IE 211

INTRODUCTION TO ENGINEERING THERMODYNAMICS

Chapter2-Part1

‘’Energy, Energy Transfer and General Energy Analysis’’

‘’Energy cannot be created or destroyed during a process; it can only

change from one form to another’’ (1. Law of Thermodynamics)

A refrigerator in a well insulated room Temp. will rise or not?

SYSTEM: air+ refrigerator

ADIABATIC CLOSED SYSTEM

(no mass and heat flow)

Only electrical energy(work) crosses the

boundary

Electrical Energy (input)

Conservation of energy requires;

Energy content of the room increases by an

equal amount to energy drawn by refrigerator

What about a fan running in a well-insulated room?

FORMS of ENERGY

Total energy of a system can be examined in two groups

1) Macroscopic Forms: Related to motion and the influence of some

external effects such as gravity, magnetism, electricity, and surface

tension

(kinetic (K.E.) and potential energy(P.E.))

2) Microscopic Forms: related to the molecular structure of a system and

the degree of the molecular activity, independent of outside reference

frames (internal energy,(U),i.e inner work, internal work, intrinsic energy)

-Thermal

- Mechanical

- Electrical

- Magnetic

- Chemical

- Nuclear

Their sum constitutes the total energy, E, of system

Total energy of a system on a unit mass (e);

e= E/m (kJ/kg)

The effect of magnetic,electric, and surface tension are significant in some

specialized cases only and usually ignored

TOTAL ENERGY of a SYSTEM

E = microscopic energy+ macroscopic energy

E = U + K.E. + P.E. = U + mV2/2 + mgh

Energy on a unit mass; e = u + V2/2 + gh

Mechanical energyInternal energy

Thermodynamics deals only with the total energy

change of the system, (E)

E = U + K.E. + P.E

MECHANICAL (macroscopic) ENERGY: Form of energy that can be

converted to mechanical work completely and directly by an ideal mechanical

device such as an ideal turbine

Examples: Kinetic and potential energy (thermal energy is not, since it cannot be

converted to work completely)

Pressure itself is not a form of energy. But a pressure force acting on a fluid

through a distance produces work, called flow work in the amount of P/ (J/kg)

Mechanical energy of a flowing fluid on a unit mass;

emech = P/ + V2/2 + gh

Energy in rate form;

Emech = memech = m (P/ + V2/2 + gh). . .

(J/s = Watts, W)

Mechanical energy change of a fluid during incompressible (: const.) flow;

emech = (P2-P1)/ + [(V2)2 – (V1)

2]/2 + g(h2-h1)

Mechanical energy of flowing Fluid:

Wind turbine

Water turbine

Generate mechanical work

No conservation of nuclear, chemical or thermal energy to mechanical

energy and no heat transfer. Only Mechanical forms of energy!

pump

Fan

Consume mechanical work

(Turbine extracts mechanical

energy from fluid by dropping its

pressure)

(A pump transfers mechanical

energy to a fluid by raising its

pressure)

MECHANICAL ENERGY

STATIONARY SYSTEMS: Closed systems (no mass transfer) whose

velocity and elevation of the center of gravity remain constant during the

process (K.E., P.E.=0)

Total energy change in stationary systems;

E = U (Internal energy change of the system depens on heat,Q and work transfer,W)

(velocity) V1= V2

(altitude) h1 = h2

+ +

INTERNAL ENERGY,(U)

Atoms may also posses electric and magnetic dipole-moment energies when

subjected to external electric and magnetic fields due to twisting of the

magnetic dipoles produced by the small electric currents associated with the

orbiting electrons

1) Internal energy due to kinetic energy:

+++++

SENSIBLE ENERGY

The protion of the internal energy of a system associated with the kinetic

energies of the molecules

2) Internal energy due to binding forces:

Binding forces exist between the molecules of a substance, between the atoms

within a molecule and between the particles within an atoms and its nucleus.

If sufficient energy is given to molecules of solids or liquids (molecules overcome

binding forces) solids melt and evaporate, liquids evaporate

The internal energy associated with the phase of a system is called the LATENT

ENERGY, e.g. Latent heat of melting, latent heat of evaporation

The internal energy associated with the atomic bonds in a molecule is called

CHEMICAL ENERGY. (during combustion process chemical bonds are destroyed)

The tremendous amount of energy associated with the strong bonds within the

nucleus of the atom itself is called NUCLEAR ENERGY.

STATIC and DYNAMIC FORMS of ENERGY

Static Energy : Energy that can be contained or stored in the system

Dynamic Energy : Form of energy not stored in the system and can be

viewed as energy interaction.

e.g. Heat,Q, and work, W transfer in closed system

Internal energy,U, of a system can be altered by heat and work transfer

(1) Closed system (control mass) (3) Adiabatic system (well-insulated)

NOTE: There is no mass and heat(Q)

transfer, but the energy content and thus the

temperature can be changed by WORK, W

(another form of energy)

(2) Open system (control volumes)

Exchange energy via mass transfer

ENERGY TRANSFER

(to the system or from the system)

(heat ,Q, and work,W)

(4) Isolated system

(has rigid boundary)

No Mass and energy transfer!

1) WORK(W), (Joules,J)

Work is the energy transfer associated with a force acting through

a distance

Examples:

Work per unit mass of a system, w;

w = W/m, (J/kg)

The work done per unit time is called, POWER, W;.

(J/s or W)

or

moving piston Rotating cranck shaft Electric wire crossing the system

FORMS OF WORK

Non-Mechanical

- Electrical work

- Electrical polarization work

Mechanical

- Shaft work

- Spring work

- Work done on elastic solid bars

- Work associated with the stretching of a

liquid film

- Work done to raise or to accelerate a body

NON MECHANICAL FORMS of WORK

SYSTEM: heating element + air in oven

In a electric field, electrons in a wire move

under the effect of electromotive forces,

doing work

When N coulombs of electrical charge

move through a potential difference, V:

We = V.N

(Electrical Work)

I : number of

electrical charge

moving per unit time

MECHANICAL FORMS of WORK;

1) There must be a force acting on the boundary

2) The boundary must move

Example: Expansion of a gas into an evacuated space is not a work interaction since no

energy is transferred

1) Shaft work

s = (2r).nn: number of revolutions

Shaft work: Wsh = F.s = T.2n

2) Spring work

Wspring = Fdx

Where F = kx,

k: spring constant (N/m)

3) Work done on elastic solid bar

4) Work associated with the stretching of a liquid film

Force generated per unit length is called surface tension, s

Work done is called surface tension work

dA = 2bdx

F= 2bs

5) Work done to raise or to accelerate a body

(1) Work transfer needed to raise a body is equal to the change in the potential energy

of the body

(2) Work transfer needed to accelerate a body is equal to the change in the kinetic

energy of the body

Example: generation of hydroelectric power

2) HEAT (Q), (Joules,J)

Form of energy that is transferred between two systems (or a system and its surroundings)

Energy transfer takes place until the equilibrium is maintained.

Heat Transfer mechanisms:

1) Conduction Interaction between particles(solid-solid)

2) Convection Between a solid surface and a fluid in motion

3) Radiation Due to emission of electromagnetic waves (or

photons) into open atm. or vacuum

Direction: Higher temperature body to the lower

Heat is only recognised as it crosses the boundary of a system

Example:Heat transfer through the skin (system boundary) of the hot potato

In thermodynamics HEAT is simply means HEAT TRANSFER

Heat transfer per unit mass of a system, q;

q = Q/m, (J/kg)

or

The following terms are not consistent with the thermodynamic meaning of heat;

Body heat means the thermal energy content of the body

Heat flow means the transfer of thermal energy

Heat addition the transfer of heat into a system

Heat rejection the transfer of heat out of a system

Heat transfer rate, Q (J/s or W);.

(Total heat transfer)

Heat and work are path functions (magnitude depends on the path followed

during process)

Path functions have inexact differentials designated by symbol,

* Differential amount of work: W

* Differential amount of heat: Q

Properties, e.g. Volume, are point funtions (they depend state only, not on

how system reaches that state)

Point functions have exact differentials designated by symbol, d

* Small change in volume: dV

Adding differential amounts of work;

SIMILARITIES BETWEEN HEAT(Q) AND WORK (W)

Both heat and work are recognized at the boundaries of a system

System posses energy, but not heat or work

Both are associated with a process, not a state

Both are path functions

FORMAL SIGN CONVENTION

Heat and work are directional quantities (both magnitude and direction

should be specified)

(+), Heat transfer to the system

(-), Heat transfer from the system

(+), Work done by the system

(-), Work done on the system

(U = Q – W)

EXAMPLES

The potato baked in an oven

Increase in the total energy of the potato

becomes equal to the amount of heat transfer. (If

we disregard any mass transfer;i.e. Moisture loss from

potato)

Heating of water in a pan

Increase in the total energy of the water is

12 kJ

1) Processes that involve only heat trasfer(Q)

2) Processes that involve only work trasfer(W), works are done on system (-)

Heating of well-insulated(adiabatic) room by an electrical heating

Stirring process

Compression

Conversion of non-mechanical(electrical work)

Conversion of mechanical(shaft work)

Conversion of mechanical

Boundary work is transferred to the air inside of the cylinder.

Both pressure and temperature of the air increases

ENERGY BALANCE(Conservation of Energy)

‘’ The change in the energy of a system during a process is simply equal to the

net energy transfer to(or from) the system’’

The net change in total energy (E):The difference of the total energy energy

entering and the total energy leaving the system

Esystem = Ein - Eout

Energy is a property, depends on the state of a system (E=0 if state does not change)

ENERGY FORMS Internal (sensible, latent, chemical and nuclear)

Kinetic

Potential

Electric

In the absence of electric, magnetic and surface tension effects;

Esys = Ein - Eout = U + K.E. + P.E.

For stationary systems (KE, PE = 0)

Esys = U

Esys = Ein - Eout= (Qin-Qout) + (Win-Wout) + (Emass,in- Emass,out)

Open systems : mass transfer,Q,W 0

Closed systems : mass transfer=0; Q,W 0

Adiabatic systems : mass transfer,Q=0; W 0

Isolated systems : mass transfer,Q,W = 0

(Macroscopic)

(Microscopic)