Download Temperature and Heat

Temperature and Heat
Foundation Physics
Lecture 2.4
26 Jan ‘10
Temperature, Internal Energy and Heat
• What is temperature?
• What is heat?
• What is internal energy?
Temperature
Internal Energy
Does a glass of
water sitting on a
table have any
energy?
No apparent
energy of the
glass of water
on a
macroscopic
scale.
Microscopic
kinetic energy is
part of internal
energy.
Molecular
attractive forces
are associated
with potential
energy.
Atoms, molecules, Phases of Matter
Matter (solid, liquid or gas) is made up of atoms
and molecules or particles which are in continual
motion.
Total kinetic energy of the particles in a given
body is directly proportional to the absolute
temperature of the body. Kinetic energy of the
gas molecules would become zero at absolute
zero, and molecular motion would cease.
Potential energy of the particles is due to
electrostatic interactions of the electrons and the
nuclei which exert forces on each other.
Total internal energy of a body is the sum of
potential energy and kinetic energy of the
molecules in the body.
Phases (solid)
Solid
Solid: In a solid material, the
attractive forces are strong
enough that the molecules are
packed closely in an orderly
way. At the same time, there are
also repulsive forces so that the
molecules cannot penetrate into
one another. Thus the
molecules are held in more or
less fixed positions. The
molecules in a solid vibrate
about their nearly fixed
positions, usually in an array
known as crystal lattice.
Phases (Liquid)
Liquid: In a liquid, the
molecules are moving more
rapidly, or the forces between
them are weaker, so that they
are sufficiently free to roll over
one another.
Liquid
Phases (Gas)
Gas
Gas: In a gas, the forces are so
weak, or the speeds so high,
that the molecules do not even
stay close together. They move
rapidly every which way, filling
any container and occasionally
colliding with one another. For
an ideal gas, the intermolecular
forces are assumed to be
negligible and thus, potential
energy is zero.
Microscopic Explanation
We look at the interaction potential of to neighbouring atoms
Ep
r0: Distance
between two
atoms at T0= 0K
(minimal thermal
motion of the
atoms)
r1
r0
E
r1: Average
position of the
atom at T1>T0
r1>r0: due to the
asymmetry of the
potential
Thermometers and Temperature Scales
Objectives are to:
• define what a thermometer is
• describe the physical principles on which the use of
a thermometer is based
• state the Zeroth Law of Thermodynamics, and
discuss its physical implications with respect to
thermometers
• explain how a temperature scale is constructed
• convert temperatures from one scale to another
• obtain a feel for the range of temperature values in
everyday life and throughout the Universe
Temperature and Heat
Temperature is the physical property which determines the
direction of net flow of heat.
Heat is the net energy that is transferred from one object to
another due to temperature difference between the two
objects in thermal contact.
Thermal equilibrium exists for two bodies which are in thermal
contact with no net flow of heat between them.
Zeroth Law of Thermodynamics states that if bodies A and B
are separately in thermal equilibrium with a third body C, then
A and B are in thermal equilibrium with each other.
Scales of temperature
• The Thermodynamic Scale of Temperature (also known
•
•
as Kelvin Scale) is totally independent of the properties of
any particular substance and is therefore an absolute
scale of temperature. The fixed points are the triple point
of water (273.16 K or 0.01˚C) and absolute zero (0 K or –
273.15˚C).
The kelvin is the SI unit of temperature in the
thermodynamic scale. One kelvin is thus defined to be
1/273.16 of the thermodynamic temperature of the triple
point of water.
The Celsius Scale is related to the Thermodynamic Scale
by the equation t/oC = T/K – 273.15.
Zeroth Law of Thermodynamics
If bodies A and B are separately in thermal equilibrium with
a third body, C, then A and B will be in thermal equilibrium
with each other if placed in thermal contact.
Temperature Scales
Conversion of temperature sclaes
T (°C ) = 5 [T (° F ) − 32°]
9
T (° F ) = 9 [T (°C ) + 32°]
5
T ( K ) = T (°C ) + 273.15
Problem: Frozen alcohol makes as good a candle
as wax, with one disadvantage: Alcohol melts at
-114oC. What Fahrenheit temperature is this?
Temperature Ranges
Temperature Ranges
Thermal expansion
The Golden Gate Bridge
has an over all length of
~2800m (mainly steel). If the
bridge experiences
temperature extremes from
-20oC to +40oC, what will its
change in length be?
Bimetalic strip
Nanomechanical Transducer
Peltier test for sensor
mechanical check
IBM laboratories,
Rüschlikon, Switzerland
500µm long
100µm wide
0.5µm thick
1600
1400
1200
1000
def. nm
800
600
heating test 30s, 0.75°C
400
200
0
-200
500µm
219 220 221 222 223 224 225 226 227 228 229
min
Functionalized A
Fucntionalized D
Functionalized B
Functionalized E
Functionalized C
Functionalized F
Thermal Expansion Formulae
Sample Expansion Problem
Find the coefficient of expansion for a mysterious metal bar and use the
table 5.1. to identify the metal. A bar with length 300 cm expands by 8.7mm
when heated by 100 oC
Δl = α ⋅ l ⋅ ΔT
Δl is the change in length
α is the coefficient of expansion
ΔT is the temperature change in oC
Coefficients of Linear Expansion at 2OoC
Solids
Aluminum
Brass
Gold
Iron or steel
Lead
Silver
Glass (ordinary)
Glass (Pyrex)
Quartz
Concrete, brick
Marble (average)
Liquids
Ether
Ethyl alcohol
Gasoline
Glycerin
Mercury
Water
Gases
Air and most others
at atmospheric pressure
α (1/oC)
25 x 10-6
19 x 10-6
14 x 10-6
12 x 10-6
29 x 10-6
18 x 10-6
9 x 10-6
3 x 10-6
0.4 x 10-6
12 x 10-6
2.5 x 10-6
550 x 10-6
370 x 10-6
320 x 10-6
170 x 10-6
60 x 10-6
70 x 10-6
1100 x 10-6
Problem: linear expansion
copper
?
In the bimetallic strip shown the upper material is
copper. Which of the following materials could be
used for the lower metal? a) steel; b) brass; c)
aluminum
Problem: linear expansion
The aluminum cone has been exactly fitted at 20oC to the copper
block. Then it is taken out of the hole and at 180oC again placed
inside. How much does the aluminum cone then stick out of the
copper? (αCu=14.10-6 K-1, αAl=23.10-6 K-1
h2
2o
4 cm
Al
Cu
h1
3 cm
Density
In physics, density is mass (m) per
unit volume (V) — the ratio of the
amount of matter in an object
compared to its volume. A small,
heavy object, such as a rock or a lump
of lead, is denser than a larger object
of the same mass, such as a piece of
cork or foam
ρ=
m
V
where, in SI Units:
ρ (rho) is the density of the substance,
measured in kg·m–3 m is the mass of the
substance, measured in kg V is the volume of
the substance, measured in m3
cgs units grams per cubic centimeter -> 1g/cm3=103kg/m3
Densities of various Substances
(unless otherwise specified at 0oC and 1 atm)
Solids
Aluminum
Brass
Copper' (average)
Gold
Iron or steel
Lead
Silver
Uranium
Concrete
Cork
Glass
Granite
Wood
Ice (0oC)
Bone
2.70
8.44
8.8
19.3
7.8
11.3
10.1
18.7
2.3
0.24
2.6
2.7
0.3-0.9
0.917
1.7
Liquids
Water (4oC)
Blood, plasma
Blood, whole
Seawater
Mercury
Ethyl alcohol
Gasoline
Glycerin
Olive oil
1.000
1.03
1.05
1.025
13.6
0.79
0.68
1.26
0.92
Gases (unless otherwise spec. at 0oC and 1 atm)
Air
1.29 x l0-3
Carbon dioxide
1.98 x 10-3
Carbon monoxide 1.25 x 10-3
Hydrogen
0.090 x l0-3
Helium
0.18 x 10-3
Methane
0.72 x 10-g
Nitrogen
1.25 x 10-3
Nitrous oxide
1.98 x 10-3
Oxygen
1.43 x l0-3
o
Water (100 C steam) 60 x l0-3
Heat capacity and latent Heat
Heat is defined as energy that flows as a result of temperature difference.
Heat capacity, Cp, of a body is defined as the quantity of heat absorbed or
liberated, Q, by the body per unit temperature change, . The S.I. unit for heat
capacity is J.K-1.
Q=s⋅m⋅ΔT
• s=Specific heat capacity, also known simply as specific heat, is the measure
•
•
•
of the heat energy required to increase the temperature of a unit quantity of a
substance by a certain temperature interval.
Latent heat is defined as the quantity of heat absorbed or liberated by a
substance in order to change a substance from one phase to another phase
without a temperature change. The SI unit for latent heat is J.
Specific latent heat of fusion of a substance, , is defined as the quantity of heat
required per unit mass to change the substance from the solid phase to the liquid
phase without a change in temperature.
Specific latent heat of vaporization of a substance, , is defined as the quantity of
heat required per unit mass to change a substance from the liquid phase to the
vapour phase without a change in temperature.
Specific Heats of various substances at 20° C
Substance
s (cal/g.ºC, kcal/kg.ºC)
Aluminum
Brass
Copper
Gold
Iron or steel
Lead
Silver
Glass
Ice (-5°C)
Porcelain
Wood
Human Body (average)
Protein
0.217
0.090
0.092
0.031
0.11
0.030
0.056
0.20
0.50
0.26
0.4
0.83
0.4
Substance
Ethyl Alcohol
Glycerin
Mercury
Water (15°C)
s (cal/g.ºC, kcal/kg.ºC)
0.58
0.60
0.033
1.000
Gases at Constant Pressure
Air
0.25
Carbon Dioxide
0.199
Helium
1.24
Nitrogen
0.218
Oxygen
0.218
Water (100°C steam)
0.482
Next Lecture
• To Be Covered: Phase changes and latent
heat
• Reading: Chapter 5
ƒ Section 5.3
ƒ Section 5.4