As you might have
guessed, Carnot Heat Engine's are reliant on
thermodynamics. The
first and second laws of thermodynamics are vital in
order to understand how engines
operate in general. These laws describe how heat
is related to energy and work. They
also show how energy flows from high to low, increasing
overall entropy. Carnot
discovered that engine efficiency is only reliant on the
temperature difference
between the hot and cold reservoir. While this
seems simple enough, understanding
energy is the first step to truly grasping what Carnot
discovered.
The First Law of Thermodynamics:
This law is actually just a statement about conservation
of energy. As many
physics student know, energy cannot be created or
destroyed. However, energy
can be transferred. A common phrase in any
thermodynamics class is "Energy in
equals energy out." While this is true, it is not
always clear what the term "energy"
entails. The textbook, "Physics for Scientists and
Engineers: A Strategic Report"
describes the energy of a system as mechanical energy plus
thermal energy. In
earlier chapters of the book, mechanical energy is
described as potential energy plus
kinetic energy. You also learn that thermal energy
is equal to work plus heat. With
all of these relationships in mind, the idea of working
any equations using energy
seems daunting. However, when each relationship is
dissected and looked at
separately before they are brought together, understanding
energy becomes rather
simple.
1. Energy In Equals Energy Out
This is perhaps the most straight
forward portion of the equations. In a closed
system, energy that enters the systems is equal to the
energy leaving the system,
otherwise, there is a change in the energy of the
system. While we do not have a
complete idea of what energy is at this point, the idea
is simple. You can never get
more than what you put in. Energy is always
conserved.
Ein – Eout
= ΔEsystem
2. Kinetic Energy and
Potential Energy
Kinetic energy is the energy of
motion. Objects can only have kinetic energy if
they
have mass and velocity. In addition, objects also
have potential energy which is
sometimes considered stored energy. While there
are many types of potential energy,
for the purpose of this site, we will focus on potential
energy from position. Together,
both kinetic and potential energy make up an energy type
known as mechanical energy.
This energy is typically seen on a macroscopic scale and
can be observed easily by two
equations:
Emech = KE + PE
PE= mgh
KE = 1/2mv2
PE - Potential Energy, m - mass of object, h - height
from datum,
KE - Kinetic Energy, v - velocity of object
3. Thermal Energy Equals Work
Plus Heat Along
with mechanical energy, there is also thermal
energy. Thermal Energy is
energy of heat and work. It is the most
important form of energy for
understanding thermodynamics. Unfortunately,
it can become rather complicated
under non-ideal conditions. Luckily, in order
to understand Carnot Heat Engines,
we look at idealized situations where friction and
irreversibility do not exist. This
formula for thermal energy applies when kinetic and
potential energy are negligible.
