Temperature, Heat, and the
First Law of Thermodynamics
Outline
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Temperature
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The Celsius , Fahrenheit and Kelvin
Scales
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Thermal Expansion
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Absorption of Heat
Heat capacity
specific heat,
latent heat,
18-1
Temperature p(58)
• Thermodynamics is the study and
application of the thermal energy (often
called the internal energy) of systems. One of the central concepts of thermodynamics is temperature.
• Temperature is an SI base quantity related to our sense of hot and cold.
It is measured with a thermometer, which contains a working substance
with a measurable property, such as length or pressure, that changes in a regular way as the substance becomes hotter or colder.
Physicists measure temperature on the
18-1
Temperature p(59)
The Zeroth Law of Thermodynamics
Thermometer
A thermometer is a device that measures the
temperature of things.
Thermometer works through a change in the physical properties of the temperature change, such as property of extended objects with increasing temperature,
18-1
Temperature p(59)
Triple Point of Water
• The Triple point of water is the point in which solid ice, liquid
water, and water vapor coexist in thermal equilibrium. (This does not occur at normal atmospheric pressure.)
• By international agreement, the temperature of this mixture has been defined to be 273.16 K. The bulb of a constant-volume gas thermometer is shown
18-2
The Celsius and Fahrenheit Scales p(62)
• The Celsius temperature scale is defined by
with T in kelvins.
• The Fahrenheit temperature scale is defined by
Absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance.
Example
:
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Converting to kelvin
solution
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Converting
solution
Eexample
• Convert
solution
•
Conversion
solution
Converting
Example
Converting
There are two steps to convert from Kelvin to Fahrenheit:
18-3
Thermal Expansion p(64)
• All objects change size with changes in temperature. For a
temperature change
ΔT
, a change
ΔL
in any linear dimension
L
is given by
The strip bends as shown at tempera trues above this reference temperature. Below the reference temperature the strip bends the other way. Many thermo- stats operate on this principle, making and breaking an electrical contact as the
Linear Expansion
Thermal expansion
EXAMPLE
Consider a 2 m long brass rod at temperature is 22 °C,
if temperature increased to 50 c˚. what would the length
of rod be ??take the linear expansion of brass is.
Solution
∆L= α L ∆T
∆L= 19x10
-6x 2(50-22)
18-3
Thermal Expansion p(65)
• If the temperature of a solid or liquid whose volume is
V
is
increased by an amount
ΔT
, the increase in volume is found to
be
in which β is the
coefficient of volume expansion
and is related to linear expansion in this way,Volume Expansion
Thermal expansion of a On a hot day in Las Vegas, an oil trucker loaded 37 000 Lof diesel fuel. He encountered cold weather on the way to
Payson, Utah, where the temperature was 23.0 K lower than in Las Vegas, and where he delivered his entire load. How many liters did
deliver? The coefficient of volume expansion for diesel fuel is 9.50 x 10-4/Co, and the coefficient of linear expansion for his steel truck tank is 11 x 10-6Co
Calculations:
∆V= (37000) (9.50 x 10)(-23) = -808 L
Thus, the amount delivered was
V del = ∆V+V = 37000 – 808 = 36190 L
Sample Problem 18.02
p(62)
The
specific heat capacity
is defined
as the amount of heat energy needed to
raise 1kg of sample by 1 degree Celsius.
The
heat capacity
is defined as the
amount of heat energy needed to raise
18-4
Absorption of Heat p(68)
Absorption of Heat by Solids and Liquids
• The heat capacity C of an object is the proportionality constant
between the heat Q that the object absorbs or loses and the resulting temperature changeΔTof the object; that is,
in which Ti and Tfare the initial and final temperatures of the object.
If the object has
mass m, then,
It takes 487.5 J to heat 25 grams of copper from 25 °C to
75 °C. What is the specific heat in Joules/g·°C?
Q= mcΔT
487.5 J = (25 g)c(75 °C - 25 °C)
487.5 J = (25 g)
c
(50 °C)
Solve for c:
