Lec 1 : Introduction, conservation of energy, dimensions and units, systems, properties

Lec 1: Introduction, conservation of energy, dimensions and units, systems, properties Mt St Helens, 1980

ENGR 212-503 • For next time: • Read: §1-1 to 1-10 • HW 1 --- book not available; postponed to September 8, 2003 • Outline: • Introduction • What is thermodynamics? • New concepts • Important points: • Systems (open/closed) • Properties (intensive/extensive) • Units

Laws of thermodynamics • First law of thermodynamics is same as law of conservation of energy--deals with quantities of energy • Energy can change forms: from potential energy to kinetic energy, for example--but the total amount never changes.

Laws of thermodynamics • Where a system such as a human body may undergo an energy change, the first law is written:

Units • Two systems will be used heavily: • The International System (known as the SI system from its French title--see text). • The US Customary System (USCS). (The USCS is also known as the inch-pound (IP) system and the English Engineering System).

Newton’s 2nd Law

SI system constant • In the simpler SI system the units are chosen so that the constant is unity. The primary dimensions are mass (kg), length (m), and time (s or sec). • The equation that contains four units is not over-determined because force F is then a derived or defined unit.

Force is a defined unit in SI 1 N  1 kg  1 m / s2

USCS • This unfortunate system has four applicable primary dimensions for the equation which has only four dimensions. • Mass is lbm • length is ft • time is s (sec) • force is lbf

USCS • The system is over-determined • We get around this by introducing gc, the gravitational constant, so that

Gravitational constant • In other words, the const in F=maconst is:

Gravitational constant • Please memorize this.

Gravitational constant • Your text gets around the problem of over-determination by defining:

Advice which will gain you points • 1. Write out all units in all equations to the bitterest of detail. • 2. Use a horizontal line in all equations, not a slash. e.g. Write ; Not F = ma/gc

To gain you points • Remember that a goal is to communicate that you know how to do the assignments--including obtaining the correct answer. • Keep steps in order--work from top to bottom on the page--do not jump erratically to left over open space for key steps, which may then be out of order. Circle or box key numbers and results. • Make it easy for your reader, not hard.

Notes on expressing units • The º symbol is not used with absolute temperatures K or R. • Unit names are not capitalized even if they are proper names (for example, 1,000 watts). • Abbreviations of units are capitalized if they are from proper names (1,000 W or 1.2 kWh). • Abbreviations are never plural (4 joules but 4 J; 4 kilojoules but 4 kJ).

TEAMPLAY WHEN A TEAM PLAY EXERCISE APPEARS, IT IS TO BE SOLVED FIRST BY YOU, THEN BY A PARTNER, AND FINALLY AS A GROUP OF FOUR.

TEAMPLAY In the old movie Armageddon, actor Bruce Willis flew to an asteroid: • What was the force on Bruce Willis, whom we shall assume weighs perhaps 200 lbm, when he stepped on the asteroid where the acceleration was an assumed 1.8 ft/s2?

System • A system is a quantity of matter or region of space we want to study. • Two types of systems (closed and open). • The first is known as a closed system (or control mass)--because it involves a fixed amount of matter and is closed to matter (mass) transfer across the system boundary.

System • Closed system--no mass crosses boundary indicated by dashed line. • Mass indicated by gray is system to study. • Boundary can move (piston could go in and out)

System • A system is a quantity of matter or region of space we want to study. • The second is known as an open system (or control volume)--because it involves a fixed region in space and is open to matter (mass) transfer across the system boundary.

System Fluid velocity=V0 • Open system--boundary generally does not move (it could) and mass crosses the boundary. • We are just interested in theregion bounded by the dashed lines.

System System boundaries are often defined by dashed lines, as in the previous pipe flow example. The dashed lines define a control surface.

TEAMPLAY • A bottle of Coke is placed in a refrigerator that is at 3C: • Should the bottle of Coke be treated as a closed or open system?

Definitions • Property--A characteristic of a system to which numerical values can be assigned to describe the system.

Definitions • State--Condition of a system defined by its properties.

TEAMPLAY • What are some properties that define the liquid in the previous Coke example of a system?

Definitions • Extensive properties--properties which can be counted, and for which their value for the whole system is the sum of the value for subsystems into which the overall system might be divided. • They depend on the extent of the system. • Often symbolized with capital letters.

Definitions • Intensive properties are independent of the size (mass or volume) of the system. • Often symbolized by lower case letters.

Energy • Energy = sum of internal energy, kinetic energy, and potential energy. • (Note energy E here is an extensive property).

Energy • On a unit mass basis, or intensive basis • Every term in the previous equation has been divided through by the mass m.

Other units • Volume V will be in m3 and ft3 • Density  is in kg/ m3 or lbm/ ft3 • Specific volume v is new and powerful; the inverse of density, it is in units of ft3/lbm or m3/kg.

Specific gravity • Also known as relative density--it is the ratio of the density of a substance to some reference density. • Reference density is usually water at 4 ºC. • Sometimes, for gases, “gas gravity” is used, and in that case the reference density is for air at 60 ºF and 14.696 psia.

TEAMPLAY Sketch possible system boundaries for the following and classify them as open or closed: • Bicycle tire deflating. • Kettle of water heating (but not boiling). • Kettle of water boiling. • Jet engine in flight. • Pentium IV, 2.4 Ghz chip in operation.

Lec 1 : Introduction, conservation of energy, dimensions and units, systems, properties