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Physics 213: Unit 2
Temperature, Heat, and
the Zeroth Law of Thermodynamics
Thermodynamics
Thermodynamics deals with an internal
energy of the systems, the thermal
energy, and is governed by the Laws of
Thermodynamics
Thermal
Temperature
Zeroth Law of Thermodynamics
Temperature in daily life:
HOT
WARM
COOL
COLD
What is the definition of temperature ?
How to measure the temperature?
0th Law
0th Law of Thermodynamics
If body A and B are each in thermal
equilibrium with a third body T, then they
are in thermal equilibrium with each other.
Thermal equilibrium: two objects in thermal contact cease to
have any exchange of heat.
Thermal contact : Heat can be exchanged.
Heat: energy exchanged between objects due to their
temperature difference.
Temperature : two objects in thermal equilibrium with
each other are at the same temperature
Measuring Temperature
Bring a thermometer to a thermal
equilibrium with the object
Thermometer: physical property changes
with temperature
— pressure of gas at constant volume
— length of solid
— Electric resistance
— Volume of liquid
Temperature Scales
DEFINE:
Freezing temperature of water: 0˚C
Celsius scale
Boiling temperature of water: 100˚C
Constant-volume gas thermometer:
Measure Pressure
Temperature
Different gases all extrapolate to zero pressure
at -273˚C:
Kelvin scale
Temperature Scales
Kelvin scale:
T = 0 K for P = 0 point
Celsius scale:
Tc = T - 273.15˚
Fahrenheit scale: TF = 9/5 Tc + 32˚
Thermodynamic scale: triple point of water
T3 = 273.16 K
Thermal Expansion
When heated, objects expand.
Linear expansion:
L LT
Length changes Original
this much
length
Volume expansion:
3
Temperature
changes this much
Coefficient of L / L
T
linear expansion
V VT
V / V
Coefficient of
T
volume expansion
Unusual Behavior of Water
The maximum density occurs at 4 ˚C:
Above 4 ˚C, expands when heated
until 100 ˚C;
Below 4 ˚C, expands when cooled
until 0 ˚C.
Temperature and Heat
Internal energy: the energy of a system when
it is stationary - nuclear, chemical, strain,
etc.
Thermal energy: the energy that changes
when the temperature changes, associated
with motions of atoms, molecules, etc.
Heat Q: the transferred energy.
Units of Heat
Calorie: amount of heat to heat 1 g of
water from 14.5 to 15.5 ˚C
BTU: amount of heat to heat 1 lb of water
from 63 to 64 ˚F
SI: Joule (the same as energy)
Joule’s experiment:
1 cal = 4.186 J
HRW 2E (5th ed.). Suppose the temperature of a gas at the boiling
point of water is 373.15 K. What then is the limiting value of the
ratio of the pressure of the gas at that boiling point to its pressure at
the triple point of water? (Assume the volume of the gas is the same
at both temperatures.)
P
0
T(K)
0
Constant-volume gas thermometer: P-T curve
extrapolates to origin.
pb Tb (K ) 373.15
p3 T3 (K ) 273.16
HRW 9E (5th ed.). At what temperature do the following pairs of
scales read the same: (a) Fahrenheit and Celsius, (b) Fahrenheit and
Kelvin, and (c) Celsius and Kelvin?
(a)
TF = (9/5)TC + 32˚
For
TF = (9/5)TF + 32˚
we get TF = -40 ˚F
(b)
TF = (9/5)TC + 32˚
= (9/5)(T - 273.15) + 32˚
For TF = (9/5) (TF - 273.15) + 32˚
we get TF = 575 ˚F
(c) Since
TC = T - 273.15
the Kelvin and Celsius
temperatures can never have the
same numerical value.
HRW 28P (5th ed.). At 20˚C, a rod is exactly 20.05 cm long on a
steel ruler. Both the rod and the ruler are placed in an oven at 270˚C,
where the rod now measures 20.11 cm on the same ruler. What is the
coefficient of thermal expansion for the material of which the rod is
made of?
The change in length for the rod is 20.11cm-20.05cm plus
the expansion of the steel ruler at its 20.11cm mark:
∆Ls = Lss∆T = (20.11 cm)(11 x 10-6 /C˚)(270˚C-20˚C)
= 0.055 cm
∆L = (20.11cm-20.05cm) + 0.055 cm = 0.115 cm
The coefficient of thermal expansion of the material the rod
is made of is:
= ∆L/L∆T=23 x 10-6 /C˚
L LT