# Thermodynamics

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28 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Thermodynamics

Chapters 12 and 13

Thermal Energy

Chapter 12

Thermal Energy

The overall energy of motion of the particles
of an object make up its thermal energy.

total amount of energy in all the particles

Kinetic
-
molecular Theory

At the atomic level there is always motion

Hotter particles are moving faster and thus have
more kinetic energy.

Temperature

Average Kinetic energy in the object

Temperature Scales

Celsius

based on the boiling and freezing
points of water.

Fahrenheit

loosely based on the average
temperature of the human body

Kelvin

most common scientific research
scale. 0 Kelvin is absolute zero, or the point at
which all movement stops.

Misconception

Objects can be cold.

Physics Reality

We describe the state of an object by the amount
of thermal energy or temperature the object has

Heat is really how much energy is being
transferred between two objects.

If something feels cold to you it is pulling heat
from your body. Hot is just the opposite.

Heat

Heat is energy that flows from one substance
to another. If two substances have the same
temp. then there is no heat between them.

In short heat is energy moving.

Matter does not contain heat.

Heat Transfer

(Amount of Energy being transferred)

Methods

Convection

by currents in fluids

Conduction

by contact in solids

by waves in any substance

Specific Heat (C)

The amount of energy that must be added to the
material to raise the temperature of a unit mass
by one degree, usually in the units J/(
kgK
)

Heat Transfer Formula

Conservation of Energy and Thermal
Energy

If no energy is added or removed,
E
A

+ E
B

= E
A

+ E
B

If no energy is added or removed from the
system then the following applies

Change of State

Every substance has a unique melting or boiling
point and these numbers change based on the
amount of air pressure.

For this class we will assume that all substances are at
the standard air pressure for sea level.

An object can be melting or freezing at its melting
point.

At the boiling point the object is constantly
changing from a gas to a liquid and from a liquid
to a gas.

Heat Required formulas

Heat of Fusion

Heat of Vaporization

First Law of Thermodynamics

Really a restating of the conservation of
energy.

Energy can not be created or destroyed on
transferred into other forms or objects.

Whenever heat is added to a system, it transforms
to an equal amount of some other form of energy.

Heat added = increase in internal energy +
external work done by the system

The process of expansion or compression of a
gas so that no heat leaves or enters a system.

Usually done rapidly or by insulating the
system.

Examples: cylinders of an automobile engine,
bicycle pump

Second Law of Thermodynamics

Heat flows spontaneously from hot bodies to
cold bodies.

Heat will never, of itself, flow from a cold object to
a hot object.

The Law states that natural processes go in a
direction that maintains or increases the total
amount of entropy of the universe.

All things will become disordered unless
something is done to put them in order.

Heat Engines

A heat engine is any device that transfers
internal energy into mechanical work.

It is easy to change work into heat. (Try
rubbing your hands together.) You can never
completely change heat into work.

Examples: Jet engine, internal combustion
engine, steam engine, any engine that uses

Heat Engines and the 2
nd

Law

When work is done by a heat engine running
between two temps,
T
hot

and
T
cold
, only some
of the input heat at
T
hot

can be converted to
work, and the rest is expelled as heat at
T
cold
.

Every heat engine will

Absorb heat from a reservoir of higher temp

Convert some of its energy into mechanical work

Expel the remaining energy as heat to some lower
temp reservoir

Heat Engines and the 2
nd

Law

Expelled heat can be desirable or repulsive.

There is an ideal efficiency of heat engines
called the Carnot Efficiency

Formula

Ideal efficiency = (
T
hot

T
cold
)/
T
hot

States of Matter

Chapter 13

Properties of Fluids

Applying a force to a surface is pressure

Formula:

Units:

Fluid particles apply pressure to a object by
bouncing off the surface.

Atmospheric Pressure (STP)

101.4
kPa

at sea
level and room temperature

Pascal’s Principle

Static Fluids

Applies only to fluids at rest.

The pressure you exert on one portion of the
fluid is transferred to the rest of the fluid.

The container’s shape does not effect the
pressure.

Examples: hydraulics, tube of toothpaste, etc.

Formula:

Buoyancy

Archimedes Principle

Pressure of water on a body

Buoyant Force

the upward force on an
object that is provided by a fluid

Formula:

Principle

an object immersed in a fluid has
an upward force on it equal to the weight of
the fluid displaced.

Apparent Weight:

Bernoulli’s Principle

Fluids in Motion

As the velocity of a fluid increases the
pressure exerted by that fluid decreases.

When a fluid flows through an constriction its
velocity increases.

This can relate back to energy, net work and
forces.

The net work on the fluid is proportional to the
force that is placed on the fluid.

Flight

In your notes, explain how Bernoulli’s
Principle aids in flight.

Evaporation and Condensation

Evaporation is a cooling process.

Explain

Condensation is a warming process.

Explain

Thermal Expansion

We can calculate how much a substance will
expand linearly or volumetrically due to heat
or an increase in thermal energy.

Formulas

Alpha is specific to each substance. Pg 317.