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SPH 302

THERMODYNAMICS

Nyongesa F. W., PhD. e-mail: fnyongesa@uonbi.ac.ke

UNIVERSITY OF NAIROBI 6/21/2014 9:13 PM SPH 302

Nyongesa F. W., Ph.D 2

Course Outline

Lecture 1: Thermodynamic

concepts & Zeroth Law

Lecture 2: 1ST Law

Lecture 3: 2ND Law

Lecture 4: Entropy and 2ND Law

Lecture 5: Thermodynamic potentials

Lecture 6: Phase Changes & Equlibria

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Prerequisites

SPH 203

Calculus

ODE

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Introduction & Overview

Before industrial revolution, machinery were powered by animals

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James watt Steam Engine

1ST Engine (invented1790) to

convert steam to mechanical work - lead to industrial revolution

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Steam Engines of Industrial Revolution

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Steam powered cars

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Steam powered Industries

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Work output

Steam (Heat)

By 19th cent, a new science was born – THERMODYNAMICS

GOAL: Asses efficiency of devices that convert heat work e.g., heat engines

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1ST Diesel Engines -1930s

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Hybrids & gas turbines

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What is Thermodynamics?

Thermo Energy transfer in form of heat

Dynamics Energy transfer in form of mechanical work

Thermodynamics = science that govern energy conversions processes (heat work)

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Why Thermodynamics ?

It gives Laws that govern energy conversion processes

ENERGY

Chemical Thermal

Solar Mechanical

Energy 1 (heat)

Energy 2 (work)

Laws of Thermodynamics

Relationship?

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Food (Energy) input

Waste

Work out

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W

waste

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Heat pumps

Work in

Heat out

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Laws of Thermodynamics

Energy conversions processes are governed by 4 Laws

Zeroth Law:

Gives condition for

thermal equilibrium

between 2 bodies

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First Law :

Defines relationship between heat input (dQ), work (dW) and internal energy (dU) in a system

dQ = dU + dW

dQ dU

dW

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“No animal works continously without eating or wearing out”

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Second Law:

(i) Defines efficiency of processes that convert heat work. i.e., No heat engine with 100% efficiency.

“If you eat, you must shit”.

Fuel Tank Engine

No exhaust

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Food input)

Impossible

Work out

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(ii) Gives direction of natural occurring processes - “Arrow of time”

“Naturally processes proceed in a direction such as to increase Total Entropy (disorder)

of system” Disorder

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Disorder

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Third Law:

Explains behaviour of systems as T absolute zero

As T 0, Entropy change (dS) 0 system becomes orderly E.g., water ice

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Thermodynamic Concepts & Zeroth Law

Thermodynamic concepts Ideal Gas Laws Zeroth Law

Lecture 1

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Objectives

Define thermodynamics

concepts

Explain ideal gas Laws

Explain Zeroth Law and its

significance

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Thermodynamic concepts

Energy conversion processes are studied using concept of thermodynamic system

System – Part of material universe that can be isolated completely from the rest for investigation

SYSTEM + SURROUNDING = UNIVERSE

system sorrounding boundary

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Types of systems

3 main types

(a) Open systems:- Allows exchange of both heat and matter through the boundary. E.g. human digestive system.

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(b) Closed systems:- Allows only exchange of heat with surrounding E.g. refrigerator or Engine

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(c) Isolated System:- No heat or matter exchange occurs with surrounding. The walls are Adiabatic (Adiathermal) E.g. Vacuum flask.

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Thermodynamic Variables:- parameters that describe behavior or state of system i.e., P, T, V & composition ()

Extensive variable:- Dependent on mass/size of the substance present in the system e.g., U, S, V etc.

Intensive variable:- opposite of Extensive

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Heat:- Transfer of thermal energy between systems as result of temp difference

Work:-Transfer of mechanical energy

Both Heat & Work = ways of

transferring energy

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Thermodynamic equilibrium:- When system experiences thermal, mechanical and chemical equilibrium S.T. state variables (P, V, T & ) are const.

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Working substance:- Fluid enclosed in the system that either receives or transfers energy to the surrounding in the form of heat or work.

In Thermodynamics, we use ideal gas as the working substance. WHY?

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Activity

Name the working substance in these systems

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Lecture Evaluation

Explain the difference between

Open & Closed system

Adiabatic & Diathermal Wall

Extensive & Intensive variables

State function and state variable

Heat and work

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1.2

Behavior of Ideal gases

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An ideal gas is an abstraction whose properties represent the limiting behavior of real gases at very low densities/high P.

Assumptions in ideal gas

(i) no intermolecular attractions S.T internal energy is entirely K.E Temp

(ii) Molecules occupy negligible volume

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Behavior of ideal gas depends on T, P and V and obey gas laws:

(a) Boyle’s law:

Compressing a gas

increases P

VP

1

V

P

PV = c

CPV

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(b) Pressure law: Heating a gas increases P

(c) Charles’ Law: Heating a gas increases V

CT

PTP

CT

VTV

T

P

T

V

-273 K

-273 K

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(d) V No. of moles (n) of the gas i.e.

V n

Combining (a) to (d) gives

PV = nRT ……………………..(1.1)

State of a system is given by Eqn of State

Equation of state

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P-V Diagram

From PV = nRT Behaviour/processes of ideal gas is represented on PV diagrams

V

P

PV = c

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Non Ideal (Real) Gas

Characterized by

Gas cannot be compressed to zero volume since gas molecules occupy finite volume and liquefy at low P

Intermolecular attractions

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Eqn of state of real gas is given by

Where a & b molecular attractions & v = molecular volume

nRTnbVv

aP

2

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Activity

State difference between Ideal & Non-ideal gas

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Zeroth law of Thermodynamics

A hot object looses heat to attain thermal equilibrium with surrounding

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“Statement of Zeroth Law”

"If objects A and B are separately in thermal equilibrium with third object C, then A and B are in thermal equilibrium with each other”.

A C

B

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Significance of Zeroth Law

Gives condition for thermal equilibrium between 2 bodies

Introduces concept of temperature and how it is determined.

Temperature = measure of degree of hotness

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Lecture -Evaluation

1. Explain an ideal gas and ideal gas laws

2. State Equation of State

3. State Zeroth Law and its significance