Acknowledgement

1
Acknowledgement
While some of these slides are mine, many have
been taken from the Internet. My contribution
will be mainly in providing the discussion that
will follow most of the slides.
2
Conservation of Energy? What is that?
What is the most important concept in physics?
Fma
--- Newtons second law of motion
--- Special Relativity of Einstein
Emc2
Conservation of Energy
3
Energy
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4
Conservation of mechanical energy
Robert H. March, Physics for Poets
equals
the kinetic energy ½ mv2 at points A, , D and E.
PE mgh
M100.0 kg, h20.0 m, and g10.0 m/s/s.
PE mgh 20,000 J.
5
Conservation of mechanical energy
Robert H. March, Physics for Poets
Where did this PE mgh come from?
6
Conservation of mechanical energy
Robert H. March, Physics for Poets
We pushed and moved the carts from the bottom to
the top. We did work!
7
Mechanical energies

• Energy stored by a change in configuration -
  Potential energy mgh

• Where do we measure this height from?

• From anywhere we want!

• Energy of Motion KE ½ mv2

• Energy of Motion is independent of which way
  you are going!

8
Kinetic Energy Energy of Motion

• Kinetic energy (KE) refers to the energy
  associated with the motion of an object. The
  kinetic energy is simply
• KE (½)mv2 where
• m mass in kg, and
• v velocity of object in m/sec
• What are the units of KE?
• KE mass velocity2 kgm2/s2
  Joule or just, J
• A Joule is a substantial amount of energy!

9
How is energy stored in a configuration?

• Work displacement ? force in direction of
  the displacement.
• Doing work pumping in mech. Energy
• Sometimes it changes PE only, while at some other
  times it is changing the KE.
• Mechanical energy is conserved!

10
Conservation of energy?
Robert H. March, Physics for Poets
11
Conservation of energy
Robert H. March, Physics for Poets
Energy is again conserved if you consider heat as
another form of energy.
12
Energy

• The unit, Joules applies to all forms of
  energy, not just KE.
• As well see later, there are sometimes more
  convenient units to use for energy.
• You have probably heard of the unit Watt. For
  example,a 100 Watt light bulb? A Watt W is
  simply energy usage per unit time, or J/s.
• So, 100 W means the bulb uses 100 J per
  second!
• How many J are used by a 100 W bulb in 2
  minute? A) 200 J B) 1200 J
  C) 12000 J D) 2000 J

13
What is a Force ?
Force is simply
A PUSH
A PULL
or
Forces have both magnitude and direction
14
Force and Acceleration

• Experimentally, we find that if we apply a
  forceto an object, it accelerates.
• We also find that the acceleration (a) is
  directlyproportional to the applied force (F)
  and inversely proportional to the mass (m) .
  That is

Isaac Newton
a F / m

• This means
• Increasing the force increases the acceleration
  decreasing the force results in a lower
  acceleration.

15
Newtons Second Law

• Since two or more objects must be involved, a
  force intimately tied to the notion of an
  interaction.
• Interactions are now believed to occur through
  the exchange of force carriers. This is a very
  important point, and well come back to it
  later
• So far, we know only of four types of
  fundamental forces in nature
• Gravity, Electromagnetic, Weak, and Strong
• We will come back to each of these
• All other forces in nature are understood to be
  the residual effects of these fundamental forces

16
Newtons Third Law
For every action, there is an equal and opposite
reaction.
What does it mean?
When you pay 22,000 for a Toyota Camry it is a
Transaction
I.e., it is give and take.
What happens to each others bank account?
Yours goes down and the dealers goes up. For
every action there is an equal and opposite
reaction.
17
Momentum (I)
What is momentum?
Momentum is simply the product of the mass and
the velocity.Denoting momentum as p, it is
simply

• The units of momentum are kgm/sec kg
  m/sec
• Momentum is a very important subject in physics
  because it is what we call a conserved quantity.
  What does this mean?
• We will come back to the idea of conserved
  quantities in physics. They play a very
  important role in understanding the world around
  us!

18
Energy Conservation
Like momentum, energy is a conserved
quantity.This provides powerful constraints on
what can and cannot happenin nature.This is an
extremely important concept, and we will come
back tothis over and over throughout the
remainder of the course.
19
Blah, blah, blah
20
Temperature and Heat
Temperature is a fundamental quantity which
characterizes the physical state of a substance.
In the microscopic statistical theory, we
understand temperature as the average energy per
degree of freedom of motion of the
substance. Heat is an interaction between two
objects, particularly the flow of energy from one
object to another. When two objects are placed
in thermal contact (so that heat is able to flow
from one to the other), heat will flow until the
temperatures of the two objects are the same.
Then the two objects are in thermal equilibrium.
21
Thermometers Thermostats

• Use the expansion of Hg to define a temperature
  scale.
• Use the differential expansion of two dissimilar
  metals to make either a thermometer or a
  thermostat (temperature activated switch)

22
Water is special!
Thermal Expansion.
Water is an exception to the rule. Between 0 and
4 C it contracts. Above 4 C it expands. Water
is most dense at 4 C.
23
Temperature Scales
Celsius water freezes at 0 C and boils at 100
C Fahrenheit water freezes at 32 F and boils
at 212 F Kelvin - water freezes at 273.15 K and
boils at 373.15 K. But how do we determine the
equal divisions between these calibration points?
Absolute Zero the lowest possible temperature
0 K 273.15 C TK TC 273.15
TF 1.8 TC 32
24
Heat
Heat Q is the energy transferred between one
object and another due to temperature
differences. Heat is measured in calories (cal).
1 cal 4.186 J the energy needed to increase
the temperature of one gram of water by one
degree C.
A Calorie (C) is a kilocalorie.
Salad oil 8.6kC / kg ? 8.6 kC /liter36106
J/liter Gasoline has only slightly greater energy
density
25
Specific Heat
If you add heat to a substance its temperature
will increase. But how much? That depends on
the specific heat of the substance.
Q mcDT Q heat added m mass c
specific heat DT change in temperature Water
has a very large heat capacity a lot of energy
transfer (heat) is required to change its
temperature. This has a major impact on the
climate. Water c 1.0 cal /(ºC g) 1.0 Cal
/(ºC kg) It takes one calorie to raise the
temperature of 1 gm of water by 1 degree Celsius.
26
Specific Heat, values
27
Calorimetry
1.0-g lead pellets at 75 C are to be added to
180 g of water at 22 C. How many pellets are
needed to increase the equilibrium temperature to
25 C?
Heat gained by water (180g)?(1.0
cal/g/degreeC)?(25-22)degrees
540 c
Heat lost by pellets m ? (.0305
cal/g/degreeC)?(75-25)degrees
By setting them equal, i.e. m ? 2.2875 c/g 540
c we get m236 grams, 236
pellets
28
Latent Heat heat of phase change

• Ice gt Water requires 80 c/g

• Water gt Steam requires 540 c/g

This is why steam burns are terrible!
29
How much energy in a gallon of boiling water?
To heat up a gallon of water from 30 deg C to
100 deg C, you need (4000g)(70
deg)(1cal/deg/g)(4.2 J/c) 1,000,000 Joules
To accelerate a 1000-kg car from 0 mph to 100
mph (45 m/s), you need ½ (1000)(45) (45) J
1,000,000 J.
30
Heat Transfer

• How does the energy move from a hotter to a
  colder object?
• Three mechanisms
• Conduction
• Convection
• Radiation

31
Thermal Conductivities Table

• Metals have high thermal conductivity, most
  electrical insulators also have low thermal
  conductivity.
• Air is a great insulator, except that large air
  spaces allow heat flow by convection.

32
Thermal Conductivities Table
High conductivity
High conductivity
High conductivity
33
Convection and Radiation

• Convection when heat is carried by a moving
  fluid
• Example heat house with radiator
• Gulf stream transports Heat from Caribbean to
  Europe
• Radiation when electromagnetic waves
  (radiation) carry heat from one object to
  another.
• Example heat you feel when you are near a fire
• Example Heat from the sun
• Formation of frost (ice) at night, T(air) gt 0ºC

34
Conduction
Conduction is the process whereby heat is
transferred directly through a material, any bulk
motion of the material playing no role in the
transfer. Those materials that conduct heat well
are called thermal conductors, while those that
conduct heat poorly are known as thermal
insulators. Most metals are excellent thermal
conductors, while wood, glass, and most plastics
are common thermal insulators. The free electrons
in metals are responsible for the excellent
thermal conductivity of metals.
35
Conduction
36
Conduction Of Heat Through A Material
37
Conduction
Rate of heat transfer by conduction, Q/t through
the length, L across the cross-sectional area, A
is given by the following equation, where k is
the thermal conductivity and ?T is the
temperature difference between the two ends.
SI Unit of Thermal Conductivity J/(s m C)
38
Heat Transfer

• There are three ways in which heat can be
  transferred from one object to another
• Conduction when two objects are in physical
  contact.

k thermal conductivity Q heat transferred A
cross sectional area t duration of
heat transfer L length
DT temperature difference between two ends
In a hot oven the air and the metal rack are at
the same temperature, but which one feels hotter
and why?
39
Convection and Radiation

• Convection when heat is carried by a moving
  fluid
• Example heat house with radiator
• Gulf stream transports Heat from Caribbean to
  Europe
• Radiation when electromagnetic waves
  (radiation) carry heat from one object to
  another.
• Example heat you feel when you are near a fire
• Example Heat from the sun
• Formation of frost (ice) at night, T(air) gt 0ºC

40
Styrofoam
41
Mother shielding her cub?
Is this cuddling just a mothers instinct?
Doesnt she realize that the cub is protected by
the same type of fur as she is?
42
Owens Corning
43
How do fish in a lake survive?
Fruit growers sometimes spray water to protect
their crops against freezing. After a subzero
night, these berries are visible in their
insulating jackets of ice.
44
Convection
Convection is the process in which heat is
carried from place to place by the bulk movement
of a fluid.
Convection currents are set up when a pan of
water is heated.
45
Volcanic Eruption
During a volcanic eruption, smoke at the top of
the plume rises thousands of meters because of
convection.
46
Convection
Explains why breezes come from the ocean in the
day and from the land at night
47
Radiation
Radiation is the process in which energy is
transferred by means of electromagnetic
waves. Heat transfer by radiation can take place
through vacuum. This is because electromagnetic
waves are involved in radiation and they can
propagate through empty space.
48
Radiation
Q In the living room, the heating unit is placed
in the floor but the the refrigerator has a
top-mounted cooling coil. Why?
A Air warmed by the baseboard heating unit is
pushed to the top of the room by the cooler and
denser air. Air cooled by the cooling coil sinks
to the bottom of the refrigerator.
49
Radiation
Radiation is the process in which energy is
transferred by means of electromagnetic
waves. Heat transfer by radiation can take place
through vacuum. This is because electromagnetic
waves are involved in radiation and they can
propagate through empty space.
50
Radiation

• Energy carried by electromagnetic waves
• Light, microwaves, radio waves, x-rays
• Wavelength is related to vibration frequency

51
Radiation
52
Black Body
A material that is a good absorber, like
lampblack, is also a good emitter, and a material
that is a poor absorber, like polished silver, is
also a poor emitter.
53
Suntans
Suntans are produced by radiation, ultraviolet
rays.
54
Summer Clothing
Q People are uncomfortable wearing dark clothes
during the summer. Why?
A Dark clothes absorb a large fraction of the
sun's radiation and then reemit it in all
directions. About one-half of the emitted
radiation is directed inward toward the body and
creates the sensation of warmth. Light-colored
clothes, in contrast, are cooler to wear, since
they absorb and reemit relatively little of the
incident radiation.
55
A White sifaka Lemur
To warm up in the morning, they turn their dark
bellies toward the sun.
56
Why is the mother shielding her cub?
Havent we seen this before?
Ratio of the surface area of a cub to its volume
is much larger than for its mother.
To cool food, we cut it into smaller pieces, why?
57
Thermos Bottle
A thermos bottle minimizes energy transfer due to
convection, conduction, and radiation. Stopper-
minimize conduction. Double-walled glass vessel
with the space between the walls is evacuated to
minimize energy losses due to conduction and
convection. The silvered surfaces reflect most of
the radiant energy that would otherwise enter or
leave the liquid in the thermos.
58
Halogen Cooktop
In a halogen cooktop, quartz-iodine lamps emit a
large amount of electromagnetic energy that is
absorbed directly by a pot or pan.
59
Metal foil
Highly reflective metal foil covering this
satellite minimizes heat transfer by radiation.
60
The StefanBOLTZMANN Law Of Radiation
The rate at which an object emits radiant energy
is proportional to the fourth power of its
absolute temperature. This is known as Stefans
law and is expressed as follows, where s is the
Stefan-Boltzmann constant, s 5.67 ? 10-8
W/m2.K4.
The factor e is called the emissivity, which is a
number between 0 and 1. Perfect radiators have a
value of 1 for e. A is the surface area and T is
the temperature of the radiator in Kelvin.
61
Black Body Radiation

• Any object heated to a temperature T (on an
  absolute scale) radiates Electromagnetic Energy
  (light) with total power
• P e s A T4
• 0ltelt1 emissivity property of material
• s 5.67 10 8 W/(m2 K4)
• A surface area of object
• Early triumph of quantum theory (M. Planck) to
  predict this equation, including the value of s.
• Peak wavelength occurs at l (5.110-3 m K ) /
  T (Chap 30)
• If the surroundings have temperature TS, then the
  net power radiated is
• P e s A T4 - TS4
• Dark, dry, night, TS 3 K, Black body radiation
  cools the surface faster than conduction can
  transport heat from the ground or air. Frost can
  form even if air temperature gt 0C.