Download Measuring Temperature

Measuring Temperature
The standard metric unit
of temperature is the
degree Celsius (˚C).
Water freezes at 0˚C.
Water boils at 100˚C.
The Fahrenheit scale
is used only in the
United States.
Why Do We Need Temperature
Scales?
Our sense of how hot or cold something
feels cannot be trusted.
Try this!
Put one hand in hot
water and the other in
cold. Then put them
both into the same
container of warm
water. Conflicting
messages will be
sent to your brain.
Absolute Zero
When a gas is held at constant pressure and
is cooled, its volume decreases by 1/273 for
each 1˚C the temperature goes down.
By the time you'd get down to a temperature of
-273˚C, the volume would decrease to zero.
Therefore, -273˚C must be the coldest possible
temperature. We call it absolute zero.
The Kelvin Scale
The zero point on the Kelvin
temperature scale is
absolute zero. In this way,
there are no negative
numbers on the Kelvin scale.
Scale divisions on the Kelvin
scale are identical in size to
those on the Celsius scale.
Heat and Temperature
Heat and temperature are not the same thing.
Temperature is
related to the
average kinetic
energy of moving
molecules.
The faster the molecules move, the higher
the temperature.
Heat and Temperature
Heat depends on temperature, but also on the
mass of the object, and its heat capacity.
Even though Lake Ontario
is at a colder temperature
than your cup of coffee, it
contains a lot more heat.
The reason is that Lake
Ontario is so much bigger
(more massive) than your
morning beverage.
Thermodynamics
The first law of thermodynamics states that
the heat added to a system will show up as an
increase in the system's internal energy plus
any external work done by the system.
A good example of this would be an automobile engine.
nd
2 Law of Thermodynamics
Heat always flows
spontaneously from an
area of higher
temperature to an an area
of lower temperature.
(Never the other way
around.)
In picture (a) the cup holds
hot coffee; in picture (b)
the cup holds ice water.
Thermodynamics
The Second Law of Thermodynamics
Stated as the Law of Heat Engines:
Any cyclic process that uses thermal energy to
do work must also have a thermal energy
exhaust. In other words, heat engines are always
less than 100% efficient at using thermal energy
to do work.
Thermodynamics
All heat engines depend
on the spontaneous
flow of thermal energy
from high temperature
to low temperature.
There is a limit to how
efficient heat engines can
be. Their efficiency is
theoretically restricted to
60% or less.
Thermodynamics
Sometimes, the waste heat can be
(partially) made use of.
An example: A steam-electric power generation
plant can make use of its waste thermal energy to
heat buildings. It can even be sold to nearby
property owners for heating. This is called
cogeneration.
Waste heat from your car's engine is used to heat
the interior of your car on a cold winter day!
Things Run Down
The 2nd law of Thermodynamics applies to
more than just heat engines.
When a pendulum swings,
each swing is a little lower
and slower than the last.
Friction keeps shaving off
some of its PE and KE
and turning it into waste
thermal energy which
dissipates.
The Laws of Thermodynamics & Us
When we use the earth's energy resources, we
do not decrease the earth's total energy.
This is just another way of stating the
Law of Conservation of Energy.
Instead, we are changing highly useful forms
of energy into less useful forms.
And this restates the 2nd Law of
Thermodynamics.
Entropy
Suppose a box full of hot gas is touched to a
box full of cold gas.
cold
hot
Thermal energy will flow from the high
temperature box to the low temperature box.
What will it look like after this has happened?
Entropy
After some time has passed, the temperatures
will equalize. The boxes will now look like this:
The fast and slow moving molecules are no
longer segregated. They are mixed.
Did the molecules actually move from one box
to the other?
Entropy
This was the more
organized state.
This is the more
disorganized
state.
The amount of disorganization is called entropy.
Entropy
The 2nd Law of Thermodynamics
stated as the Law of Entropy:
The total entropy (or microscopic
disorganization) of the participants in any
physical process cannot decrease during that
process, but it can increase.
That's not to say that you can't create a little more
order in some part of your world, but the price you
pay is to create more disorder somewhere else.
Entropy
In other words, disorganization or randomness
must increase.
Specific Heat Capacity
Different substances have different thermal
capacities for storing energy.
A substance with a low
specific heat, (like the pie
crust) heats up quickly
and cools down quickly
because it required little
energy to change its
temperature. On the
other hand, the filling has
a high specific heat and is
still too hot to eat!
Calculating Heat
ΔQ = c m ΔT
Heat gained
or lost
Specific Heat
Capacity
mass
temperature
change
Heat energy is measured in Joules.
Thermal Expansion
As the temperature of most substances
increases, its molecules move faster and farther
apart. Most substances expand when heated
and contract when cooled.
Extreme heat on a
July day caused the
buckling of these
railroad tracks.
Thermal Expansion
Brass expands more when heated than iron
does, and it contracts more when cooled.
Because of this behavior, the strip bends.
Bi-metallic strips like these are used in
thermostats.
Water as an Exception
Most liquids contract when they turn solid.
Water actually expands when it turns to ice.
As a result, ice floats on liquid water.
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