Download Temperature, Heat, and Expansion

Temperature, Heat,
and Expansion
MR. PADILLA
PHYSICS H
Temperature
 Quantity that tells how hot or cold something is
compared with a standard.
 Measured in degrees (°)
 We use devices such as a thermometer to measure
temperature.
 Temp. is proportional to the average kinetic energy
of an objects molecules.
3 Major Temperature Scales
 Fahrenheit (F) – used only in the US
 Freezing = 32°
 Boiling = 212°
 Celsius (C) – most widely used scale
 Freezing = 0°
 Boiling = 100°
 To convert between F and C use the equation
9
C  32  F
5
 Kelvin (K)– scale used in scientific research
 Degrees are the same size as the Celsius degree
 Zero = absolute zero
 Absolute zero – lowest possible temperature, the
temperature at which all motion stops.
 Absolute zero about = -273.15° C.

K = C + 273.15
Freezing = 273K
 Boiling = 373K

Sample Problem 10A
 What are the equivalent Celsius and Kelvin
temperatures of 50.0oF?
Heat
 Heat is the energy that
transfers from one
object to another because
of temperature difference
between them.
 An object does not contain
heat because heat is
energy in transfer.
 When objects touch to
transfer heat they are in
thermal contact.
 Two objects in contact
will reach the same temp.
– they are in thermal
equilibrium.

Which direction does heat
move?

Describe the heat transfer
in two cases:
 Hand on a burner
 ice in a glass of coke
Internal Energy
 The law of conservation of
 Every molecule contains
potential and kinetic
energy. The sum of all
these energies is called
internal energy.
 When a substance takes
in or gives off heat any
of its internal energies
can change.
energy must be adapted to
include this
 ΔPE + Δ KE + Δ U = 0
 The total energy remains
the same
 PEi + KEi + Ui =
PEf + KEf + Uf
Sample Problem 10B
 A vessel contains water. Paddles that are propelled
by falling masses turn in the water. The agitation
warms the water and increases its internal energy.
The temperature of the water is then measured,
giving and indication of the water’s internal energy
increase. If a total mass of 11.5 kg falls 1.3 m and all
of the mechanical energy is converted to internal
energy, by how much will the internal energy of the
water increase? (Assume no energy is transferred as
heat out of the vessel to the surroundings or from the
surroundings to the vessel’s interior.)
Example
 A .10kg ball falls 10.0m onto a hard floor, then
bounces back up 9.0m. How much of it’s mechanical
energy is transformed to the internal energy of the
ball and the floor?
 Let’s see how you do…
 PEi + KEi + Ui = PEf + KEf + Uf
 PEi = PEf + Uf
 (.10kg)(9.8 m/s2)(10.0m) = (.10kg)(9.8 m/s2)(9.0m) + Uf
 Uf = 0.98 J
Measuring Heat
 Heat is measured in calories.
 calorie – the amount of heat required to raise
the temperature of 1 g of water by 1°C
 1000 calories = 1 kilocalorie
 The unit for measuring a food,
the Calorie = 1 kilocalorie
1 calorie = 4.186 Joules
Calculating Heat
 Heat – Q
measured in calories (cal) or Joules (J)
 Specific Heat – c
 measured in cal/(g°C) or J/kg°C
 Temperature – T
 Celsius – C
 Kelvin – K
 Fahrenheit - F
Specific Heat Capacity
 An objects capacity for storing internal energy.
 Quantity of heat required to raise the temperature of
a unit mass of the substance by 1 degree.
 Often referred to as specific heat.
 Water has a much higher specific heat than most
common materials.
 Table 10.4 in your book will give you some common heat capacities.
Easy Problem
 Calculating Heat
 Q = mcT
 How many calories are
needed to raise the
temperature of 1kg of
water by 15°C?
 c for water = 1cal/g°C
Q=(1000g)(1cal/g°C)(15°C)
Q = 15000 calories
Or 15 kcal
Or 15 Cal
 How many joules is 15000 calories?
 (15000cal)(4.186J/cal) = 63000 J
Harder Ex.
 When heat is lost from one substance, an equal amount
is gained by another substance(s).
 What is the final temperature when a 3.0 kg gold bar at
99°C is dropped into 0.22kg of water at 25°C?
 Heat lost = heat gained
so…
 cgmgΔTg = cwmwΔTw
 (1.29x102)(3.0kg)(99°C-Tf)=(4.186x103)(.22kg)(Tf-25°C)
 Solve
 Tf = 47°C
Sample Problem 10C
 A 0.050 kg metal bolt is heated to an unknown initial
temperature. It is then dropped into a beaker
containing 0.15 kg of water with an initial
temperature of 21.0 oC. The bolt and the water then
reach a final temperature of 25.0 oC. If the metal has
a specific heat capacity of 889 J/kg x oC, find the
initial temperature of the metal.
Change of Phase
Phases
 Matter exists in three phases
 Solid
 Liquid
 Gas
 Matter can change from one phase to another.
 Changes involve a transfer of energy
Evaporation
 Change in phase from a liquid to a gas that takes
place at the surface of the liquid.
 What are some examples of evaporation taking
place?
 Molecules at the surface gain enough kinetic energy
to break free of the liquid
 Become a vapor.
Condensation
 Changing of a gas to a
 Saturated: air contains
liquid.
 Molecule loses kinetic
energy.
 It is a warming process.
its limit of water vapor
for a temp.
 Relative Humidity:
indicates how much
water vapor is in the air
compared to the max for
that temp.
Condensation in the Atmosphere
 When particles move slowly they are more likely to
stick together.
 This is when condensation occurs.
 Clouds form when water vapor condenses upon other
particles or ions.
 Fog is basically a cloud near the ground.

Occurs when moist air near the ground cools.
Evaporation & Condensation Rates
 When evaporation and condensation take place at
the same time, at equal rates they are in
equilibrium.
 If there is more evaporation than condensation, a
liquid is cooled.
 If there is more condensation than evaporation, a
liquid is warmed.
 This heat is usually transferred to the environment.
Boiling & Freezing
 Bubbles of gas from
 Liquid gives way to the
below the surface escape
into the surrounding air.
 The pressure in the
bubbles must be great
enough to resist the
pressure of the
surrounding liquid.
solid phase
 Foreign molecules may
interfere with freezing,
making it more difficult.
 Water freezes together to
form a 6 sided crystal
structure.
Boiling & Freezing
 At the same time…
 When water boils it takes energy, leaving the
remaining water cooler.
 Lessening the air pressure causes water to boil at a
lower temperature.
 With low enough air pressure, the water’s surface
will freeze while it is still bubbling.
Energy
 When a material
changes from a solid
to a liquid to a gas it
gains energy
 When it changes
from a gas to a
liquid, to a solid to
loses energy.
Energy & Phase Change
 Heat is required to
change matter from one
phase to another.
 This heat is absorbed
without changing the
temp.
 Water


Latent Heat of Fusion –
80 cal/g (3.33x105 J/kg)
Latent Heat of
Vaporization – 540 cal/g
(2.26 x 106 J/kg)
Q = mcT + mLv + mLf
 How much energy is
released from 50 g of
water at 20° freezing to 10° ice?
 How much energy is
required to raise 30g of
water at 40°C to steam at
110°C?
Q = (50g)(1cal/g°C)(20°C) +
(80cal/g)(50g) +
(50g)(0.50cal/goC)(10oC)
Q = 5250 cal
Q = (30g)(1cal/g°C)(60°C) +
(540 cal/g)(30g) +
(30g)(0.48cal/goC)(10oC)
Q = 18144 cal
Sample Problem 10D
 How much energy is removed when 10.0 g of water is
cooled from steam at 133.0 oC to liquid at 53.0 oC?
Assignment
 Practice 10D – all
Thermal Expansion
 When the temperature of a substance is
increased, its molecules tend to move farther
apart.
 This results in expansion.
 Almost all forms of matter will expand when
heated and contract when cooled.
More Thermal Expansion
 Normally, a gas will expand/contract more than a
liquid and a liquid will expand/contract more than
a solid.
 The amount of expansion of a substance depends
on its change in temperature.
 Different substances expand at different rates.
 Ex. Jar lid, thermostat, roads
Expansion of Water
 Water at a temperature
of 0°C contracts when
heated.
 Water is most dense at
4°C.
 Ice, then, is less dense
than water.
 This is why ice floats.
Conduction
 Materials that conduct (or transmit) heat well are
called conductors.
 This means energy is moving from one place to
another.
 Metals are the best conductors.
 If you touch a piece of metal and a piece of wood
that have been outside, which will feel colder?
Which is really colder?
 The metal will feel colder because it is a better
conductor (heat moving out of you)
 Wood is a poor conductor.
 Poor conductors are said to be good insulators.
 In general liquids and gasses are good insulators.
Convection
 Convection occurs by
currents in a fluid.
 A fluid will move from
one place to another,
releasing energy as it
goes.
 Convection occurs in all
fluids, liquid or gas.
 Convection follows
Archimede’s principle


A substance that is less
dense will rise to the top.
Cold air will move toward
areas of hot air
 Convection is
responsible for
producing winds.
Wind
 Convection currents can form in areas of uneven
heating, such as a shoreline.
Rising Warm Air
 As warm air rises, the atmospheric pressure around
it decreases.
 Since there is less pressure, it will expand, and cool.
 When there is more pressure it will become
compressed and heat up.
 Ex. Blowing air, Candle
Radiation
 The sun transmits heat through radiation.
 Radiation is neither convection or conduction.
 Any energy that is transmitted by radiation is
called radiant energy.
 Radiant energy is in the form of electromagnetic
waves (radio, microwaves, light, x-ray)
Cooling
 If you want to cool off a
 Newton’s Law of
warm can of Coke,
would it cool faster in
the fridge or in the
freezer?
 Why?
Cooling
 The rate of cooling of
an object is approx.
proportional to the
temp. difference
between the object and
its surroundings.
Greenhouse Effect
 What is the greenhouse effect?
 Short wavelengths of energy enter in and get
absorbed. Long wavelengths get emitted from inside
but get reflected back in.
 Energy the Earth radiates = terrestrial radiation
 pg 387-389: 5-19 odd, 20, 34-40 even