Transcript Thermal Energy - OWU Online | Go OWU

Heat and Internal Energy
• Internal Energy U is the total energy associated with
the microscopic components of the system
– Includes kinetic and potential energy associated with the
random translational, rotational and vibrational motion of
the atoms or molecules
– Also includes the intermolecular potential energy
– Does not include macroscopic kinetic energy or external
potential energy
• Heat refers to the transfer of energy between a
system and its environment due to a temperature
difference between them
– Amount of energy transferred by heat designated by
symbol Q
– A system does not have heat, just like it does not have
work (heat and work speak to transfer of energy)
Units of Heat
• The historical unit of heat was the calorie
– A calorie is the amount of energy necessary to raise the
temperature of 1 g of water from 14.5°C to 15.5°C
– A Calorie (food calorie, with a capital C) is 1000 cal
• Since heat (like work) is a measure of energy
transfer, its SI unit is the joule
– 1 cal = 4.186 J (“Mechanical Equivalent of Heat”)
– New definition of the calorie
• The unit of heat in the U.S. customary system is the
British thermal unit (BTU)
– Defined as the amount of energy necessary to raise the
temperature of 1 lb of water from 63°F to 64°F
More About Heat
• Heat is a microscopic form of energy transfer
involving large numbers of particles
• Energy exchange occurs due to individual
interactions of the particles
– No macroscopic displacements or forces involved
• Heat flow is from a system at higher temperature to
one at lower temperature
– Flow of heat tends to equalize average microscopic kinetic
energy of molecules
• When 2 systems are in thermal equilibrium, they are
at the same temperature and there is no net heat
flow
• Energy transferred by heat does not always mean
there is a temperature change (see phase changes)
Heat Transfer Simulation
Simulation presented in class.
(ActivPhysics Online Exercise #8.6, copyright Addison Wesley publishing)
Specific Heat
• Every substance requires a unique amount of
energy per unit mass to change the temperature of
that substance by 1°C
• The specific heat c of a substance is a measure of
Q
this amount, defined as:
(units of J / kgoC)
c
m DT
• Or Q  mc DT
– DT is always the final temperature minus the initial
temperature
– When the temperature increases, DT and Q are
considered to be positive and energy flows into the system
– When the temperature decreases, DT and Q are
considered to be negative and energy flows out of the
system
– c varies slightly with temperature
Consequences of Different Specific Heats
• Air circulation at the beach
– Water has a high specific heat
compared to land
– On a hot day, the air above the
land warms faster
– The warmer air flows upward
and cooler air moves toward the
beach, creating air circulation pattern
• Moderate winter temperatures in regions near large
bodies of water
– Water transfers energy to air, which carries energy toward
land (predominant on west coast rather than east coast)
• Similar effect creates thermals (rising layers of air)
which help flight of eagles and hang gliders
– Sections of land are at higher temp. than other areas
Calorimetry
• Calorimetry means “measuring heat”
– In practice, it is a technique used to measure specific heat
• Technique involves:
– Raising temperature of object(s) to some value
– Place object(s) in vessel containing cold water of known
mass and temperature
– Measure temperature of object(s) + water after
equilibrium is reached
• A calorimeter is a vessel providing good insulation
that allows a thermal equilibrium to be achieved
between substances without any energy loss to the
environment (styrofoam cup or thermos with lid)
• Conservation of energy requires that:  Qk  0
(Q > 0 (< 0) when energy is gained (lost))
Example Problem #11.17
An aluminum cup contains 225 g of water and
a 40-g copper stirrer, all at 27°C. A 400-g
sample of silver at an initial temperature of
87°C is placed in the water. The stirrer is
used to stir the mixture until it reaches its final
equilibrium temperature of 32°C. Calculate
the mass of the aluminum cup.
Solution (details given in class):
80 g
CQ1: Interactive Example Problem:
Calorimetry
Part (a): What is the energy released via heat
by the block?
A)
B)
C)
D)
E)
193 J
–193 J
193 kJ
–193 kJ
4186 kJ
(Physlet Physics Exploration #19.3, copyright Prentice–Hall publishing)
CQ2: Interactive Example Problem:
Calorimetry
Part (c): What is the equilibrium temperature
of the system?
A)
B)
C)
D)
E)
300.0 K
304.6 K
319.0 K
327.1 K
1000 K
(Physlet Physics Exploration #19.3, copyright Prentice–Hall publishing)
Phase Transitions
• A phase transition occurs when the physical
characteristics of the substance change from one
form to another
• Common phase transitions are
– Solid  liquid (melting)
– Liquid  gas (boiling)
• Phase transitions involve a change in the internal
energy, but no change in temperature
– Kinetic energy of molecules (which is related to
temperature) is not changing, but their potential energy
changes as work is done to change their positions
• Energy required to change the phase of a given
mass m of a pure substance is: Q   mL
– L = latent heat – depends on substance and nature of
phase transition
– + (–) sign used if energy is added (removed)
Phase Transitions
• All phase changes can go in either direction
– Heat flowing into a substance can cause melting (solid to
liquid) or boiling (liquid to gas)
– Heat flowing out of a substance can cause freezing (liquid
to solid) or condensation (gas to liquid)
• Latent heat of fusion Lf is used for melting or
freezing
• Latent heat of vaporization Lv is used for boiling or
condensing (somewhat larger for lower pressures)
• Table 11.2 gives the latent heats for various
substances
• Large Lf of water is partly why spraying fruit trees
with water can protect the buds from freezing
– In process of freezing, water gives up a large amount of
energy and keeps bud temperature from going below 0°C
T vs. Q for Transition from Ice to Steam
Initial state: 1 g
of ice at –30°C
Final state: 1 g
of steam at
120°C
Qtot = 3.11  103 J
• Part A: Temperature of ice changes from –30°C to 0°C
– Q = mcice DT = (1.00  10–3 kg)(2090 J/kg°C)(30.0°C) = 62.7 J
• Part B: Ice melts to water at 0°C
– Q = mLf = (1.00  10–3 kg)(3.33  105 J/kg) = 333 J
• Part C: Temperature of water changes from 0°C to 100°C
– Q = mcwater DT = (1.00  10–3 kg)(4.19  103 J/kg°C)(100°C) = 419 J
• Part D: Water changes to steam at 100°C
– Q = mLv = (1.00  10–3 kg)(2.26  106 J/kg) = 2.26  103 J
• Part E: Temperature of steam changes from 100°C to 120°C
– Q = mcsteam DT = (1.00  10–3 kg)(2.01  103 J/kg°C)(20°C) = 40.2 J
Evaporation and Condensation
• The previous example shows why a burn caused by
100°C steam is much more severe than a burn
caused by 100°C water
– Steam releases large amount of energy through heat as it
condenses to form water on the skin
– Much more energy is transferred to the skin than would
be the case for same amount of water at 100°C
• Evaporation is similar to boiling
– Molecular bonds are being broken by the most energetic
molecules
– Average kinetic energy is lowered as a result, which is
why evaporation is a cooling process
– Approximately the same latent heat of vaporization
applies
– Reason why you feel cool after stepping out from a
swimming pool
Example Problem #11.31
A 40-g block of ice is cooled to –78°C and is then
added to 560 g of water in an 80-g copper
calorimeter at a temperature of 25°C. Determine the
final temperature of the system consisting of the ice,
water, and calorimeter. (If not all the ice melts,
determine how much ice is left.) Remember that the
ice must first warm to 0°C, melt, and then continue
warming as water. The specific heat of ice is 0.500
cal/g°C = 2090 J/kg°C.
Solution (details given in class):
16°C
Conduction
• Energy can be transferred via heat in one of three
ways: conduction, convection, radiation
• Conduction occurs with temperature differences
• Transfer by conduction can be understood on an
atomic scale
– It is an exchange of energy between microscopic particles
by collisions
– Less energetic particles gain energy during collisions with
more energetic particles
– Net result is heat flow from higher temperature region to
lower temperature region
• Rate of conduction depends upon the characteristics
of the substance
– Metals are good conductors due to loosely-bound
electrons
Conduction
• Consider the flow of heat by
conduction through a slab of crosssectional area A and width L
• The rate of energy transfer (power)
is given by:
Th  Tc
Q
P
 kA
Dt
L
L
– Assumes that slab is insulated so that energy cannot
escape by conduction from its surface except at the ends
– k is the thermal conductivity and depends on the material
– Substances that are good (poor) conductors have large
(small) thermal conductivities (see Table 11.3)
– P is in Watts when Q is in Joules and Dt is in seconds
Home Insulation
• In engineering, the insulating quality of materials
are rated according to their R value: R = L / k
• R values have strange units: °Fft2 / (Btu/h)
– That’s why units are not usually given!
• Substances with larger R value are better insulators
• For multiple layers, the total R value is the sum of
the R values of each layer
• Still air provides good insulation, but moving air
increases the energy loss by conduction in a home
– Much of the thermal resistance of a window is due to the
stagnant air layers rather than to the glass
Convection
• Convection is heat flow by the movement of a fluid
• When the movement results from differences in
density, it is called natural convection (fluid currents
are due to gravity)
– Air currents at the beach
– Water currents in a saucepan while heating
• When the movement is forced by a fan or a pump, it
is called forced convection (fluid is pushed around
by mechanical means – fan or pump)
– Forced-air heating systems  (although air currents move
under natural convection)
– Hot-water baseboard heating 

– Blood circulation in the body
Thermal Radiation
• Thermal radiation transfers energy through
emission of electromagnetic waves – does not
require physical contact
• All objects radiate energy continuously in the form
of electromagnetic waves due to thermal vibrations
of the molecules
– At ordinary temperatures (~20°C) nearly all the radiation
is in the infrared (wavelengths longer than visible light)
– At 800°C a body emits enough visible radiation to be selfluminous and appears “red-hot”
– At 3000°C (incandescent lamp filament) the radiation
contains enough visible light so the body appears “whitehot”
• An ideal emitter and absorber of radiation is called a
blackbody (would appear black)
Thermal Radiation
• The rate at which energy is radiated is given by
Stefan’s Law:
P  AeT 4
–
–
–
–
P is the rate of energy transfer (power), in Watts
σ = Stefan-Boltzmann constant = 5.6696 x 10–8 W/m2K4
A is the surface area of the object
e is a constant called the emissivity, and ranges from 0
to 1 depending on the properties of the object’s surface
– T is the temperature in Kelvin
• Objects absorb radiation as well
• Net rate of energy gained or lost given by:
Pnet  Ae T 4  T04

– T0 = temperature of environment

Applications of Thermal Radiation
• Choice of clothing
– Black fabric acts as a good absorber, so about half of the
emitted energy radiates toward the body
– White fabric reflects thermal radiation well
• Thermography as medical diagnostic tool
– Measurement of emitted thermal energy using infrared
detectors, producing a visual display (see Fig. 11.13)
– Areas of high temperature are indicated, showing regions
of abnormal cellular activity
• Measuring body temperature
– Radiation thermometer measures the intensity of the
infrared radiation from the eardrum (see Fig. 11.14)
– Eardrum is good location to measure temperature since it
is near hypothalamus (body’s temperature control center)
Resisting Energy Transfer
• Dewar flask/thermos bottle
• Designed to minimize energy transfer to
surroundings
• Space between walls is evacuated to
minimize conduction and convection
• Silvered surface minimizes energy
transfer by radiation
• Neck size is reduced
• Same principle behind dressing in coats
and sweaters to keep warm
– Warmer air is trapped close to our bodies, reducing
energy loss by convection and conduction
Global Warming
• Analogous to a greenhouse
– Visible light and short-wavelength infrared radiation are
absorbed by contents of greenhouse, resulting in the
emission of longer-wavelength infrared radiation (IR)
– Longer-wavelength IR absorbed by glass
– Glass emits IR, half of which is emitted back inside the
greenhouse
– Convection currents are inhibited by the glass (although
this is not mirrored in Earth’s atmosphere)
• Earth’s atmosphere fills role of glass roof in
greenhouse
– “Greenhouse gasses” like CO2 are particularly good
absorbers of IR
– More greenhouse gasses in the atmosphere means more
IR is absorbed and Earth’s surface becomes warmer