1

Work

Heat

Adiabatic Process

Reversible Process

Irreversible Process

Isobaric process

Isochoric process

Isothermal process

Polytropic process

Modes of Heat Transfer

Chapter 4 : Work and Heat

Work – the energy transfer associated with a force acting through a distance

- a form of energy

- has unit such as kJ

- the work done during a process between states 1 and 2 is denoted 1W2 or simply W

The work done per unit mass of a system is denoted w and is defined as

(kJ/kg)

The work done per unit time of a system is called power and

denoted as . The unit of power is kJ/s, or kW

WORK

m

Ww

W.

Work Sign Convention

work done by a system : +ve

e.g. Work produced by car engines, hydraulic, steam, or gas

turbines

Work done on the system : -ve

e.g. Work consumed by compressors, pumps, and mixers : negative

System System

Surroundings

(+)

(+)

(-)

(-) W

Q

Work done by system

• Therefore;

• or in terms of the specific volume, v:

where m is the mass of the system.

Work = Area under the curve

LETS DO

Quiz 1

Quiz 3

• If data PV is not available but an analytical

expression is given, we can integrate directly;

• Example: PV = constant

P1V1 = P2V2 = PV = constant

P = V

VP 11

• Substitute,

1W2 = P1V1

1W2 = P1V1

2

1V

dV

1

2lnV

V

• A common PV relation in a process is called a polytrophic process.

PVn = constant or P = nV

constant

•Substitute into work equation,

1W2 = n

VPVP

11122 (4.4)

Eq(4.4) is not applicable for n=1

Polytrophic

process ,

PVn = k

Q 5

For ideal gas,

n=0 Isobaric n=1 Isothermal

n= γ Adiabatic n= α Isohoric

Exercises

Lets look at tutorial problem 5

Exercises

Lets look at tutorial problem 5

Exercises

Please try out Quiz 9

• Equation of state for thermally perfect gas

pV = nRT

n = number of moles

R = Universal gas constant (8.314J/mol.K)

V = total system volume

also for T = constant, p1V1 = p2V2 , work done

by the system is

or in terms of the specific volume and the system

mass

Isothermal

process

Isothermal

process

Combined Gas Law

PV

R T

PV

R T

1 1

1

2 2

2

Chapter 3

More Exercises

Lets look at tutorial problem 4 Cengel text book 3-7 compressibility factor

Quiz 8

Quiz 10

Heat, Q

– Heat is energy transferred between a system

and its surroundings by virtue of a

temperature difference only.

• "Adiabatic" means no heat is transferred (Q=0).

• Unit : Joule, Calorie, BTU etc

Heat Sign convection

(+) heat transferred to the system

(-) heat transferred from the system

System System

Surroundings

(+)

(+)

(-)

(-)

W

Q

Reversible process

• Defined as a process which can be

reversed – both the system and

surrounding are returned to their initial

state at the end of the reverse process

Irreversible process

• Process that are not reversible

Quasi-equilibrium process

• A process during which the system

remains nearly in equilibrium at all times

• A slow process that allows system to

adjust itself internally (1-7)

Isobaric process

• Constant pressure

• W12 = P (V2-V1)

Isochoric process

• Constant volume, V1 = V2

• W12 = P (V2-V1)

• W = 0

Heat transfer mode

• Heat transfer by conduction: Heat transfer

by energy exchange between adjacent

molecules.

Given by Fourier’s Law: Q = -kA dx

dT

Heat transfer mode

Heat transfer by convection:

HT by the bulk motion of fluid

over or near surface with a

different tempt

Given by Newton’s Law of Cooling:

Q = Ah∆T

Where h is heat transfer coefficient,

∆T is temperature difference between solid surface and fluid.

Combined conduction and convection:

Q = UA∆Toverall

Heat transfer mode

• Heat transfer by radiation: heat transfer by

the radiation of hot surface

•Energy is transmitted as electromagnetic wave in

space.

Given by:

Q = εσATs4

Where ε - emissivity

σ - Stefan-Boltzman constant

Ts - surface temperature.

More Exercises

Quiz 5

Quiz 4

Quiz 2

Quiz 6

Quiz 7