Question

Energy: Some Basic Principles Define the terms system and surroundings. What does it mean to say that a system and its surroundings are in thermal equilibrium?

Short answer

A system is the part of the universe being studied, the surroundings are everything else, and thermal equilibrium means no net heat flow between them.

Step-by-step solution

Define 'System'

In thermodynamics, a 'system' refers to the portion of the universe that is being studied or observed. It can be as large as a container of gas or as small as a single atom. The system is separated from the rest of the universe by a boundary, which may be real and fixed or simply an imaginary line.

Define 'Surroundings'

The 'surroundings' refer to everything outside the system's boundary. Essentially, the surroundings encompass the rest of the universe that interacts with the system. Any exchange of energy or matter between the system and its surroundings occurs through the boundary.

Explain Thermal Equilibrium

When we say that a system and its surroundings are in thermal equilibrium, it means that there is no net flow of thermal energy between them. This occurs when both the system and its surroundings have reached the same temperature, and as a result, the macroscopic properties related to temperature remain constant over time.

Key concepts

System

In the realm of thermodynamics, a 'system' is essentially the specific part of the universe you are focusing on. Imagine it as your subject of study. This could range from a large container filled with gas to a minute molecule, depending on what you wish to analyze.
The system is defined by a boundary. This boundary can be something physical and tangible—a wall, a container—or it can be an imaginary line that separates it from everything else. For example:
- A pot of boiling water on the stove can be seen as a system, with the pot acting as the boundary. - A sealed can of soda could be a system, where the can itself serves as the boundary. Identifying the system is crucial for understanding how energy and matter interact within and outside this boundary.

Surroundings

Now, let's turn our focus to the 'surroundings.' Everything outside the system's boundary is considered the surroundings. Essentially, this means the rest of the universe that has the potential to interact with the system.
The surroundings are significant because they dictate how the system exchanges energy and matter. Any interaction across the boundary occurs with the surroundings. For example:
- In the case of our boiling pot, the surrounding includes the air in the kitchen and even the room itself. - The surroundings for the sealed soda can would involve the ambient environment around it, including the air temperature and possible heat sources.
Understanding the surroundings is key to predicting how the system will respond to external changes. This allows us to analyze processes like heating, cooling, and chemical reactions.

Thermal Equilibrium

'Thermal Equilibrium' is a state of balance. When a system and its surroundings are at thermal equilibrium, it means that both have reached a temperature where there is no more net heat flow between them. They are, quite literally, at the same temperature. This situation can be visualized as follows:
- Consider a cup of coffee left on a table. Initially, the coffee (system) is hotter than the room air (surroundings). Over time, the coffee cools as heat transfers to the air until both the coffee and the room air reach the same temperature. - A similar process happens with ice cubes melting in water. Initially, the water is warmer, but as energy flows to the ice, both eventually reach the same temperature, achieving thermal equilibrium.
Reaching thermal equilibrium is important because once achieved, macroscopic properties related to temperature, such as pressure and volume, stop changing over time. This gives us a stable system to study energy exchanges.