Thermal Energy
 Chapter 6


 Joseph F. Alward, PhD
 Department of Physics
 University of the Pacific

 

 

Temperature is a measure of
the average kinetic energy
of the molecules in the object.
-----------------------------------------
Will both fingers feel the same
temperature when they're
put in the warm water?

 

 

 

 

 Fahrenheit and Celsius
 Temperature Scales
F = (9/5) C + 32
--------------------------
Example:  C = 100

F = (9/5) 100 + 32
 
   = 180 + 32

    = 212

   

 

  Absolute Zero and the Kelvin Scale
Pressure remains the same, volume changes by 1/273 for each Celsius degree change.

 

 

 

 

 Temperature, Thermal Energy, and Heat
Temperature

Measures average
kinetic energy.		 Thermal Energy

The sum of all the
kinetic energies.		 Heat

The flow of thermal
energy

 

 

 

 Temperature, Heat, and Thermal Energy
The temperature of a spark
is about 2000 C.

Is its thermal energy large?

Does it have "heat"?

What happens when the
spark lands on the boy's
skin?

 

 

 

  Thermal Equilibrium
Two objects in contact are said
to be in thermal equilibrium if
their temperatures are the same.
---------------------------------------------
Does the thermometer measure
the temperature the water had
before the thermometer was
put in the water?

 

 

 

 

  Heat Units
One calorie is the amount of
thermal energy required to
raise the temperature of
one gram of water by one
Celsius degree.
--------------------------------------
 3000 calories of thermal
energy enters each cup.
The temperature of the
water on the left rises by
30 Celsius degrees.

By how much does the
temperature of the
water in the cup on the
right rise?

 

 

 

   Calories 
The scientific "calorie" is spelled
with a lower-case "c".

One "calorie" = 4.184 Joules
---------------------------------------------
The "dieter's" calorie is spelled
with an upper-case "C".

One "Calorie" = 1000 calories

 

 

 

   Specific Heat Capacity of Water
The specific heat of water is
one calorie per gram per
degree Celsius.

Apple is mostly water;
by how much will 200 g
of apple filling cool if it
loses 2000 calories of
heat?

 

 

  Specific Heat Capacity of Crust
The specific heat  capacity
of crust is 0.2 calorie per
gram per degree Celsius
(five times smaller than
water).

By how much will 200 g
of crust cool if it loses
2000 calories of heat?
------------------------------------
Which stays hotter longer?

Apple, or crust?

 

 

 

   Water Holds Thermal Energy at Lower Temperature
Specific heat capacity of water is larger than soil,
so the rise of temperature is less for water, all
other things being equal.  But, there's another
effect:

Sun's energy penetrates farther into water than
it does into earth, so the temperature rise per
square meter of exposed surface  is less for
water.

 

 

  Thermal Expansion 
Metals, like most objects,
expand when they warm,
contract when they cool.

 

 

 

 

 

 

 

 

    Thermal Expansion

 

   

 

    Bimetallic Coil in Thermostats
When temperature falls,
coil contracts.

Mercury inside bulb
rolls to the right,
completing an electrical
circuit.

 

 

      Ice is Less Dense than Water

Water molecules in crystal form have an open-structured hexagonal
arrangement, so water expands upon freezing (water becomes less dense).

(From Conceptual Physics, by Paul G. Hewitt)

 Water molecules
in solid water (ice)
are farther apart.
Ice is less dense
than water.

0 - 4 C: molecules
move closer.

Above 4 C:  
molecules move
apart. 

 

    A Frozen Lake

  Why do lakes freeze on top first?

 

 

 Why Do Lakes Freeze on the Top First?
Freezing of Lakes        

As air above lake cools, the
water at the top contracts
and becomes more dense
than the water below.  This
cooler water sinks and warmer
water takes its place at the top.

When the lake is at 4 C, further
lowering of the air temperature
causes an expansion of the water
on top.  This top water is now
less dense than the water below,
so it stays on top, where it
freezes.

 

  

 

   The Three Common Phases of Matter

A fourth phase, plasma--ionized gas (not shown), is the most
common phase of matter in the universe. 

 

 

 

  

 

   Change of Phase 

Moistened canvas canteen
cover		 Water molecules
in canvas take
momentum from
warm metal,
gaining sufficient
kinetic energy to
fly away from
canvas, taking
thermal energy
with it.
 

 Panting dog.

 

 

 

 

 Condensation
Water gives up thermal energy
when it condenses--goes from
vapor to liquid.
------------------------------------------
Steam condenses in colder
metal pipes, giving up heat to
the pipes.
------------------------------------------

 

 

  

   Condensation and Evaporation
The air in the shower stall
is at the same temperature as
the air outside, but there's
less water vapor outside
to condense on the skin.

 

   

 

 

 

  Condensation 
Why is a 95-degree day in Phoenix, Arizona,
so much more comfortable than a 95-degree
day in New Orleans?

 

 

 

 Boiling

 

 

 

   The Pressure Cooker

 

 

    Boiling on a Mountain
At higher altitudes the atmospheric
pressure is less, so water molecules
need less kinetic energy to fight
against the force of the air on their
way off the surface of the liquid.

 

  

 Why Boiling Occurs at
 Constant Temperature

 

 

 

 

 

 

 

 

 Boiling and Freezing in a Vacuum
Reduced pressure allows
room temperature water
to boil, taking thermal
energy out of the water.

 

 

 

 

 

 

 

 

  Melting and Freezing
Pressure crushes open structure
of ice crystals, causing snowball
to melt.

Remove the pressure, water on
snowball re-freezes, forming
a hard crust.

The process of melting because
of applied pressure, then re-
freezing when pressure is
removed is called regelation.

 

 

 

  

   Energy and Changes of Phase 

 

 

 

 

  H20 Transformations 

   Specific heat of ice = 0.5 cal/g-C             Specific heat of water = 1 cal/g-C
   Heat of fusion = 80 cal/g                            Heat of vaporizaton = 540 cal/g

 

 

   

 

 

 Molten Lead Bath
Water on wet finger absorbs the
heat from the molten lead.