Download 09 Thermal Energy

Thermal Energy
• Homework handed out in class today
• Lab today Energy content of food
• Test on H day, Jan. 28
Thermal Energy Topics
• Temperature
• Energy Content
• Stored energy in foods and fuels
• Materials and Thermal Energy
• Relationship between energy and temperature
• Heat Transfer
• How thermal energy moves
Creating Thermal Energy
KE
Work by friction
Thermal
E
• This is not the only way
• But it is the way we have discussed so far
• That makes it a good place to start
• What happens when friction transfers energy to an
object? (rub your hands together)
• It gets hot!
• Increase in Temperature = increase in Thermal E
• Thermal Energy is Temperature!!!
Thermal Energy is Temperature?
• Thermal energy is not a new type of energy!
• Thermal energy is kinetic energy!
• It is the kinetic energy of the particles that make up a
substance!
• Gas—the gas particles move, spin and vibrate
• Liquid—the particles slide and jiggle past one another
• Solid—the particles vibrate and jiggle in place
Translating
(moving)
Spinning
Vibrating
Temperature and Kinetic energy
• Temperature of a substance is defined as:
The average kinetic energy per particle
• Colder: particles move more slowly
• Take up less space
• Cold gases are more dense (cold air sinks to the floor)
• Cold objects shrink (a little bit)
• Hotter: particles move more quickly
• Take up more space
• Hot gases are less dense (hot air balloons rise)
• Hot objects expand (a little bit)
Temperature and Kinetic Energy
• Energy is measured in Joules
• Temperature is measured in degrees
• How are they the same thing?
• They are not!!
• Write average KE of a particle as ⟨𝐾𝐸⟩, then
⟨𝐾𝐸⟩ = 𝑘𝐵 𝑇
• Do not worry about this equation. We won’t use it.
• Just know this: when we say “temperature”…
• …we mean average kinetic energy per particle
Temperature and Kinetic Energy
• Remember: temperature is an average
• In a substance at a specific temperature, some particles
will be moving slower, some will be moving faster.
• Temperature describes the average over very large
numbers of particles
• Most substances contain roughly 1023 particles per cubic cm.
Temperature Scales
• Temperature is average
KE only if we are using
absolute temperature
• Absolute temperature is
measured in Kelvin degrees
• Not the Fahrenheit degrees we use
in the U.S.
• We just call them “Kelvins”
• If the average KE per particle is
zero, then the absolute
temperature is zero
Fahrenheit Temperature Scale
• Absolute temperature is not convenient for everyday use
• Fahrenheit: most familiar to us
• units are degrees (°F)
• Water freezes at: 32 degrees
• Water boils at: 212 degrees
• Average human body temperature: 98.6
• No other country in the world wastes their time with this
temperature scale any more.
Celsius temperature scale
• Used in the rest of the world
• Used in the science lab
• Units are degrees Celsius (°C)
• Body temperature: 36.8 °Celsius
• Water freezing point: 0 °Celsius
• Water boiling point: 100 °Celsius
100 degrees apart
• The Celsius degree is the same size as the Kelvin.
Kelvin scale: absolute temperature
• Absolute zero: the temperature at which the particles in
a substance would have zero kinetic energy
• No negative numbers!
• Because no such thing as negative kinetic energy!!!
• Water freezing point: 273 K
• Water boiling point: 373 K
• Used in:
100 degrees apart
• thermal physics
• Thermodynamics
• If you need to work with the total energy contained in a
substance, you need to use this temperature scale.
Converting between scales
• Fahrenheit temperature = (1.8 X Celsius Temp.) + 32
• °F = 1.8 °C + 32°
• Celsius temperature = (Fahrenheit temp. – 32) / 1.8
• °C = (°F – 32°)/1.8
• Kelvin temp. = Celsius temp + 273
• K = °C + 273
Convert the following:
• 74 degrees Fahrenheit to Celsius and Kelvin
• 23 °C, 296 K
• 43 degrees Celsius to Fahrenheit and Kelvin
• 109 °F, 316 K
• 5 degrees Celsius to Fahrenheit and Kelvin
• 41 °F 278 K
• 278 degrees Kelvin to Fahrenheit and Celsius
• 41 °F, 5 °C
Momentum Makeup
• Up to 10 points
• In class, Feb. 4
• 15 minutes
Review: Thermal Energy is Temperature
• Thermal energy is not a new type of energy!
• Thermal energy is kinetic energy!
• It is the kinetic energy of the particles that make up a
substance!
• Gas—the gas particles move, spin and vibrate
• Liquid—the particles slide and jiggle past one another
• Solid—the particles vibrate and jiggle in place
Translating
(moving)
Spinning
Vibrating
Temperature Scales
• Temperature is average
KE only if we are using
absolute temperature
• Absolute temperature: Kelvin scale
• If the average KE per particle is zero,
then the absolute temperature is
zero
• “Practical” scales:
• Celsius
• Fahrenheit
• You MUST convert these to Kelvin
in order for temperature to be
equivalent to energy!
Example: Absolute Temperature
• A sample of helium gas is at a temperature of 27 °C
• If it were to have twice as much energy, what would its
temperature be in °C?
• This is not an absolute temperature. We must convert to
Kelvin first:
• 27 °C + 273 = 300 K
• Absolute temperature: twice as much energy means
twice as high a temperature:
• Twice as much energy as 300 K is 2 x 300 K = 600 K
• Now convert back to °C:
• 600 K – 273 = 327 °C
Measuring Temperature: Thermometers
• A thermometer is a device that
measures temperature
• Thermometers use substances that
have physical properties that change a
lot with changes in temperature
• most substances expand as their
temperature increases and contract as
their temperature decreases
• Common thermometers use colored
alcohol or mercury
• These substances expand and contract a
very large amount when their
temperature changes
Other Types of Thermometers
• Bimetallic strip: thermometer made from two different
types of metal that expand and contract at different rates
• Household thermostats used to work this way
• Digital thermometer: measures changes in the electrical
resistance of a special material
• How thermostats work today
Other Types of Thermometers
• Infrared Thermometer:
measures the infrared radiation
given off by an object
• All objects give off this radiation!
• We’ll talk about this radiation
later in the unit
• Gas Thermometer: measures changes
in the pressure in a gas
• Earliest type
• Can be very accurate
• Not used very often
Energy Content
• Substances can contain stored energy
• Often this energy is stored in chemical bonds
• Food
• Gasoline
• Explosives
Creating Thermal Energy
Kinetic
Energy
Chemical
Energy
• Burn fuel
• Metabolize food
• Ignite explosives
Work by friction
Thermal
Energy
Chemical Reaction
Thermal
Energy
Energy from Food
• We are powered by the energy contained on food
• It is stored as chemical energy in the food
• The energy content in food is measured in Calories
• The capital “C” is really important!! These are food Calories
• 1 (food) Calorie = 4190 J
• A Big Mac contains 563 Calories
• This would power a 60 W light bulb for 11 hours!
• (4200 J/Cal) x
563 Cal =
2,365,000 J
• (2,365,000 J)/60 W
= 11 h
Calories and calories
• The calorie is a unit of energy
• It is the energy required to raise the temperature of
1 gram of water by 1 °C (or 1 K, same thing)
• 1 calorie = 4.2 J
• The food Calorie = 1000 calories
• Pay attention to big C and small c!!!
• The food Calorie is also called the kilocalorie and
abbreviated kcal
Thermal Energy and Temperature
• If we add thermal energy to an object…
• …will it get colder?
• …or hotter?
• How is the temperature change related to the energy?
Energy and Temperature
• When thermal energy is transferred to a substance, we
say the energy was “absorbed”
• If energy is absorbed, the temperature goes up
• The exact temperature rise Δ𝑇 depends on
1. The mass of the substance
2. What substance it is
• Different substances have different specific heats
• We use the symbol 𝐶 for specific heat
• For thermal energy transferred between substances we
use the symbol 𝑄
Specific Heat
• Specific heat describes the ability of a substance to absorb
thermal energy
• Temperature change of an object depends on
• The amount of energy transferred
• The mass of the object
• The specific heat of the objects substance
Energy transferred
Temperature change =
(mass) X (specific heat)
Δ𝑄
Δ𝑇 =
𝑚𝐶
Does this make sense?
Δ𝑄
Δ𝑇 =
𝑚𝐶
Energy transferred
Temperature change =
(mass) X (specific heat)
• Transfer more energy ⇒ A bigger temperature rise
• More mass ⇒ A smaller temperature rise
• Compare giving the same energy to a pebble or a boulder
• More specific heat ⇒ A smaller temperature rise
Example Specific Heats
Material
Aluminum
Brick
Copper
Granite
Nylon
Paper
Wood
Water
C (J/g/°C)
.88
.84
.38
.80
1.60
1.34
1.72
4.19
Energy Change: What scale to use?
• Because we are talking about change in temperature…
• (that is: adding or subtracting)
• …we can use either K or °C
• For energy transfer calculations, we do not need to
convert to absolute temperature.
Specific Heat and the calorie
• The calorie is an old-fashioned unit of energy that is still
widely used in science and nutrition
• The calorie is defined using the specific heat of water:
• 1 calorie of energy given to 1 gram of water will raise its
temperature by 1 °C
• 1 cal = (1 g) x (1 cal/g/°C) x (1 °C) (𝑄 = 𝑚𝐶 Δ𝑇)
• The specific heat of water is 1
cal
g °C
• 1 cal = 4.19 J
• We can use K instead of °C because they are the same size
•1
cal
gK
= 4.19
J
gK
Specific Heat and the calorie
• Water has an unusually high specific heat
• 4.19 J (1 calorie) of energy given to a gram of water raises
its temperature by 1 °C
• The same amount of energy given to a gram of a
different substance will raise its temperature by a
different amount.
• 1 calorie given to 1 gram of copper will raise its temperature by
9 °C!!
• Different substances have different specific heat capacity
• Usually shortened to “specific heat”
Specific Heat Capacity
• Substances have their own specific heat capacities.
• Example:
• Filling in a hot apple pie has a greater specific heat capacity
than the crust.
• The filling has lots of water in it
• Water has a very high heat capacity
• As the pie cools, it loses thermal energy
• If a gram of crust and a gram of filling lose the same
amount of energy, the filling will stay hotter!
• That’s why you can burn your mouth on a
pie that doesn’t burn your hand!
Specific heat equation
Δ𝑄
•𝐶 =
𝑚 Δ𝑇
• 𝐶 is specific heat
• Δ𝑄 is energy lost or gained
• 𝑚 is mass
• Δ𝑇 is change of temperature
• Joules per gram per Celsius: J/gK or J/g°C
• Calories per gram per Celsius: cal/g°C
Example
• How much energy must be transferred to 200 kg of water
in a bathtub to raise the water’s temperature from 25 °C
to 37 °C?
• 𝑄 = 𝑚𝐶 Δ𝑇
• 𝑄 = (200,000 g) (4.2 J/g/°C) (37 °C – 25 °C)
• 𝑄 = 10,080,000 J
• How much energy is needed to increase the temperature
of 755 g of Iron from 283K to 403K?
[C of iron is 0.45 J/(g K)]
• 𝑄 = 𝑚𝐶 Δ𝑇
• 𝑄 = (755 g)(0.45 J/g/K)(120 K)
• 𝑄 = 40,770 J
Example
• How much energy must a refrigerator absorb from 225 g
of water to decrease the temperature of water from 35C
to 5C?
• 𝑄 = 𝑚𝐶Δ𝑇
• 𝑄 = (225 g)(1 cal/g/°C)(30 K)
• 𝑄 = 6750 cal × 4.2 J/cal = 28,350 J
Practice Problems
• A 4.0 g sample of glass was heated from 274 K to 314 K,
and was found to have absorbed 32 J of energy as heat.
• Determine the specific heat of the glass
• 𝑄 = 𝑚𝐶 ΔT
•𝐶=
•𝐶=
𝑄
𝑚Δ𝑇
32 J
4.0 g 40 K
=
J
0.2
gK
• How much energy will the same glass sample gain when
it is heated from 314 K to 344 K?
• 𝑄 = 𝑚𝐶Δ𝑇
• 𝑄 = 4.0 g
J
0.2
gK
30 K = 24 J
Practice Problems
• Determine the specific heat of a material if a 35 g sample
absorbed 96 J as it was heated from 292 K to 313 K.
•𝐶=
•𝐶=
𝑄
𝑚Δ𝑇
96 J
35 g 21 K
=
J
0.13
gK
• 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?
• 𝑄 = 𝑚𝐶Δ𝑇
• Δ𝑇 =
• Δ𝑇 =
𝑄
𝑚𝐶
980,000 J
6200 g
J
4.2g °C
• 291 K + 38 K = 329 K
= 38 K
Review: Specific heat equation
𝑄
•𝐶 =
(a rearrangement of 𝑄 = 𝑚𝐶Δ𝑇)
𝑚 Δ𝑇
• 𝐶 is specific heat
• 𝑄 is energy lost or gained
• 𝑚 is mass
• Δ𝑇 is change of temperature
• Joules per gram per Celsius: J/gK or J/g°C
• Calories per gram per Celsius: cal/g°C
• The specific heat of water is 1
• 1 cal = 4.2 J
cal
g °C
or 4.2
J
gK
Example
• How much energy—in calories—must be transferred to
20 g of water to raise the water’s temperature by 7 °C?
•
•
•
•
•
𝑄 = 𝑚𝐶 Δ𝑇
𝑄 = (20 g) (1 cal/g/°C) (7 °C)
𝑄 = 140 cal
How many Joules is this?
140 cal X 4.2 J/cal = 588 J
Thermal Energy Transfer
Flow of Thermal Energy
• Thermal energy will move from a hot region to a cold
region
• We call this movement “the flow of heat”
• Thermal energy will NEVER move from cold to hot!!!
• Never means never, ever, ever!
• Objects feel “hot” or “cold” because
of energy transfer
• If thermal energy flows out of your hand,
the object feels cold
• If thermal energy flows into your hand,
the object feels hot
Energy never moves from cold to hot?
• What about refirgerators
and air conditioners?
• Thermal energy moves
from the cold inside to
the warm outside!
• This only happens if
extra energy is added in
the form of mechanical
work
• More energy is dumped
to outside than is taken
from inside
Types of Thermal Transfer
• Conduction
• Convection
• Radiation
Conduction
• The flow of energy within a
substance or between two
objects in contact
• Results from transfer of energy
between particles
• Example: transfer of heat
along a piece of metal or wire
• Energy (heat) moves down the
rod
• Eventually the energy starts to
move into your hand, causing
you to drop the rod
Conduction
• Conduction is how thermal energy moves in solids
• The particles vibrate, but they can’t move around.
• Hot particles have lot’s of KE
• They bang into their neighbors.
• The banging gives the neighbor particles more KE
• Now they are hot too!
• Note: It’s kinetic energy that is flowing!!
• Not the particles!!
Conduction
• Good conductors:
• Most dense solids are good thermal conductors
• Metals are the best of the solids
• The electrons that can carry electrical current are also good at carrying
kinetic energy
• Poor conductors:
• Solids with weak bonds between the molecules
• They have low density (wood, cork)
• Solids with trapped air
• Wool yarn, sytrofoam (air is not a solid and is a lousy conductor)
• We put these materials in the walls of our house and call it
“insulation”
• Allows the inside temperature to be very different than the outside
temperature without using too much energy.
Convection
Convection
• Heat flow due to the movement of the particles
• Seen in fluids (liquids and gases
• Example: Plate tectonics: hot magma rises, transferring
heat from the core of the Earth to the surface. Cooler
magma sinks back down.
• The flow under the
crustal plates makes
them move (slowly)
• Convection can occur
in both gases and
liquids.
• To have one word for
both, we call them
“fluids”
Convection Currents
• Transfer of heat involving bulk motion of fluids.
• The particles move in a current
2. Hot air blob
expands and
floats up, creating
a current. Energy
spreads by
conduction.
1. Energy moves
from radiator to
air by conduction.
3. Pushed by the
currents, air blob
continues to
move, releasing
energy to
surrounding air.
4. Now cool, the
air blob is carried
by the current
back to the
radiator.
Convection
• In a fluid, a chunk of cold material can move into a region
of warmer material.
• This does NOT violate the Never Rule!!!
• Heat will flow into this cold chunk from the surrounding
warmer fluid.
Convection Examples
• Heating water
• Heated water expands, rises
• Cools at the top, becomes more
dense, and sinks
• The particles move!!
• A campfire
• Heated air expands, rises
• Can carry embers with it
• Away from the fire, the air cools
and sinks
• If the embers are still burning, it
can start a forest fire
Radiation
• The transfer of energy by
electromagnetic waves
• Example: heat given off from a fire
• Energy leaves hot fire
• Energy is carried through space
• Energy is absorbed by cooler hands
All Objects Emit Radiation
• We just can’t see most of it
Hot objects radiate long
wavelengths that are just
outside what we can see.
Really hot objects radiate
visible light.
Even cold objects radiate,
but at very long wavelengths that we can’t see.
Wave Frequency - Temperature
• Frequency = “color” of light
a) A low-temperature (cool)
source emits primarily lowfrequency, long
wavelength waves.
b) A medium-temperature
source emits primarily
medium-frequency.
c) A high-temperature source
emits primarily highfrequency, short
wavelength waves. If the
object is hot enough, it can
emit visible light.
Wave Frequency - Temperature
Radiation
• All objects emit thermal radiation
• If it has a temperature, then it emits radiation
• Hotter objects emit more
• And the wavelengths are shorter
• Cooler objects emit less
• And the wavelengths are longer
• It you can touch it, the thermal radiation it emits is
invisible to your eye: infrared light
• Night vision cameras can detect some of this light
Radiation
• The surface properties of an object affect how much
radiation is emitted at a given temperature
• Non-shiny black objects emit more (good radiators)
• Shiny silver objects emit the least (poor radiators)
• Good radiators are good absorbers
• Poor radiators are poor absorbers
Radiation
• Emission of radiant energy
• Every object above absolute zero radiates
• From the Sun's surface comes a really
huge amount of light, or solar radiation
• The Earth also emits radiation in the
form of infrared waves that are not
visible to our eyes
• Radiation must balance!
• Earth radiates the same amount of
energy as it absorbs from the Sun
• If not, it would be a molten blob of lava!
Global Warming
• The amount of energy radiated
by the Earth must equal what is
absorbed from the Sun
• CO2 in the atmosphere is making
the Earth less like a black pot,
and more like a silver pot.
• The Earth has to get hotter in
order to radiate away the same
amount of energy it always did.
• Exactly how much hotter (and when) is hard to figure out.
Climate denialists use this difficulty to claim that that
global warming is not real. It is real.
Review
• What is Thermal Energy
• Celsius, Kelvin, and Fahrenheit scales
• Calories, calories, and joules
• Energy and temperature change
• Energy transfer
• These review slides are NOT what you should study!
• They are a reminder of what you should study
Thermal Energy is Kinetic energy
• Temperature of a substance is defined as:
The average kinetic energy per particle
• Colder: particles move more slowly
• Take up less space
• Cold gases are more dense (cold air sinks to the floor)
• Cold objects shrink (a little bit)
• Hotter: particles move more quickly
• Take up more space
• Hot gases are less dense (hot air balloons rise)
• Hot objects expand (a little bit)
Temperature Scales
• Temperature is average
KE only if we are using
absolute temperature
• Absolute temperature is
measured in Kelvin degrees
• If no more energy can be
removed from the particles, then
the absolute temperature is zero
Converting between scales
• Fahrenheit temperature = (1.8 X Celsius Temp.) + 32
• °F = 1.8 °C + 32°
• Celsius temperature = (Fahrenheit temp. – 32) / 1.8
• °C = (°F – 32°)/1.8
• Kelvin temp. = Celsius temp + 273
• K = °C + 273
• Fixed points:
• Freezing:
• Boiling:
0 °C
100 °C
273 K
373 K
32 °F
212 °F
Definition of the calorie (small c)
• The calorie is the amount of energy required to:
raise the temperature of
1 gram of water
by
1 °C
• It is a unit of energy, like the joule
• But one calorie is more energy than one joule:
1 cal = 4.19 J
Energy absorbed or released and ΔT
• Thermal energy is temperature
• Adding or removing energy changes temperature
• For a fixed energy move (like, 1 J or 1 cal), the size of the
ΔT is different for different materials
• Different materials have different specific heat capacity:
• Symbol: C
energy
• Units:
mass degrees
• Energy can be joules or calories
• Mass is usually grams, but can be kilograms
• Degrees can be °C or K
The Formula
Δ𝑄 = 𝑚𝐶Δ𝑇
• Δ𝑄: energy absorbed or released
• 𝑚: the mass of the material that the energy entered/left
• Δ𝑇 = 𝑇𝑓 − 𝑇𝑖 : the change in the material’s temperature
• You need to be able to find Δ𝑄, 𝑚, 𝐶, or Δ𝑇
• You need to be able to algebra!
• Algebra first, then put in numbers!
Δ𝑄
𝑚=
𝐶Δ𝑇
Δ𝑄
Δ𝑇 =
𝑚𝐶
Δ𝑄
𝐶=
𝑚Δ𝑇
Energy Transfer: Conduction
• Energy moves by the flow of kinetic energy
• Particles do not flow
• This is how energy moves in solids
Energy Transfer: Convection
• Energy moves by the movement of hot material
• The particles do flow
• This is how energy moves in fluids
• Liquids
• Gases
Energy Transfer: Radiation
• Energy is carried by electromagnetic waves (light)
• This is how energy moves from the Sun to the Earth
• This is how energy moves from a fire to your hands
Homework Problems
• Whether one object is warmer than another object has most to do
with molecular
A. kinetic energy.
B. potential energy.
C. masses.
• Room temperature on the Kelvin scale is about
A.
B.
C.
D.
E.
100 K.
200 K.
300 K.
400 K.
more than 400 K.
• Absolute zero corresponds to a temperature of
A.
B.
C.
D.
0 K.
-273° C.
both of these
none of the above
• At absolute zero, a substance has
A.
B.
C.
D.
absolutely no molecular motion.
no more energy to give up.
no volume.
all of the above
• Thermal energy is measured in units of
A.
B.
C.
D.
degrees.
joules.
calories.
both joules and calories.
• When you touch a piece of ice with your finger, energy flows
A.
B.
C.
D.
from your finger to the ice.
from the ice to your finger.
both of these
none of the above
• The number of joules in 1 kilocalorie is
A.
B.
C.
D.
4.2
42
4200
1054
• Increasing the temperature of 1 gram of ice-cold water to the
temperature of boiling water requires
A.
B.
C.
D.
80 calories.
100 calories.
540 calories.
none of the above
• To say that water has a high specific heat capacity is to say that
water
A.
B.
C.
D.
requires a lot of energy for an increase in temperature.
releases a lot of energy in cooling.
absorbs a lot of energy for an increase in temperature.
all of the above
• Give the conversion formulas for all three temperature scales.
• °F = °C x 1.8 + 32
• °C = (°F – 32) / 1.8
• K = °C + 273
• Define conduction. In what kind of substances is conduction the
most important thermal transfer mechanism?
• The flow of thermal energy without the flow of particles.
• Solids
• Define convection. In what kind of substances is convection the most
important thermal transfer mechanism?
• The movement of thermal energy by the flow of particles
• Liquids and gases (fluids)
• Define radiation. When does an object emit thermal radiation?
• Thermal energy that flows as electromagnetic waves (light)
• All objects emit thermal radiation. The hotter they are, the more they emit.
• How much energy is needed to increase the temperature
of 100 g of water from 22 °C to 72 °C?
• 𝑄 = 𝑚𝐶Δ𝑇
• 𝑄 = 100 g
1
cal
g °C
50 °𝐶 = 5,000 cal
• Convert to Joules
• 5,000 cal x 4.2 J/cal = 21,000 J
• A 10 kg ball of iron is dropped onto hard pavement from
a height of 100 m and comes to rest. Suppose that half
of the heat generated goes into warming the ball. How
many degrees Celsius does the temperature of the ball
rise? (The specific heat capacity of iron is 0.45 J/g/K)
1
2
• 𝑚𝑔ℎ = 𝑄 = 5,000 J
• Δ𝑇 =
𝑄
𝑚𝐶
=
5000 J
10,000 g 0.45 J/g/K
= 1.1 K
Quiz
• Name the three temperature scales (2 pts)
• Fahrenheit, Celsius, Kelvin
• Convert 65 °F to Celsius and Kelvin (3 pts)
• (65 – 32)/1.8 = 18 °C, 18 + 273 = 291 K
• Extra credit: How many significant figures does 13070
have? (1 pt)
• Four
Thermal Conductors and Insulators
• If a tile floor and a wood floor are at the same
temperature, why does the tile feel cold under your feet
while the wood feels warm?
(Thermal) Conductor
• a material in which energy can be easily transferred as
heat
• gases are very poor conductors because particles are far
apart
• In liquids the particles are closer, so they conduct better
than gases, but they are still not great conductors
• Solids conduct best. The atoms are tightly packed
allowing easy transfer of energy
• In metals, the electrons can also help
Some Thermal Conductivities
Material
Thermal Cond. (W/m/K)
Copper
400
Aluminum
200
Brass
100
Steel
50
Ceramic floor tiles
1.5
Concrete
1
Window glass
1
Typical plastic
0.25
Most wood
0.15
Fiberglass insulation
0.04
Styrofoam
0.03
Air
0.025
Used as thermal insulation
Quiz
• Name the three methods of heat transfer. Give a
brief (1 sentence) description of each. (5 pts)
• Conduction: heat transfer through spreading kinetic
energy. Most important mechanism in solids, where the KE
is all vibrations.
• Convection: physical motion of a fluid, carrying heat with
it.
• Radiation: heat transfer by electromagnetic waves, either
emission or absorption
• Extra credit: Write .00000372 in scientific notation.
(1 pt)
• 3.72 X 10-6
Specific heat equation
𝑄
• 𝐶𝑝 =
𝑚 Δ𝑇
• 𝐶𝑝 is specific heat
• 𝑄 is energy lost or gained
• 𝑚 is mass
• Δ𝑇 is change of temperature
• Joules per gram per Celsius: J/gK or J/g°C
• Calories per gram per Celsius: cal/g°C
Specific heat equation using the
definition of the calorie
• 1 calorie = heats 1 gram of water 1 degree
𝑄
𝑚
𝐶𝑝
Δ𝑇
Quiz
• How much energy is required to raise the temperature of
25 g of water from 22 °C to 47 °C? (2 pts)
• 𝑄 = (25 g)(1 cal/(g °C))(25 °C) = 652 cal
• It takes 180 J of energy to warm 15 g of chalk from 299 K
to 311 K. What is the heat capacity of chalk? (2 pts)
• 𝐶𝑝 = (180 J)/[(15 g) x (12 K)] = 1 J/(g K)
• Convert 7.00 calories to joules. (1 pt)
• (7.00 cal) x (4.18 J/cal) = 29.3 J
• Extra credit: express 3.14159 so it has two significant
figures. 3.1
Law of Thermodynamics
• 1st Law: Energy Conservation
• 2nd Law: Energy flows from hot to cold, not the other way
around.
• 3rd Law: No system can reach absolute zero.
• This law is the easiest to understand. You just have to accept
that getting all the way to zero temperature can’t be done.
1st Law of Thermodynamics
• The total energy used in any process is conserved
• The energy can be transferred as heat
• Or the energy be transferred by work
• Or both
• An engine that does work (like in a car)
• Heat in = Work out + Heat out
• An engine that cools (like in a refrigerator)
• Work in (from a motor) + Heat in (from food) = Heat out
The First Law
• Heat in = Work out +
heat out
• All the energy is
accounted for
• Hot reservoir at TH
• Source of energy
• Cold reservoir at TL
• Place to dump waste heat
• Heat engine
• Uses thermal energy to
do useful work
The First Law
• Heat removed + Work
input = Heat dumped
• All the energy is
accounted for
• Cold reservoir at TL
• What we are cooling
• Work input
• Energy to drive the
process
• Hot reservoir
• Where the removed heat goes
(“outside”)
The Laws of Thermodynamics
When work is done on a system, compressing air in a
tire pump for example, the temperature of the system
A. increases.
B. decreases.
C. remains unchanged.
D. is no longer evident.
The Laws of Thermodynamics
When work is done on a system, compressing air in a
tire pump for example, the temperature of the system
A. increases.
B. decreases.
C. remains unchanged.
D. is no longer evident.
Explanation:
In accord with the first law of thermodynamics, work
input increases the energy of the system.
2nd Law of Thermodynamics
• By itself, energy transferred as heat always moves from
an object at a higher temperature to a lower
temperature
• No added work, like in a refrigerator
The Laws of Thermodynamics
When a hot cup is filled with cold water, the direction of heat flow is
A. from the cup to the water.
B. from the water to the cup.
C. random, in no particular direction.
D. nonexistent.
The Laws of Thermodynamics
When a hot cup is filled with cold water, the direction of heat flow is
A. from the cup to the water.
B. from the water to the cup.
C. random, in no particular direction.
D. nonexistent.
Explanation:
The second law of thermodynamics tells us that the direction of
unassisted heat flow is from hot to cold.
(If assisted with energy input, as with an air conditioner for example,
then heat can flow from cold to hot.)
2nd Law of Thermodynamics
• By itself, energy transferred as heat always moves from
an object at a higher temperature to a lower
temperature
• No added work, like in a refrigerator
A more precise statement:
• Entropy always increases
Entropy
• Measure of disorder or
randomness in a system
• A system will move from
high energy(order) to low
energy (disorder)
• House of cards falling is an
increase in entropy
Entropy
• Entropy is about the
number of ways to
arrange the particles of a
system
• There are only a few
ways to arrange the
cards to make this
“house”
• We could swap the 6 of
hearts with the king of
spades
• Still the same house
Entropy
• But knock the house
down…
• There are lots and lots of
ways to arrange a
random pile of cards
• But all of those random
piles are
equivalent…they are just
random piles
More arrangements that are the same = more entropy
Entropy
• One molecule of gasoline becomes 8 molecules of
CO2 plus 9 molecules of H2O
• From 1 house of cards to a random pile!
Entropy
• Second law of thermodynamics—restatement:
• Natural systems tend to disperse from concentrated and
organized-energy states toward diffuse and disorganized
states.
• Energy tends to degrade and disperse with time.
• The total amount of entropy in any system tends to
increase with time.
Entropy and the 2nd Law
Required!!
Required!!
Entropy
Your garage gets messier each week. In this case, the
entropy of your garage is
A. increasing.
B. decreasing.
C. hanging steady.
D. nonexistent.
© 2017 Pearson Education, Inc.
Entropy
Your garage gets messier each week. In this case, the
entropy of your garage is
A. increasing.
B. decreasing.
C. hanging steady.
D. nonexistent.
Explanation:
If your garage became more organized each week, then
entropy would decrease in proportion to the effort
expended.
What Is Heat?
• Heat
• defined as a flow of thermal energy due to a temperature
difference.
• natural direction of heat flow is from a higher-temperature
substance to a lower-temperature substance.
• Hot coffee cools as heat
flows out into the room
• Cold water warms as heat
flows in from the room in
Energy and Heat Summarized
• Temperature is kinetic energy of particles in a substance
• Energy Content is potential energy stored in the particles of a
substance
• Heat is energy in transit.
• Heat and Energy Content are measured in joules, calories, or
Calories.
• All three are forms of Energy
• 1 food Calorie equals 1000 calories. To the
weight watcher, the peanut contains 10 Calories.
• To the scientist, the peanut releases 10,000
calories (41,800 joules) of energy when burned
or digested.
Quantity of Heat
The quantity of heat needed to raise the temperature of a
certain substance a specific amount is 1 Calorie. This is the
same amount of energy as
A. 1000 calories.
B. 4180 joules.
C. Both of these.
D. Neither of these.
© 2017 Pearson Education, Inc.
Quantity of Heat
The quantity of heat needed to raise the temperature of a
certain substance a specific amount is 1 Calorie. This is the
same amount of energy as
A. 1000 calories.
B. 4180 joules.
C. Both of these.
D. Neither of these.
© 2017 Pearson Education, Inc.
Calories Reminder
• calorie (small c): amount of energy needed to
raise 1 gram of water 1 degree Celsius
• Calorie (big C): amount of heat needed to raise 1
kg of water
1 degree Celsius. The Calories
used on food labels are big-C Calories (kilocalories
• one gram of glucose contains 3811 calories of
energy (sometimes called heat)
• Cells do not burn glucose in a flame. They convert the
stored energy to stored energy in another molecule
Low Temperature Experiments
Quiz
• Define temperature (2 pts)
• The average kinetic energy of a particle in a substance (all the
particles have this KE)
• What is thermal energy? (2 pts)
• The sum of the kinetic energy and stored (potential) energy in
a substance
• What is a calorie? (1 pt)
• The energy required to warm 1 g of H2O by
1 °C
• Extra credit: write 4.3178 so that it has four significant
figures (1 pt) 4.318
Temperature Scales Reminder
• Celsius
• Named after Anders Celsius (1701–1744)
• Water freezes: 0 ºC for
• Water boils: 100 ºC
• Fahrenheit
• Named after G. D. Fahrenheit (1686–1736)
• Water freezes: 32 ºF
• Water boils: 212 ºF
• Kelvin
•
•
•
•
Absolute temperature
Named after Lord Kelvin (1824–1907)
Water freezes: 273 K
Water boils: 373 K
• Say “kelvins,” not “degrees”
• No negative Kelvin temperatures!!!
Heat Transfer: Conduction
If you hold one end of a metal bar against a piece of ice, the end in
your hand will soon become cold. Does cold flow from the ice to
your hand?
A. Yes.
B. In some cases, yes.
C. No.
D. In some cases, no.
Heat Transfer: Conduction
CHECK YOUR ANSWER
If you hold one end of a metal bar against a piece of ice, the end
in your hand will soon become cold. Does cold flow from the ice
to your hand?
A. Yes.
B. In some cases, yes.
C. No.
D. In some cases, no.
Explanation:
Cold does not flow from the ice to your hand. Heat flows from
your hand to the ice. The metal is cold to your touch, because
you are transferring heat to the metal.
© 2017 Pearson Education, Inc.
Quiz
1. Temperature is a measure of the average ___________
per ____________ in a substance (3 pts).
2. A material is at a temperature of 50 K. If the same
material were to have twice as much thermal energy,
its temperature would be __________. (2 pts)
Quiz
1. How many calories must be added to 3 g of water to
raise the water’s temperature by 3 °C? (3 pt)
2. How many Joules in 1 calorie? (2 pt)
Energy Content
• Potential Energy stored in food or fuel
• measured by energy released when stuff is metabolized (food)
or burned (fuel)
• The Kilocalorie
• Used for energy content in food
• One kilocalorie or Calorie (with a capital C) is the heat needed to
change the temperature of 1 kilogram of water by 1 degree Celsius.
• There are 1,000 calories in a Calorie
• Calorie can also be called kcal
Energy Content and Food
• Food supplies the human body with the energy it needs
• An average person expends roughly 2400 Calories a day
• Note: capital C Calories!
• Most of this energy is released as heat!
2400 Cal × 4180 J
= 116 Watts
24 h × 3600 s h