文档介绍：PART 1
THE SECOND LAW OF THERMODYNAMICS
PART 1 - THE SECOND LAW OF THERMODYNAMICS
. Background to the Second Law of Thermodynamics
[IAW 23-31 (see IAW for detailed VWB&S references); VN Chapters 2, 3, 4]
Some Properties of Engineering Cycles; Work and Efficiency
As motivation for the development of the second law, we examine two types of processes that
concern interactions between heat and work. The first of these represents the conversion of work
into heat. The second, which is much more useful, concerns the conversion of heat into work. The
question we will pose is how efficient can this conversion be in the two cases.
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Block on rough surface
Viscous liquid Resistive heating
Figure A-1: Examples of the conversion of work into heat
Three examples of the first process are given above. The first is the pulling of a block on a rough
horizontal surface by a force which moves through some distance. Friction resists the pulling.
After the force has moved through the distance, it is removed. The block then has no ic
energy and the same potential energy it had when the force started to act. If we measured the
temperature of the block and the surface we would find that it was higher than when we started.
(High temperatures can be reached if the velocities of pulling are high; this is the basis of inertia
welding.) The work done to move the block has been converted totally to heat.
The second example concerns the stirring of a viscous liquid. There is work associated with the
torque exerted on the shaft turning through an angle. When the stirring stops, the es to
rest and there is (again) no change in ic or potential energy from the initial state. The fluid
and the paddle wheels will be found to be hotter than when we started, however.
The final example is the passage of a current through a resistance. This is a case of electrical work
being converted to heat, indeed it models operation of an electri