http://www.ck12.org Chapter 14. Thermodynamics
14.3 Second Law of Thermodynamics
Objectives
The student will:
- Understand the second law of thermodynamics
- Understand how to calculate the efficiency of a heat engine
- Understand how a Carnot engine operates
- Understand that entropy is a measure of disorder
Vocabulary
- Heat engine
- Reversible process
- Irreversible process
- Efficiency
- Carnot engine
- Entropy
Introduction
The first law of thermodynamics is a statement of the conservation of energy. But not all processes that conserve
energy are physically possible. For example, it would not violate the law of conservation of energy if a glass of water
at room temperature were to freeze. Nothing in the first law prevents the molecular motion of the water molecules
from transferring heat through the glass to the atmosphere and therefore lowering the water temperature enough for
it to freeze. But this never happens.
The second law of thermodynamics describes the direction in which physical phenomena can occur.One statement
of the second law is: Heat can flow spontaneously from hot to cold but never from cold to hot.
http://demonstrations.wolfram.com/HeatTransferAndTheSecondLawOfThermodynamics/
There are certain processes that we know can never happen in reverse: A tea cup falls and shatters after hitting the
floor. But it does not put itself back together again and float back up from where it fell. All natural processes are
irreversible. Even after one swing, a pendulum is not back to the height from where it was released. Friction ensures
that some of the gravitational potential energy of the pendulum is converted into heat. On the other hand, heat does
not spontaneously convert into potential energy: the pendulum would never start to swing wider while cooling down,
will it?
Nature’s tendency is to go from a more ordered state to a less ordered state.