Lecture 33
Temperature and Thermal Energy
Ozgur Unal
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Remember a form of energy, thermal energy, we briefly
mentioned few chapters ago.
Do you think the gases in the air are moving? Or, are they
suspended in the air?
How about the particles (atoms or molecules) in a liquid
and solid?
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Particles (atoms and molecules) in solids, liquids and gases
are randomly moving around.
The motion of these particles are described by the kinetic
theory.
According to the kinetic theory, the particles also collide
with each other.
Since these particles are moving, they have kinetic energy.
As the particles collide, the kinetic energy can be transferred
from one particle to another particle.
http://www.chm.davidson.edu/vce/kineticmoleculartheo
ry/basicconcepts.html
http://preparatorychemistry.com/KMT.html
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How does the temperature of an object change if you give
energy to it (heating, leaving under the Sun etc.)?
How do you think the energies of the particles change if you
increase the temperature of an object?
The temperature of a substance is a measure of the average
kinetic energy of its particles.
As the average speed of the
particles in random motion
increases, the temperature of
the substance increases.
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The SI unit of temperature is Kelvin (K).
Celsius scale (C) and Fahrenheit (F) scale are commonly
used.
Conversion equations:
Fahrenheit to Celsius  oC = 5*( oF – 32)/9
Celsius to Fahrenheit  oF = 9*oC/5 + 32
Celsius to Kelvin  oC = K – 273
Example: What is a temperature of -40 oF in degrees
Celsius?
Example: What is a temperature of 300 K in degrees
Fahrenheit?
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Particles in an object do not only have kinetic energy, but
also potential energy.
This potential energy is due to the interaction of particles.
The potential energy of particles increase as they get farther
apart  Figure 2 on page 256.
The sum of kinetic and potential energies of all the particles
in an object is the thermal energy of the object.
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Which ball has more thermal energy: A ball at rest at 300 K
or a rolling ball at 300 K?
Thermal energy is the sum of the potential and kinetic
energies of the particles in the ball. Since the balls have the
same temperature, their thermal energies are the same. The
rolling ball’s kinetic energy is NOT included in thermal
energy.
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What is heat?
Heat is the thermal energy that flows from something at a
higher temperature to something at a lower temperature.
Heat is a form of energy  Unit of heat is Joules.
Heat flows from hot objects to cold objects.
State in which direction heat flows for the following
examples:
1- An object at 10 K in contact with another object at 20 K.
2- Ice in a cup of liquid water.
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Lecture 34
Specific Heat
Ozgur Unal
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Have you ever noticed that some materials heat up faster
than other materials?
Can you provide some examples?
In a hot summer day, although the sea and the sand gets
equal amount of sunlight, sand gets hotter than sea.
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As a substance is heated, its temperature change depends on
the amount of thermal energy added and the nature of the
substance.
Example: For an equal amount of water and iron, it takes
about 6 times more thermal energy to increase the
temperature equal amounts.
The amount of thermal energy needed to raise the
temperature of 1 kg of some material by 1 K is called the
specific heat.
The unit of specific heat is J/(kg*K)
Check out Table 1!
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For a given amount of substance, if you increase its thermal
energy, how does the temperature change?
For a given amount of thermal energy, if you increase the
mass, how does the temperature change?
change in thermal energy = mass*change in temperature* specific heat
Q = m*(Tf - Ti)*C
Q = Change in thermal energy
m = mass
Tf = Final temperature
Tf = Initial temperature
C = Specific heat
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Example: Find the change in thermal energy of a 20-kg
wooden chair that warms from 15 K to 25 K, if the specific heat
of wood is 1,700 J/(kg*K).
Example: The air in a living room has a mass of 72 kg and a
specific heat of 1,010 J/(kg*K). What is the change in thermal
energy of the air when it warms from 20 oC to 25 oC?
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In order to find the specific heat of an object, C, we must
know Q, m and ΔT.
Specific heat is measured using a device called calorimeter.
In a calorimeter, a heated sample of substance transfers
thermal energy to a known mass of water.
The energy absorbed by water can be calculated
by measuring the water’s temperature.
The thermal energy released by the sample is
equal to the thermal energy absorbed by the
water.
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Develop a procedure to find the specific heat of water.
Use microwave to heat up the water.
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