Entropy: A Measure of Disorder
Entropy is a fundamental concept in thermodynamics that describes the degree of disorder or randomness in a system.
Entropy
Entropy is a measure of the disorder or randomness in a system. It quantifies the number of possible ways a system can be arranged.
Calculating Entropy Change
The change in entropy, $ \Delta S $, is defined as:
$$
\Delta S = \frac{\Delta Q}{T}
$$
where:
- $ΔQ$ is the heat added to the system.
- $T$ is the absolute temperature (in Kelvin) at which the heat is added.
If 200 J of heat is added to a system at 400 K, the change in entropy is:
$$
\Delta S = \frac{200 \, \text{J}}{400 \, \text{K}} = 0.5 \, \text{J K}^{-1}
$$
Entropy increases when heat is added to a system and decreases when heat is removed.
The Second Law of Thermodynamics
The second law of thermodynamics introduces the concept of entropy to explain why certain processes occur spontaneously while others do not.
The second law of thermodynamics
The second law of thermodynamics states that in any natural process, the total entropy of an isolated system always increases or remains constant in ideal reversible processes.
Entropy Increases in Isolated Systems
- In an isolated system (where no energy or matter is exchanged with the surroundings), the total entropy always increases or remains constant.
- This is often stated as:
- Entropy increases in realistic, irreversible processes.
- Entropy remains constant in idealized, reversible processes.
- When a hot object is placed in contact with a cold object, heat flows from the hot object to the cold one.
- The entropy of the hot object decreases, but the entropy of the cold object increases by a larger amount, leading to a net increase in entropy.
- A common misconception is that entropy always increases.
- In fact, entropy can decrease locally (e.g., when a gas is compressed), but the total entropy of the system and its surroundings will always increase or remain constant.
Reversible and Irreversible Processes
Reversible processes
Reversible processes are idealized processes that occur in such a way that the system and its surroundings can be returned to their original states without any net change.
In these processes, the entropy change of the system is exactly balanced by the entropy change of the surroundings, resulting in no net change in the total entropy.
Irreversible processes
Irreversible processes are real-world processes that cannot be undone without leaving a net change in the system or surroundings.
