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