Physics 1010:
The Physics of Everyday Life
TODAY
• Heat and Thermodynamics
Thermometers, temperature scales; conduction,
convection, radiation.
1
Today’s topics
• Heat and thermometers
• Burning - conversion of chemical energy
to thermal (chemical bonds)
• Heat exchangers - getting the heat
into the room: open fires, wood stoves,
and furnaces
• Conduction of heat
• Convection of heat
• Radiative heat transport
2
Heat Flow
Three lead bricks are in contact as shown.
Heat flows:
A) From the hot to the cold
B) From the cold to the medium
C) From the hot to the medium
D) From the medium to the cold
E) All but B
1. Hot Brick
2. Med. Brick
3. Cold Brick
3
Heat Flow
Three lead bricks are in contact as shown.
Heat flows:
A) From the hot to the cold
B) From the cold to the medium
C) From the hot to the medium
D) From the medium to the cold
E) All but B
1. Hot Brick
2. Med. Brick
3. Cold Brick
4
Temperature - something has heat.
Caloric. (Why not?)
• “Heat flows” from hot to
cold
• Some early physics
beliefs: conserved
quantity - a fluid called
caloric
Hot Brick
Cold Brick
5
Heat is Energy (same a mechanical)
• Count Rumford (Benjamin
Thompson) demonstrated the
heating of water by boring cannon
for the elector of Munich
• Joule (1818 - 1889) measure increase in
temperature due to friction
• Able to equate loss of mechanical energy by
friction to heat
• Famous experiment with weights moving fins in
water (he measured the change in temperature of
the water)
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Heat flows between 3 bodies of different
temperatures, as shown, in contact. When
does heat stop flowing?
a) When brick 1 has same
temp as brick 3
b) When brick 1 has same
temp as brick 2
c) When brick 2 has same
temp as brick 3
1. Hot Brick
2. Med. Brick
3. Cold Brick
d) When all bricks have
the same temperature
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Heat flows between 3 bodies of different
temperatures, as shown, in contact. When
does heat stop flowing?
a) When brick 1 has same
temp as brick 3
b) When brick 1 has same
temp as brick 2
c) When brick 2 has same
temp as brick 3
1. Hot Brick
2. Med. Brick
3. Cold Brick
d) When all bricks have the
same temperature
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Fundamental Principle of Thermodynamics
(used for measuring temperature)
Bodies in thermodynamic contact will
eventually all have the same temperature
Hot Brick
Med. Brick Cold Brick
Med. Brick
Med. Brick Med. Brick
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Origin of the Fahrenheit scale
• “He <Fahrenheit> proposed (rather arbitrarily) a
zero for the freezing point of a brine solution, a
value of 32 for the melting point of ice, and the body
temperature at 100 units (this should be about 96,
100 is feverish)”
• Actually, water freezes at a lower temperature with
salt. Fahrenheit chose the salt solution to make
water freeze at the coldest possible temperature.
This was the coldest he could get something stable.
• Fahrenheit Scale:
Water freezes at 32oF, boils at 212oF
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Celsius temperature scale (1742)
Scale based on freezing and boiling points of water.
Boiling point depends on pressure
Must set the pressure for boiling
1. Put the cylinder AB of the thermometer (i.e. the bulb) in
thawing snow and mark the freezing point of water C, which
should be at such a height over the cylinder at A that the
distance AC is half the distance between C and the water
boiling point mark D
2. Mark the boiling point of water D at a pressure of "25 tum 3
linier" (approximately 755 mm)
3. Divide the distance in 100 equal parts or degrees; so that 0
degree corresponds to the boiling point of water D, and 100
to the freezing point of water C. When the same degrees
have been continued below C all the way down to A the
thermometer is ready.
4. Celsius Scale:
Water freezes at 0oC, boils at 100oC
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Kelvin (absolute) scale
• Increase in temperature gives
increase in pressure, so decrease
enougn and no pressure!
• Increase in density (compression)
gives increase in pressure
• Can have ideal gas law: P = nkT if
the temperature is measured
relative to something very cold:
Tkelvin = Tcelsius + 273
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Temperature - the various scales and
conversions
Tk = Tc + 273
Tf = (9/5)Tc + 32
NOTATION
0 °C = 32 °F = 273 °K
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Production of heat from chemical energy burning, activation energy, exothermic
Carbon Oxygen
reactions
O
C
Mass falling to earth, lower potential energy
Converts to kinetic energy
Bounces until friction changes energy to heat
Carbon Monoxide O
• Carbon and oxygen like to join
 Carbon Monoxide or CO
 Carbon Dioxide or CO2.
Carbon Dioxide
O
C
C
O
• Microscopic view: atoms falling
together, means they gave up
potential energy
• Kinetic energy (HEAT) acquired
by these and nearby molecules
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In most burning, weaker bonds (less
potential energy) broken
• Methane and oxygen molecules are more weakly bound
• Come out of a shallow valley, go into a deeper one
Carbon Oxygen Hydrogen
O
C
H
H
H
C
H
+5
O
O
H
O
C
+4
O
H
O
O
Activation energy: energy needed to
get first reaction going
The released energy of first one (or
more) reactions gets others going
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Lennard-Jones Potential
Same shape for all molecules
Distance, change in potential energy
different for different substances
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Lennard-Jones Potential also explains
Hook’s Law
Force on atom is the negative of the slope of graph:
For small displacements from minimum (slope=force=0) force is the same
for compression and extension.
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Heating, Cooling
Wood contains resins - SMOKE
Burning creates carcinogens
Need to move heat without the smoke
-> Heat Exchangers
• To heat (or cool) our homes, we move heat in
three ways:
• Convection
• Conduction
• Radiation
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The furnace and heat exchangers
• Combustion air brought
into heat room (not
through house)
To outside
To house
• Hot gas burns, heating
metal tubes
• Fans push house air
through tubes and to
rest of house
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Transport of heat
• Conduction: The heat is carried by
molecules that move, and then move those
next to them, but there is no net
movement of molecules
• Convection: Hot molecules are carried to a
cold region
• Radiation: electromagnetic radiation
carries the heat
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Conduction
Heat one end, the other gets hot.
Diffusion: nothing moves, but heat gets
there (“like” sound, but not a wave)
• Hot pad
• Ice pack
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Convection: bulk movement of hot fluid
Hot air
• Hot air rises
• Pushes cold air
down
Cold air
• “Convective cell”
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Radiative transport:
• All bodies radiate electromagnetic energy over a
range of frequencies. Light - for very hot
objects (10,000 °K)
• More radiation for larger temperature
• Room temperature (300 °K), light at wavelengths
longer by 30x are emitted. “Infra-red light”
• Works even in vaccum!
HOT
COLD
Radiation
Less
More
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Summary
• Heat “flows” from hot to cold
• Bodies in thermodynamic contact will all reach
the same temperature
• HEAT IS ENERGY (same as mechanical energy)
• Burning releases chemical (electrostatic)
potential energy
• We use convection, conduction, and radiation to
heat (or cool) a house
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Exam Tonight
• 7:30 pm, in this room.
• Closed Book.
• One 3x5 note card with own notes allowed.
• Calculators allowed.
• Cumulative, but emphasis on new material.
• Multiple choice plus one essay.
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Practice Questions; Work
You are using a frictionless ramp to move a 200kg
filing cabinet onto a truck. The bed of the truck is 2m
above the ground, and the ramp is 8m long.
How much work will you do moving the cabinet onto
the truck
A) 4000N B) 2000N C) 4000J D) 2000J E) 200N
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Practice Questions; Work
You are using a frictionless ramp to move a 200kg
filing cabinet onto a truck. The bed of the truck is 2m
above the ground, and the ramp is 8m long.
How much work will you do moving the cabinet onto
the truck
A) 4000N B) 2000N C) 4000J D) 2000J E) 200N
Anwer is C: W=mgh=200kg*9.8m/s2*2m=3920J
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Practice Questions; Work
You are using a frictionless ramp to move a 200kg filing
cabinet onto a truck. The bed of the truck is 2m above
the ground, and the ramp is 8m long.
Once you get the cabinet moving at a constant speed,
how much force will you exert to move the cabinet onto
the truck
A) 200N B) 200kg C) 500N D) 25kg E) 25N
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Practice Questions; Work
You are using a frictionless ramp to move a 200kg filing
cabinet onto a truck. The bed of the truck is 2m above
the ground, and the ramp is 8m long.
Once you get the cabinet moving at a constant speed,
how much force will you exert to move the cabinet onto
the truck
A) 200N B) 200kg C) 500N D) 25kg E) 25N
Anwer is C: mgh=FL, L=mg(h/L)=
200kg*9.8m/s2*(2m/8m)=490N
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Practice Questions; Oscillators
A mass of 10kg is hooked to a spring with spring
constant k=100N/m.
What is the period of the oscillator
A) 20s B) 1s C) 5s D) 10s E) 2s
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Practice Questions; Oscillators
A mass of 10kg is hooked to a spring with spring
constant k=100N/m.
What is the period of the oscillator
A) 20s B) 1s C) 5s D) 10s E) 2s
Anwer is E: T=2π sqrt(m/k)=
=2 π sqrt(10kg/100N/m)=1.99 s
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Practice Questions; Oscillators
A mass of 10kg is hooked to a horizontal spring with
spring constant k=100N/m.
If I stretch the spring horizontally by 0.2m and let it go,
how fast will the mass be moving as it crosses the
equilibrium point?
A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s
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Practice Questions; Oscillators
A mass of 10kg is hooked to a horizontal spring with
spring constant k=100N/m.
If I stretch the spring horizontally by 0.2m and let it go,
how fast will the mass be moving as it crosses the
equilibrium point?
A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s
Anwer is A: (1/2)mv2=(1/2)kx2 so v=x*sqrt(k/m)
v= 0.2m sqrt(100N/m / 10kg)=0.63 m/s
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Practice Questions; Newton’s Laws
David (in red, with sling) is trying to hit Goliath (in blue).
The velocity of the stone is shown by the arrows.
He should release the stone at approximately:
B
A
C
D
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Practice Questions; Newton’s Laws
David (in red, with sling) is trying to hit Goliath (in blue).
The velocity of the stone is shown by the arrows.
He should release the stone at approximately:
B
A
C
D
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Practice Questions; Kinematics
v start
The car is subjected to a constant force in the direction away from
the motion detector.
Sketch your predictions for the velocity and acceleration of the cart
moving toward the motion detector, slowing down at a steady rate,
and then reversing direction and speeding up. (Start your graph after
the push that gets the cart moving; + is to the right)
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+
0
time
+
0
#1
time
+
-
#1
time
B
0
time
+
0
time
-
C
-
0
+
Velocity
Velocity
Sketch your
predictions for the
velocity and
acceleration of the
cart moving toward
the motion detector,
slowing down at a
steady rate, and
then reversing
direction and
speeding up.
+ is to the right
Acceleration
-
Velocity
time
Acceleration
0
Acceleration
Velocity
+
A
+
0
D
#1
time
+
0
-
#1
time
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+
0
time
+
0
#1
time
+
-
#1
time
B
0
time
+
0
time
-
C
-
0
+
Velocity
Velocity
Answer is D
Acceleration is
constant, and
positive
Acceleration
-
Velocity
time
Acceleration
0
Acceleration
Velocity
+
A
+
0
D
#1
time
+
0
-
#1
time
38
A skier goes down a frictionless slope,
decending 5m. She hits a level stretch
with friction, and the friction force is
50N. How far will she travel before she
stops. The mass of the skier is 50kg.
• A 4.9m
• B 49m
• C 98m
• D 9.8 m
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A skier goes down a frictionless slope,
decending 5m. She hits a level stretch
with friction, and the friction force is
50N. How far will she travel before she
stops. The mass of the skier is 50kg.
• A 4.9m
• B 49m
• C 98m
• D 9.8 m
Concserv of energy: mgh=FL L=(mgh)/F
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