conservation. Energy cannot be created by a process; it must stay constant.
The second law is a little more dire: It says that during an engine cycle, engines do less
work than the energy transferred into them. In principle, the work could equal 99.9% of
the energy that flows into an engine, but it can never equal 100%.
This law is not about designing an efficient engine. It is instead a physical law that limits
the efficiency of any engine. No engine can be 100% efficient.
Another way to state the second law, called the Clausius statement, is: There can be no
process whose sole final result is the transfer of heat from a cooler object to a warmer
object. Heat flows spontaneously from an object at a higher temperature to an object at
a lower temperature. Heat will not flow in the other direction unless compelled to do so.
This direction of heat flow agrees with your everyday experience. If you place a quart of
ice cream in a hot car, you expect the ice cream to get warmer, not colder. Energy flows
from the hotter air to the cooler ice cream.
Stating the law in this fashion may help you to understand why no engine can be 100%
efficient. The illustrations at the right show a conceptual diagram of a heat engine. Heat
is allowed to flow into the engine from the hot reservoir. Some heat also flows out to the
cold reservoir. The maximum amount of work the engine can do during a cycle is the
net flow of the heat, the heat in minus the heat out.
An enterprising engineer might think she could “recycle” the heat absorbed from the
engine by the cold reservoir, say by connecting the cold reservoir to the hot reservoir. The Clausius statement of the second law says that heat
will not flow spontaneously from the cold reservoir to the hot.
To solve this problem, she might attach another engine to force heat to flow from the cold reservoir to the hot. This is indeed possible: Air
conditioners and refrigerators cause heat to flow from a cooler region to a warmer one. However, this heat flow is not spontaneous or free; it
requires work and comes at a price, as the electrical bills for air conditioners and refrigerators indicate.
Heat flows spontaneously only from hot
to coldNon-spontaneous heat flow
Work required to force heat to flow from
cold to hot21.3 - Reversible and irreversible processes
Reversible process: A process in which a
system can be returned to its initial state without
the addition of energy.
Irreversible process: Energy must be added to
a system to return it to its initial state after an
irreversible process occurs.
Some of the crucial underpinnings of the theories concerning engine efficiency rely on
the concepts of reversible and irreversible processes. In general, a process is a series
of steps that move a system from one state to another. When a system undergoes a
reversible process, it can be returned to its initial state without the addition of energy.
Every real process is irreversible, although some are close to being reversible.
Consider, for example, a puck attached to a spring on an air hockey table, as shown in
Concept 1. Imagine the spring is initially compressed and the puck is held in place.
When the puck is released, the compressed spring will push the puck to the right until it
pauses momentarily. This is a process: The spring pushing the puck until it pauses. In
an ideal system (no friction, no air resistance), the puck will return to its initial position
as the spring contracts with no additional energy required. The process of spring
expansion can be thought of as reversible.
In contrast, if you drop an egg and it breaks, several changes take place that cannot be
easily reversed (that is quite an understatement). For example, the breaking of the egg
creates sound energy through vibrations in the air. This reduces the energy of the egg.
You do not expect to be able to put it back together again without extraordinary effort.
This is an irreversible process. It would take an extreme amount of time and effort
(Humpty Dumpty inevitably comes to mind) to return the egg to its initial state.
One way to think about reversibility is to imagine videotaping a process. If you can
easily decide whether the tape is being played forward or backward, the process is
irreversible. If you watched a videotape of an egg shattering, you would know the
direction the tape is being played. On the other hand, with the puck and the spring, you would not be able to discern easily if the tape were
being played forward or backwards. (If you watched it for several cycles of motion, however, you would note that the extreme positions of the
puck were less far from equilibrium as the system lost energy to dissipative forces like air resistance and friction.)
Reversible process
System can be returned to initial state
without adding energyIrreversible process
Cannot be reversed without adding
energy
Video runs only "one way"