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Learn about properties of systems, mechanical forms of work, laws of thermodynamics, and system limitations. Understand the concept of closed, open, and isolated systems. Explore the application areas and measurement aspects of thermodynamics.
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Unit 5 Thermal Engineering Thermodynamics: Measurement:
SYSTEMS AND CONTROL VOLUMES A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary. Note that the boundary is the contact surface shared by both the system and the surroundings. Mathematically speaking, the boundary has zero thickness, and thus it can neither contain any mass nor occupy any volume in space.
PROPERTIES OF A SYSTEM Any characteristic of a system is called a property. Some familiar properties are pressure P, temperature T, volume V, and mass m. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size-or extent-of the system. Total mass, total volume, and total momentum are some examples of extensive properties.
Examples of extensive properties include: Energy , entropy, Gibbs energy, length, Mass, particle number, Momentum, number of moles, Volume, magnetic moment, electrical charge, Weight, Examples of intensive properties include: chemical potential concentration density (or specific gravity) ductility elasticity electrical resistivity hardness magnetic field malleability melting point and boiling point molar absorptivity pressure specific energy specific heat capacity specific volume temperature viscosity
MECHANICAL FORMS OF WORK If there is no movement, no work is done. In elementary mechanics, the work done by a constant force ‘F’ on a body displaced a distance ‘s’ in the direction of the force is given by Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy E of a system.
Systems may be considered to be closed or open, A closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. An open system, or a control volume, as it is often called, is a properly selected region in space. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system. A closed system with a moving boundary. Mass cannot cross the boundaries of a closed system, but energy can.
An open system (a control volume) with one inlet and one exit. A control volume can involve fixed, moving, real, and imaginary boundaries.
Thermodynamics can be defined as the science of energy. • The name thermodynamics systems from the Greek words therme (heat) and dynamis(power), which is most descriptive of the early efforts to convert heat into power. ZEROTH LAW OF THERMODYNAMICS ‘Zeroth law of thermodynamics’ states that if two systems are each equal in temperature to a third, they are equal in temperature to each other.
THE FIRST LAW OF THERMODYNAMICS One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. A rock falling off a cliff, for example, picks up speed as a result of its potential energy being converted to kinetic energy • The first law of thermodynamics states that energy can be neither created nor destroyed during a process; it can only change forms.
LIMITATIONS OF FIRST LAW OF THERMODYNAMICS AND INTRODUCTION TO SECOND LAW It has been observed that energy can flow from a system in the form of heat or work. The first law of thermodynamics sets no limit to the amount of the total energy of a system which can be caused to flow out as work. A limit is imposed, however, as a result of the principle enunciated in the second law of thermodynamics which states that heat will flow naturally from one energy reservoir to another at a lower temperature, but not in opposite direction without assistance. This is very important because a heat engine operates between two energy reservoirs at different temperatures. Further the first law of thermodynamics establishes equivalence between the quantity of heat used and the mechanical work but does not specify the conditions under which conversion of heat into work is possible, neither the direction in which heat transfer can take place. This gap has been bridged by the second law of thermodynamics.
Kelvin-Planck Statement “It is impossible to construct an engine, which while operating in a cycle produces no other effect except to extract heat from a single reservoir and do equivalent amount of work”. Although the Clausius and Kelvin-Planck statements appear to be different, they are really equivalent in the sense that a violation of either statement implies violation of other.
Clausius Statement “It is impossible for a self acting machine working in a cyclic process unaided by any external agency, to convey heat from a body at a lower temperature to a body at a higher temperature”. In other words, heat of, itself, cannot flow from a colder to a hotter body.
ENERGY CONVERSION EFFICIENCIES The efficiency of a cooking appliance represents the fraction of the energy supplied to the appliance that is transferred to the food.
Thermocouple A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces avoltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control.
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure
What Is a Barometer? A barometer is a widely used weather instrument that measures atmospheric pressure (also known as air pressure or barometric pressure) - the weight of the air in the atmosphere.
