I. Energy
A.
The ability of an object to produce a
change.
B. Energy is measured in Joules.
C. In this unit we cover 3 types of energy:
1. Potential (stored) energy
2. Kinetic (motion) energy
3. Thermal (heat) energy
Types of KE and PE
Potential Energy
Gravitational
Elastic
Chemical
Nuclear
Magnetic
Electrostatic
Kinetic Energy
Mechanical
Thermal
Electrical
Sound
Radiant
http://www.nsf.gov/news/special_rep
orts/olympics/snowboarding.jsp
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II. Work
A. Work is the transfer of energy by
mechanical means.
1. This is why work and energy are both measured
in Joules
1 J = 1 kgm2 / s2
B. Other units for energy
1. Calorie (1 cal = 4.184 J)
• A food calorie is actually 1000 calories
2. British Thermal Unit (1 BTU = 1055 J)
III. Kinetic Energy
A. Energy of motion.
B. Any mass that is moving has kinetic
energy.
C. KE depends on the amount of mass and the
velocity at which it is moving. (Both have a
direct relationship with KE.)
1
2
KE mv
2
IV. Potential Energy
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A. PE is stored energy.
B. There are many types of PE:
1. Gravitational Potential Energy.
ex. Anytime an object has height.
2. Elastic/Spring Potential Energy.
ex. Rubber band, bouncy ball
3. Chemical Potential Energy.
ex. Bonds in food and gasoline
V. Gravitational Potential Energy
A. Stored energy because an object has
height.
1. You must set a zero height that we call
our reference level.
a. Most of the time the surface of the Earth is
our reference level.
PE mgh
What is gravity?
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VI. Other types of Potential Energy
A. Elastic Potential Energy - Energy stored in the
bending or stretching of an object
1. Ex: rubber band, Trampoline
B. Chemical Potential Energy - Energy stored in
chemical bonds
1. Ex: gasoline, food
C. Electric Potential Energy - Energy stored because of
an electric charge
1. Ex: static electricity, magnets
http://www.physicsclassroom.com/
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Use this principle to determine the blanks in the
following diagram. Knowing that the potential energy at
the top of the tall platform is 50 J, what is the potential
energy at the other positions shown on the stair steps
and the incline?
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VII. Example Problems
A. A shot-putter heaves a 7.26-kg shot with a final
velocity of 7.50 m/s. What is the kinetic energy of
the shot? (204 J)
B. A 2.00-kg text book is lifted from the floor to a
shelf 2.10 m above the floor.
1. What is the gravitational potential energy relative to
the floor? (41.2 J)
2. What is the gravitational potential energy relative to
the head of a 1.65 m tall person? (8.8 J) (hint, the
person’s head is now the reference point).
VIII. Work-Energy Theorem
A. The net work done an object is equal to the
change in energy of that object.
B.
W


E
If the work done is positive, the object gains
energy.
If the work done is negative, the object loses
energy.
p. 259: Work is the transfer of energy by
mechanical means.
http://www.physicsclassroom.com/Class/energy/U5l2a.cfm
IX. Example Problems
A 150-kg roller-coaster car climbs to the top
of a 91-m high hill. How much work is done
to lift the car to the top of the hill? (133770 J)
You do 13500 J of work in accelerating a 45kg wagon from rest. What velocity does the
wagon reach? (24.5 m/s)
X. Rest Potential Energy
A. Einstein discovered that mass, by its very nature,
is energy. This energy is called rest energy.
B. To calculate the rest energy of an object E=mc2
A. E == energy
B. M == mass
C. C == speed of light
A. C = 3.0 x 108 m/s
C. Rest energy usually deals with masses on an
atomic level.
D. http://www.pbs.org/wgbh/nova/einstein/experts.html
XI. Conservation of Energy
A. Within a closed, isolated system, energy
can change form, but the total amount of
energy in the system does not change.
B. Example:
A ball thrown into the air has energy that
changes from kinetic to potential at the top and
then back to kinetic at the bottom.
http://glencoe.mcgrawhill.com/sites/0078458137/student_view0/chapter11/rollercoaster_applet
.html
Conservation of Energy
Ex.
– The initial height of a roller coaster must be more
than the loops so that there is enough energy
converted into GPE to reach the height of the
loop and enough KE to continue through the
loop. This is assuming that only GPE is the
source of the initial energy.
– Pendulums – GPE at the top of a pendulum or
swing equals the KE at the lowest point
(assuming no outside forces). At the top there is
only GPE at the lowest point there is only KE.
http://www.physicsclassroom.com/mmedia/energy/cs.cfm
Equation
–Falling down
•GPEtop = KEbottom
–Throwing up
•KEbottom = GPEtop
Drawing
–http://www.jason.org/digital_library/4851.aspx
XII. Example Problem
A. A 35-kg child is riding on a swing that rises to a
maximum height of 0.8 m. Neglecting friction,
1. What is the child's gravitational potential energy at the
top of the swing? (274.4 J)
2. What is the child’s velocity at the bottom of the swing?
(4.0 m/s)
B. A pendulum bob with a mass of 0.5 kg swings to a
maximum height of 1.0 m. What is the velocity
when the pendulum bob is at a height of 0.40 m?
(3.42 m/s)
XIII. Elastic and Inelastic Collisions
A. Elastic – kinetic energy is conserved.
1. The objects bounce off each other.
a. Example: Billiard balls
B. Inelastic – some kinetic energy is changed
into other forms of energy (heat, sound,
etc.) Almost all collisions are inelastic
1. (Remember, momentum was conserved in both
elastic and inelastic collisions, but KE is only
conserved in elastic collisions.)
C.
Superelastic- Kinetic energy is gained
1. Ex. A compressed spring is released during a
collision
XIV. Kinetic-Molecular Theory
A. Matter is made up of many tiny
particles that are in constant motion.
1. The particles are held together by
electromagnetic forces (strongest in
solids, weakest in gases) (states of matter)
2. In a hot object, the particles move
faster, therefore they have more
kinetic energy. (expand with more
thermal energy - thermal expansion)
XV. Thermal Energy vs. Temperature
A. Thermal Energy is the sum of the kinetic
and potential energies of the internal
particles in an object.
1. Thermal energy is measured in joules.
B. Temperature measures the hotness of an
object on a definite scale.
1. Temperature shows a direct relationship with
the average kinetic energy of the particles in an
object.
XVI. Temperature Scales
Kelvin (units are K)
K  C  273.15
– Based on absolute zero (the temperature were
the particles in an object stop moving)
 9

F


C

32


Celsius (units are °C)
 5
• Water – 273 k, 373 k
– Based on properties of water – 0, 100
Fahrenheit (units are °F)
– Based on coldest manmade temp of time as 0
and body temp as 100 – water 32, 212
XVII. Heat
A. Thermal energy will travel from hot to cold
until the two objects are the same
temperature. This is called thermal
equilibrium.
B. The energy that flows as a result of a
difference in temperature is called heat
(Q).
1. Heat is measured in Joules.
XVIII. Thermal Equilibrium
A. Thermal Equilibrium
is the state in which
the rate of energy
flow between two
objects is equal and
the objects are at the
same temperature.
XIX. Transfer Process
A. Conduction – the transfer of thermal energy when
particles collide
1. Ex: a metal spoon gets hot when sitting in boiling water.
B. Convection – the rising of hot air/liquids pushing
cooler air/liquids down.
1. Ex: thunderclouds
C. Radiation – the transfer of energy by
electromagnetic waves
1. Ex: sunlight heating the Earth
XX. Specific Heat
A. The specific heat (C) of a material is the
amount of energy that must be added to
raise the temperature of a 1 kg mass 1°C.
Q  mC T
B. Temperature in the above equation can be
measured in °C or K.
XXI. Example problem
A. A 0.400-kg block of iron is heated from
295K to 325 K. How much heat is absorbed
by the iron if it has a specific heat of 450
J/kgK?
(5400 J)
XXII. Latent Heat
A. Heat of Fusion (Hf) is the amount of heat
needed to melt 1 kg of a substance.
Q  mHf
B. Heat of Vaporization (Hv) is the amount of
heat needed to vaporize one kilogram of a
liquid.
Q  mH v
C. Latent heats can be negative if the object is
moving in the other direction on the heating
curve.
http://www.uwsp.edu/geo/faculty/ritter/glossary/l_n/latent_heat.html
latent heat
Latent heat is the heat energy involved in the phase change of
water. Latent heat is gained by water molecules when water
evaporates. The heat added during evaporation is used to break
hydrogen bonds between water molecules and does not raise
the temperature of the water body. The heat then is "hidden" or
stored in the water molecule until it is released during
condensation. At that point, the heat is converted into sensible
heat. Latent heat released during condensation is an important
source of energy to drive atmospheric systems like hurricanes
and cumulus clouds.
Change of State Graphs
Endothermic changes –
Exothermic changes –
XXIII. Example Problem
A. How much heat must be transferred to
100.0 g of ice at 0.0 °C until the ice melts
and the resulting temperature of the
water rises to 20.0 °C?
(41760 J)
XXIV. Heat Engine
A. A heat engine is a machine that is able to
convert thermal energy to mechanical
energy.
XXV. Thermodynamics
A. 1st Law
1. The total increase in the thermal energy of a
system is the sum of the work done on it and
the heat added to it.
2. Only restates the law of conservation of energy.
B. 2nd Law
1. Natural processes go in a direction that
increase the total entropy of the universe.
2. Entropy is the measure of disorder in an object