04-07-2012, 11:31 AM
THE SECOND LAW OF THERMODYNAMICS
[attachment=26622]
Background to the Second Law of Thermodynamics
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.
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 kinetic
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.
Difference between Free Expansion of a Gas and Reversible Isothermal Expansion
The difference between reversible and irreversible processes is brought out through
examination of the isothermal expansion of an ideal gas. The question to be asked is what is the
difference between the “free expansion” of a gas and the isothermal expansion against a piston?
To answer this, we address the steps that we would have to take to reverse, in other words, to undo
the process.
By free expansion, we mean the unrestrained
expansion of a gas into a volume as shown at the right.
Initially all the gas is in the volume designated as V1 with the
rest of the insulated enclosure a vacuum. The total volume
(V1 plus the evacuated volume) is V2 .
At a given time a hole is opened in the partition
and the gas rushes through to fill the rest of the enclosure.
[attachment=26622]
Background to the Second Law of Thermodynamics
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.
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 kinetic
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.
Difference between Free Expansion of a Gas and Reversible Isothermal Expansion
The difference between reversible and irreversible processes is brought out through
examination of the isothermal expansion of an ideal gas. The question to be asked is what is the
difference between the “free expansion” of a gas and the isothermal expansion against a piston?
To answer this, we address the steps that we would have to take to reverse, in other words, to undo
the process.
By free expansion, we mean the unrestrained
expansion of a gas into a volume as shown at the right.
Initially all the gas is in the volume designated as V1 with the
rest of the insulated enclosure a vacuum. The total volume
(V1 plus the evacuated volume) is V2 .
At a given time a hole is opened in the partition
and the gas rushes through to fill the rest of the enclosure.