Transcript Specific heat

Thermodynamics
Chapter 12
Review of Energy
• Kinetic Energy
– Energy associated with motion
• Potential Energy
– Many types of potential energy (gravitational,
chemical, etc)
– Energy at rest
– Stored energy
• Units of Energy
– joules (J)
Temperature and Heat
• The temperature of a hot cup of coffee left
sitting on the table will fall until it also
reaches thermal equilibrium with the air
temperature in the room.
• When a soda can is taken out of the
refrigerator and left on the kitchen table, its
temperature will rise – rapidly at first but
then more slowly – until the temperature of
the soda equals that of the air in the room. At
this point, the soda and the air temperature in
the room are in thermal equilibrium.
• The change in temperature is due to the
transfer of energy between object and the
environment.
Temperature and Heat
• Thermal energy: the total potential and
kinetic energy associated with the random
motion and arrangement of the particles of
a material.
• Heat, Q, is thermal energy that is
absorbed, given up, or transferred from
one body to another.
– Heat is thermal energy in motion.
– Heat is used when the transfer of
thermal energy from one body to
another body at a different temperature
is involved.
• (a) Q is negative when
heat energy is
transferred to the
environment from the
system.
• (b) Q = 0 J when the
transfer of heat energy
between the system and
the environment is equal.
• (c) Q is positive when
heat energy is
transferred to a system
from the environment.
Temperature
• Central concept of thermodynamics is
temperature.
– Our “temperature sense” is often
unreliable.
– On a cold winter day, an iron railing seems
much colder to the touch than a wooden
fence post, even though both are at the
same temperature.
– This error in perception results because
the iron removes energy from our fingers
more quickly than the wood does.
Temperature
• Temperature is a measure of
the average kinetic energy of
all the particles within an
object.
Temperature & Kinetic Energy
• The temperature of a substance
will increase if the average
kinetic energy of its particles is
increased.
• If the average kinetic energy of
particles decreases, so does the
temperature of the substance.
• Specific heat Every substance gains or
loses heat based on it’s identity. This
physical property of the substance is called
the specific heat capacity of the object.
• The specific heat capacity, C, of a solid or
liquid is defined as the heat required to raise
a unit mass of the substance by one degree
of temperature.
Define: Specific Heat
• Amount of energy required to
raise the temperature of 1kg
by 1oC
Heat
Energy =(mass)x specific x change
heat
in temp
OR
Q = m c Dt
Heat Change
• To determine the amount of thermal
energy gained or lost by a mass:
Q  m  c  ΔT
Q  m  c  (Tf  Ti )
• Heat energy is gained if Q is positive.
• Heat energy is lost if Q is negative.
Law of Heat Exchange
• For a closed system in which heat energy
cannot enter or leave, the heat lost by
objects at a higher temperature is equal
to the heat gained by objects at lower
temperature until thermal equilibrium is
reached (at which point the final
temperature of both objects is the same).
• The final temperature will be somewhere
between the initial low temperature and
the initial high temperature.
Law of Heat Exchange
• Conservation of Energy:
Q lost = Q gained
• To avoid problems with signs, for
• Q lost = Q gained problems,
• it is best to make DT = Thi – Tlo
m  c  Thi  Tlo  low temp object
 m  c  Thi  Tlo  high temp
object
Heats of Transformation
• When energy is absorbed as heat by a solid
or liquid, the temperature of the object does
not necessarily rise.
• The thermal energy may cause the mass to
change from one phase, or state, to another.
• The amount of energy per unit mass that
must be transferred as heat when a mass
undergoes a phase change is called the heat
of transformation, L.
Phase Changes
Thermal Expansion of Solids
• Solids expand when heated and contract
when cooled (with a few exceptions).
– Heated solids increase or decrease in all
dimensions (length, width, and thickness).
– When a solid is heated, the increase in
thermal energy increases the average
distance between the atoms and molecules of
the solid and it expands.
Thermal Expansion of Solids
• Thermal expansion can be
explained on a molecular
basis.
• Picture the interatomic
forces in a solid as springs,
as shown in the picture on
the right.
• Each atom vibrates about its
equilibrium position. When
the temperature increases,
the amplitude and associated
energy of the vibration also
increase.
Examples of Uses of Thermal
Expansion
•
•
Dental materials used for
fillings must be matched in
their thermal expansion
properties to those of
tooth enamel, otherwise
consuming hot drinks or
cold ice cream would be
painful.
In aircraft manufacturing,
rivets and other fasteners
are often cooled using dry
ice before insertion and
then allowed to expand to a
tight fit.
•
You can loosen a tight metal jar lid by
holding it under a stream of hot water.
Both the metal of the lid and the glass of
the jar expand as the hot water adds
energy to their atoms. With the added
energy, the atoms can move a bit farther
from each other than usual, against the
interatomic forces that hold every solid
together. However, because the atoms
in the metal move farther apart than
those in the glass, the lid expands more
than the jar and is loosened.
•
Expansions slots are often placed in
bridges to accommodate roadway
expansion on hot days. This prevents
buckling of the roadway. Driveways and
sidewalks have expansion slots for the
same reason.
Ex. A 4.0 kg sample of glass heated from
1o C to 41o C, and was found to have
absorbed 32 J of energy. What is the
specific heat of the glass?
Q = C x m x ΔT
ΔT = 41oC – 1oC = 40oC
32 J = C (4.0 kg)(40oC)
32 J = C (160 kg oC)
C = 0.2 J/kgoC
Your Turn!
• Determine the specific heat of a material if a 35 g
sample absorbed 48J as it was heated from 293K
to 313K.
• If 980 kJ of energy are added to 6.2 L of water at
291 K, what will the final temperature of the water
be?
– 1 kJ = 1000 J
– 1 L of water has 1 kg of mass
– Cwater = 4180 J/kg°K