liceo scientifico isaac newton physics course the first law of thermodynamics professor massimo...

Liceo Scientifico Isaac Newton

Physics course The first law of thermodynamics

Professor

Massimo Patrone

Read by

Cinzia Cetraro

Historical notes

James Joule (1818-1889)

• Until 1600 combustion• Industrial Revolution• Newton• Heat: source of mechanical

work• Carnot and Joule• Thermodynamics

From energy to heat energy: Joule's experiment

masses

blades

water

temperature

h

Joule's experiment

1 calorie = 4,186 joule = 4,186

The First Law of Thermodynamics The First Law of Thermodynamics

Internal Energy

molecular motion

• First Law of Thermodynamics

LsLs

QaQa

QcQc

LfLf

ΔU = Q–L

Piston or elementary machine

To understand the processes that involve real thermodynamic systems, which are rather complex, we refer to the simple model of heat engine, consisting of a cylinder with a sliding piston and containing a perfect gas.

The perfect gas

U = U(T)U = U(T)

The real thermodynamic processes give rise in the PV plane, to a graphic representation which is not possible to describe mathematically because for a given volume of fluid, the pressure has different values. So they are irreversible.

Reversible and irreversible transformations

In a very slow process, there are a succession off equilibrium states. This transformation is called quasi-static.

The quasi-static transformations can be played in reverse.

1) Transformation at constant pressure: isobaric

volume

pi=pf

Vi Vf

pre

ssu

re

ΔX

2) Transformation at constant volume: isochoric

volume

pi

Vi=Vf

pre

ssu

re

pf

L=0

ΔU=Q-L=Q

3) Transformation at constant temperature: isothermal

pre

ssu

re

volume

pi

Vi

pf

Vf

ΔU=0

ΔU=Q-L Q=L

4) Transformation without heat exchange: adiabatic

Q=0,L>0 ΔU=Q-L ΔU=-L <0 ΔT<0

Q=0,L<0 ΔU=Q-L ΔU=-L >0 ΔT>0

heat engines

Thomas Savery(1650-1715)

Thomas Newcomen (1663-1729)

James Watt (1736-1819)

Charles Parsons(1854-1931)

Thermodynamic cycles

Sadi Carnot (1796-1832) ΔU= 0 0=Q-L Q=L

A

C

CC

The Carnot engine

human body

Heat engine

Special thanks to prof. Cinzia Cetraro for linguistic supervision

the end