Transcript Slide 1
CLB 10102
PHYSICS
CHAPTER 10
Thermal Expansion
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
10.1 Expansion and Contraction
• As the average energy of particles increases, the spaces
between the particles increases. They expand (increase their
volume) as the temperature increases.
• As the average energy of particles decreases, the spaces
between the particles decreases. They contract (decrease
their volume) as the temperature decreases.
• Pure substances are matter that are made up of only one
kind of particle, which can be a solid liquid or a gas.
• These phases, or states have very specific properties in
relation to the particle model.
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
Expansion in Solids
• Most solids expand when heated and contract when cooled.
• They expand or contract in all three dimensions – length,
width and thickness.
• When a solid is heated, the expansion is due to the increased
length of the vibrations of the atoms and molecules.
• This results in the solid expanding in all directions –
volume increased, weight density decreased.
CLB 10102 Physics
Expansion and Contraction in Gases
CHAPTER 10 Thermal Expansion
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
• When the particles in a gas are heated, their
average energy increases and they need more
room, so they expand.
• When the particles in a gas are cooled, the volume
decreases, or contracts, because the particles need
less room.
• Under extremely high temperature conditions (like
the temperatures inside the Sun, particles can be
split into what makes them up (electrons and ions).
This creates a fourth state of matter called plasma.
CLB 10102 Physics
Expansion and Contraction in Liquids
CHAPTER 10 Thermal Expansion
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
• When the particles in a liquid are heated, their average
energy increases and they need more room, so they
expand.
• When the particles in a liquid are cooled, the volume
decreases, or contracts, because the particles need less
room.
• This is demonstrated by the liquid used in a
thermometer.
• As the liquid expands and contracts, it moves up and
down the inside tubing the thermometer.
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
10.2 Coefficient of Linear Expansion
• The linear expansion of a heated solid or liquid can be
measured by a quantity α, the coefficient of linear expansion.
• This coefficient is defined in such a way that it measures the
percentage change in the length per degree temperature
change.
• The change in length, per unit original length per 1oC change
in temperature, is called the coefficient of linear expansion.
L LoT
∆L = change in length
α = coefficient of linear expansion
Lo = original length
∆T = change in temperature
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
• Expansions in surface area and volume are negligible.
• A varies slightly over a range of temperatures, but the
variation is negligibly small.
• The amount that a solid expands depends on the following:
Material. Different materials expand at different rates.
Steel expands at a rate less than that of brass.
Length of the solid. The longer the solid, the larger is
the expansion.
Amount of change in temperature. The greater the
change in temperature, the greater is expansion.
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
Example:
A steel bridge is built in several segments, each 20 m long.
The gap between segments is 4 cm at 18 °C. What is the
maximum temperature that the bridge can manage before
buckling? [Coefficient of thermal expansion of steel, αsteel is
12 x 10-6 K-1]
L LoT
0.04 m = (20 m)(12 x 10-6 K-1)∆T
∆T =
0.04 m
(20 m)(12 x 10-6 K-1)
= 166.67 K
Tf = ∆T + To
= 166.67 K + (18 + 273.15 K) = 457.82 K @ 184.67oC
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
10.3 Coefficient of Superficial Expansion
• The change in surface area per unit original area per 1oC
change in temperature is called the coefficient of superficial
expansions.
A 2AoT
• Expansion in volume is negligible.
, Ao = original area
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
10.4 Coefficient of Cubic Expansion
• The change in volume per unit original volume per 1oC
change in temperature is called coefficient of cubical
expansion.
V 3VoT
, Vo = original volume
Coefficients of Thermal Expansion for Selected Materials
Material
aluminium
barium ferrite
Linear
α (10−6 K−1)
Material
Linear
α (10−6 K−1)
23.1
platinum
8.8
10
plutonium
54
brass
20.3
silicon
4.68
carbon, diamond
1.18
silver
18.9
carbon, graphite ||
6.5
solder, lead-tin
25
carbon, graphite ┴
0.5
steel, stainless
17.3
chromium
4.9
steel, structural
12
tin
22
concrete
8 ~ 12
copper
16.5
titanium
8.5
germanium
6.1
tungsten
4.5
glass
8.5
uranium
13.9
gold
14.2
water, ice (0 °C)
iron
11.8
zinc
30.2
lead
28.9
nickel
13.3
51
Coefficients of Thermal Expansion for Selected Materials
Material
Volume
β (10−6 K−1)
alcohol, ethyl
1120
gasoline
950
jet fuel, kerosene
990
mercury
181
water, liquid (1 °C)
-50
water, liquid (4 °C)
0
water, liquid (10 °C)
88
water, liquid (20 °C)
207
water, liquid (30 °C)
303
water, liquid (40 °C)
385
water, liquid (50 °C)
457
water, liquid (60 °C)
522
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
10.5 Practical Application of Thermal Expansion
Clinical Thermometer
• Another thermometer that you are probably familiar with is the
clinical thermometer.
• It also uses expansion and contraction of a liquid to measure
temperature.
• But it must be sensitive to a very small changes in temperature,
and able to measure temperatures only within a few degrees of
normal body temperatures (37ºC).
• As the liquid is warmed by the patient's body heat, it expands
past the constriction to show the patient's temperature.
• After the thermometer has been removed from the patient's
mouth, the liquid cools and contracts.
• But the liquid cannot move past the constriction; it must be
forced back by shaking the thermometer downward.
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
Thermostat
• A thermostat can be used to measure temperature in an
appliance, such as a furnace, or in a room.
• It can also switch appliances on or off at a preset temperature.
• Thermostats use the expansion and contraction of solids to
measure temperature.
• They contain a strip of two metals (called a bimetallic strip).
When heated or cooled, the two metals expand or contract by
different amounts, causing the strip to bend.
• The amount of bending depends on the temperature. It provides
a measure of the temperature.
CLB 10102 Physics
CHAPTER 10 Thermal Expansion
Outdoor Thermometer
• One thermometer that you are probably familiar with is the
outdoor thermometer.
• It uses the expansion and contraction of a liquid to measure
temperature.
• The liquid in the bulb (usually ethyl alcohol with colouring
added) expands when it is warmed and is forced up the narrow
bore on its temperature.
• The more the liquid cools. It contracts, dropping lower in the
bore.