
Question 1
Using which law are heat engines and refrigerators made?
The zeroth law gives the idea of temperature, not the principle of engines or refrigerators.
The first law keeps the energy account but does not say how much heat becomes work or which way heat goes.
Right. Engines (Planck) and refrigerators (Clausius) rest on the second law.
The third law is not part of this chapter.
In the second, do not mix up the statements.
check yourselfpick one answer
Question 2
Which statement is Clausius's?
This is Planck's statement (no heat engine can turn all the heat of a source into work); Clausius's statement is about the refrigerator.
Right. This is Clausius's statement.
This is Kelvin's statement (continuous energy cannot be had by cooling below the coldest part of the surroundings); Clausius's statement is different.
This is the first law (Joule): heat and work are equivalent; the second law speaks of the direction of heat flow.
The last question.
check yourselfpick one answer
Question 3
Which marks the difference between the first and second laws?
Right. This is the main difference: conservation against the limit and direction of conversion.
The first law says heat can become work.
The second law does not deny conservation; it gives the limit of conversion.
Defining temperature is the work of the zeroth law.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is the core idea of all forms of the second law?
AnswerHeat does not pass spontaneously from a body at a lower temperature to one at a higher temperature.
QuestionWhat is the short form of the second law?
Answer"Efficiency cannot be one": efficiency can never equal one.
QuestionState Clausius's statement.
AnswerWithout outside energy a self-acting machine cannot transfer heat from a body at a lower temperature to one at a higher temperature.
QuestionState Kelvin's statement.
AnswerIt is impossible to obtain a continuous supply of energy by cooling a body below the coldest part of its surroundings.
QuestionState Planck's statement.
AnswerIt is impossible to make a heat engine that absorbs heat continually from a source and converts all of it into work.
QuestionState Carnot's statement.
AnswerIt is impossible to build a machine that changes a given heat energy completely into mechanical energy.
QuestionWhat is the main difference between the first and second laws?
AnswerThe first is conservation and equivalence of energy; the second says only part of the heat becomes work and a hot and a cold body are needed.
QuestionWhich variable does the zeroth, first and second law point to?