第二章 The First Lw of Thermodynamics.pdf - Chapter 2...

Chapter 2
The First Lw of
Thermodynamics

Energy
• Many Different Forms
Kinetic Internal Potential
Mechanical Chemical Electrical
2.1 Introduction

2.1 Introduction
Energy
The ability to do work.
Can be converted or transported.
Cannot be created or destroyed.
• Kinetic Energy --- Energy of motion.
• K.E. = 1/2 m V
2
• Potential Energy --- Energy of position in a
potential field.
• P.E. = m g.h + ...

2.1 Introduction
Potential vs. Kinetic energy
• Potential energy, as the name implies, is energy
that has not yet been used, thus the term potential.
Kinetic energy is energy in use (or motion). A
tank of gasoline has a certain potential energy that
is converted into kinetic energy by the engine.
When the potential is used up, you're outta gas!
Batteries, when new or recharged, have a certain
potential. When placed into a tape recorder and
played at loud volume (the only settings for such
things), the potential in the batteries is transformed
into kinetic energy to drive the speakers. When the
potential energy is all used up, the batteries are
dead. In the case of rechargeable batteries, their
potential is reelevated or restored.

2.1 Introduction
What is Internal Energy ( U ) ?

2.1 Introduction
Internal Energy (U)
U ( V,T ) , U ( P,T ) , or U ( P,V ), can take on a number of
forms
• Kinetic energy of molecules in system.
• The potential energy of the constituents of the system relative
to the center of mass of the system. For example, a crystal
consisting of dipolar molecules will experience a change in its
potential energy as an electric field is applied to the system.
• The internal energy stored in the form of molecular vibration
and rotation.
• The internal energy stored in the form of chemical bonds that
can be released through a chemical reaction.
Internal Energy ( U ) --- Energy associated with the condition of
matter, not its motion or position.
Internal Energy ( U ) --- kinetic + potential energy of the
molecules or atoms of a body.

2.1 Introduction
Internal energy is the energy stored in material systems.
􀂉 Potential energy : interaction energy between electrons, ions,
atoms, and molecules
􀂉 Kinetic energy : translational, vibrational, and rotational
energy of electrons, ions, atoms, and molecules
􀂉 U is an extensive function of state
􀂉 U = nu(T, P, composition)
􀂉 U always increases with temperature (T) of the system.

2.1 Introduction
First Law of Thermodynamics

2.1 Introduction
The First Law of Thermodynamics is based on our experience that
energy can neither be created nor destroyed, if both the system and
the surroundings are taken into account.
The internal energy, U, of an isolated system is constant.
Δ U
total
=Δ U
system
+Δ U
surroundings
=0
Δ U
system
=−Δ U
surroundings
U can be changed by either heat transfer or by work “flow”
Δ U = q + w
First Law = Conservation of energy:
“The total energy (sum of all forms of energy including
q and U) of an isolated system is conserved.”