LECTURE 2

System:

We introduce boundaries in our study called the system and surroundings.

The boundaries are set up in a way most conducive to understanding the energetics of what we're studying.

Defining the system and surroundings is arbitrary, but it becomes important when we consider the exchange of energy between the system and surroundings.

Two types of exchange can occur between system and surroundings: (1) energy exchange (heat, work, friction, radiation, etc.) and (2) matter exchange (movement of molecules across the boundary of the system and surroundings).

Based on the types of exchange which take place or don't take place, we will define three types of systems:

• isolated systems: no exchange of matter or energy
• closed systems: no exchange of matter but some exchange of energy
• open systems: exchange of both matter and energy

Control Volume

control volume is defined as a volume which encloses the matter and the device inside a control surface.

Every thing external to the control volume is the surroundings with the separation given by the control surface.

The surface may be open or closed to mass flows and it may have flows from energy in terms of heat transfer and work across it.

The boundaries may be moveable or stationary.

In the case of a control surface that is closed to the mass flow, so that no mass can enter or escape the control volume, it is called a control mass containing same amount of matter at all times.

Property

In thermodynamics a property is any characteristic of a system that is associated with the energy and can be quantitatively evaluated.

The property of a system should have a definite value when the system is in a particular state.

Thermodynamic property is a point function.

Properties like volume of a system that depend on the mass of a system are called extensive properties.

Properties like pressure or temperature which do not depend on the system mass are called intensive properties.

The ratio of extensive property to the mass of the system are called specific properties and therefore become intensive properties.

Substance can be found in three states of physical aggregation namely, solid, liquid and vapor which are called its phases.

If the system consists of mixture of different phases, the phases are separated from each other by phase boundary.

The thermodynamic properties change abruptly at the phase boundary, even though the intensive properties like temperature and pressure are identical.

Equilibrium

When the property of a system is defined, it is understood that the system is in equilibrium.

If a system is in thermal equilibrium, the temperature will be same throughout the system.

If a system is in mechanical equilibrium, there is no tendency for the pressure to change. In a single phase system, if the concentration is uniform and there is no tendency for mass transfer or diffusion, the system is said to be in chemical equilibrium.

A system which is simultaneously in thermal, mechanical, and chemical equilibrium is said to be in thermal equilibrium.

Process

A process is path followed by a system in reaching a given final state of equilibrium state starting from a specified initial state.

An actual process occurs only when the equilibrium state does not exist.

An ideal process can be defined in which the deviation from thermodynamic equilibrium is infinitesimal.

All the states the system passes through during a quasi-equilibrium process may be considered equilibrium states.

For non-equilibrium processes, we are limited to a description of the system before the process occurs and after the equilibrium is restored.

Several processes are described by the fact that one property remains constant.

The prefix iso- is used to describe such processes.

A process is said to be reversible if both the system and its surroundings can be restored to their respective initial states by reversing the direction of the process.

• reversible: if the process happens slow enough to be reversed.
• irreversible: if the process cannot be reversed (like most processes).

• isobaric: process done at constant pressure
• isochoric: process done at constant volume
• isothermal: process done at constant temperature
• adiabatic: process where q=0
• cyclic: process where initial state = final state

The basic units (SI Units)

Mass---------kg.

Mole------ The mole is the amount of substance that contains as many atoms (or molecules) as there are atoms in 0.012 kg of carbon-12.

Length—m.

Time: second

SI unit of temperature is Kelvin (abbreviated as K). The Kelvin is defined as the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water. The relation between Kelvin and Celsius temperature is K = C + 273.15 (The triple point of water is at 0.01 C).

Derived units: Force: 1 N = 1 kg m/s, pressure 1 Pa = 1 N/m², 1 bar = 10 Pa, 1 atm. = 101325 Pa. In thermodynamics we are concerned with absolute pressure. Gauge pressure = absolute pressure – atmospheric pressure. Ordinary vacuum gauge pressure = atmospheric pressure – absolute pressure.