Energy, Work, Power

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Energy, Work, Power
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ENERGY

• The ability to cause CHANGE
• Anything that causes change must have ENERGY!!
• You use energy to function

• Different Forms of Energy
• Electrical
• Chemical
• Radiant
• Thermal

These are like different forms of money
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KINETIC ENERGY (KE)
-The energy of MOTION -All moving objects have
Kinetic Energy! -Depends on MASS and SPEED of
the object -Equation Kinetic Energy (KE)
Joules 1/2mass(kg) X speed2 KE
1/2mv2 -Joule (J) SI unit for Energy
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Example Problem with Kinetic Energy
-A jogger whose mass is 60kg is moving at a speed
of 3 m/s. What is the joggers Kinetic
Energy? Mass 60kg Velocity 3 m/s KE ??J
KE 1/2mv2 KE 1/2(60kg)(3m/s)2 KE 270 J
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POTENTIAL ENERGY (PE)
-The energy of REST -Objects at REST have
POTENTIAL ENERGY -Potential Energy is CHANGED
into KINETIC ENERGY when MOTION occurs
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TYPES OF POTENTIAL ENERGY

• Elastic Potential Energy Energy stored by
  something that can be STRETCHED or COMPRESSED
• Ex Rubber band
• 2. Chemical Potential Energy Energy stored in
  CHEMICAL BONDS
• Ex Food, Natural gas
• 3. Electrical Potential Energy Energy stored
  due to ELECTRICAL CHARGES
• 4. Nuclear Potential Energy Energy stored in
  the NUCLEI OF ATOMS

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TYPES OF POTENTIAL ENERGY
5. Gravitational Potential Energy Energy stored
by objects due to their POSTION ABOVE EARTH -Ex
Anything with the potential to FALL -Depends on
MASS and HEIGHT above ground -Equation GPE (J)
Mass (kg) X gravity (m/s2) X height (m) GPE
mgh
Remember Gravity on Earth 9.8 m/s2
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GPE Example Problem
-What is the GPE of a ceiling fan that has a mass
of 7kg and is 4m above the ground?
GPE mgh Gravity 9.8 m/s2 Mass 7 kg Height
4 m
GPE (7kg)(9.8m/s2)(4m) 274 kgm2/s2 274 J
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To INCREASE GPE
1 INCREASE objects HEIGHT 2. INCREASE objects
MASS
2KG
2KG
7KG
2KG
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The Change of GPE to KE
-As objects fall, GPE is changed into KE -KE is
LARGEST right before the object hits the ground,
thus GPE is the SMALLEST right before hitting the
ground -Objects with MORE GPE move FASTER because
they have more KE
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Converting between KE and PE
-MECHANICAL ENERGY Total amount of POTENTIAL
and KINETIC energy in a system Mechanical energy
PE KE
What happens to the mechanical energy as PE and
KE are converted into each other?
ME stays the same!! As PE and KE are converted,
the FORM of energy changes, but the TOTAL AMOUNT
STAYS THE SAME
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The Law of Conservation of Energy
-States that ENERGY CANT BE CREATED OR
DESTROYED!! -So does this mean the total amount
of energy in the Universe is the same at all
times???
Friction and Air Resistance
These forces can cause some mechanical energy to
change into THERMAL ENERGY!!
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TEMPERATURE
-TEMPERATURE Measure of the AVERAGE KINETIC
ENERGY of the particles in an object -As temp.
INCREASES Average SPEED of particles INCREASE
KE INCREASES -SI UNIT Kelvin (K) or Celsius (0C)
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THERMAL ENERGY
Sum of the KINETIC and POTENTIAL energy of all
the particles in an object
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THERMAL ENERGY TEMPERATURE
-As KE INCREASES TEMPERATURE INCREASES THUS -As
TEMP INCREASES THERMAL ENERGY INCREASES
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THERMAL ENERGY MASS
-As MASS INCREASES total KE INCREASES THUS -AS
MASS INCREASES THERMAL ENERGY INCREASES
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HEAT
-Thermal energy that FLOWS from something at a
HIGHER TEMP to something at a LOWER TEMP -Form of
ENERGY -Measured in JOULES (J) Always flows
from warmer to cooler
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SPECIFIC HEAT
-Amount of HEAT needed to RAISE THE TEMP of 1kg
of a material by 10C -Measured in J/(kg 0C) -As
a substance absorbs HEAT, its TEMP change depends
on the composition of the substance. This is
called the Specific Heat of the substance!
Ocean water v/s Beach Sand (see next slide)
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SPECIFIC HEAT EXAMPLE PROBLEM
1 kg of sand takes 6x less heat to raise it 10C
than 1 kg of water Which has the higher specific
heat??
WATER
Which can absorb more heat with changing its temp?
WATER
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TRANSFERRING THERMAL ENERGY
-3 Ways to transfer thermal energy from place to
place 1. CONDUCTION 2. CONVECTION 3. RADIATION
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CONDUCTION
-The transfer of energy by COLLISIONS or
TOUCH -This happens because particles are in
CONSTANT MOTION -Thermal Energy is transferred by
collisions between molecules with more KE to
molecules with less KE HEAT is transferred by
COLLISIONS, not by movement of matter
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HEAT CONDUCTORS
1. Heat moves faster by conduction in SOLIDS and
LIQUIDS than in gases WHY?? 2. The BEST
conductors METALS WHY??
Gases are more spread out/collisions are less
frequent
All the e arent bonded to atoms, so they move
more freely
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CONVECTION
-The transfer of thermal energy in a FLUID (gas
or liquid) by movement of WARMER and COOLER fluid
from place to place -Uses CONVECTION CURRENTS to
transfer the energy from WARM TO COOL -Still uses
collisions to transfer as well
-As a fluid increases its temp it EXPANDS and
its DENSITY DECREASES WHY??
-Particles have more KE move faster spread out
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RADIATION
-The transfer of energy by ELECTROMAGNETIC
WAVES -Waves can travel through space even
though there is no matter THUS Radiation is the
only form of energy transfer in space -Energy
RADIENT ENERGY
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RADIATION
-Radiation can be ABSORBED, REFLECTED, or
TRANSMITTED thru an object -This depends on
MATERIAL OF OBJECT -LIGH-COLORED Reflect
more -DARK-COLORED Absorb more

• -Radiation can be TRANSMITTED thru
• SOLIDS
• LIQUIDS
• GASES

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INSULATORS
-A material in which HEAT FLOWS SLOWLY -Good
Conductors BAD INSULATORS -Good Insulators
Wood, plastic, fiberglass, air -Gases are usually
the BEST INSULATORS
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CONTROLLING HEAT FLOW
-You can use various materials to control heat
flow Ex Jacket/sweater -Living organisms have
special features to help control the flow of
heat Ex Fur, Blubber, Scales (reflect), Color
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Transferring Energy
Review 1st Discussed transferring energy as
HEAT -Heat flows from an object that is
warmer (more ke) to an object that is cooler
(less ke) New Stuff 2nd way to transfer
energy -Work transfer of energy that
occurs when a force makes an object move
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WORK
RECALL Force push or pull - In order for WORK
to be done, a FORCE must make something
MOVE. -So If you push your hand
straight down on the table right now (apply
a force) and the table doesnt move, have you
really done any work??
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Doing Work
Two Things are Required for Work to be done -
The force must make object move - Movement must
be in the same direction as the force
Force is Up, Movement is Up
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Work and Energy
Another way to think about energy is -ENERGY is
the ability to do WORK -If something has
energy, it can transfer it to another object
by doing work on that object. -When you do
WORK on something, you INCREASE
its ENERGY -Energy is always
transferred from the object doing work to the
object on which work is done.
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CALCULATING WORK
Amount of Work done depends on 1. Amount of
Force used 2. Distance over which force was
applied (distance the object moves)
Work (in joules) applied force (newtons) X
distance (meters) W Fd
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Example Problem
You push a refrigerator with a force of 100 N. If
you move the refrigerator a distance of 5 m, how
much work do you do?
F 100 N d 5 m W ??
W Fd W (100N)(5m) W 500Nm W 500 J
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POWER
The amount of work done in one second/the RATE
at which WORK is done
For example If you and a friend push two boxes
(identical in mass) for the same distance YOUR
WORK DONE IS EQUAL BUT If your friend pushes
the box faster than you, your friend is MORE
POWERFUL!
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CALCULATING POWER
Power (in watts) Work (in Joules) time
(in seconds) P W t
SI Unit for Power Watt (W) 1 Watt 1 J of
work done in 1 sec (small unit) Use kilowatt
(kW) to often express power 1 kW 1000 W
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Example Problem You do 900 J of work in pushing a
sofa. If it took 5 sec to move the sofa, what
was your power? W 900 J t 5 sec P ??Watt
P W 900 180 J/s 180 Watts t 5
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Power and Energy
Power is also the RATE at which ENERGY is
TRANSFERRED
Power (in Watts) energy transferred (in
joules) time (in seconds) P E
t
Ex A light-bulb transfers electrical energy into
light and heat.
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Calculating Changes in Thermal Energy
Changes in thermal energy (J) Mass (kg) X
change in temp. (oC) X specific heat (J/kg oC)
Q m (Tf-Ti) C
Example Find the change in thermal energy of a
20-kg wooden chair that warms from 15oC to 25oC
if the specific heat is 700 J/(kg oC) Mass
m20kg Ti 15 oC Tf 25 oC Specific heat C
700 J/(kgoC)
Q 20(25-15)(700) 20(10)(700) 140,000 J
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THERMODYNAMICS
There is another way to increase thermal energy
besides adding heat
Thermodynamics The study of the relationship
between thermal energy, heat, and work. System
Anything you can draw a boundary around. Heat
transferred to a system is the amount of heat
flowing into the system across that
boundary. First Relationship Heat and Work both
increase Thermal Energy Ex. Situation 1 Sitting
by a fire, you warm your hands by holding them
close to the fire. Situation 2 Sitting by
a fire, you warm your hands by holding them close
to the fire and rubbing them together
Situation 2 warms your hands up more!
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Open System Thermal energy of this system can
change
Closed System processes can happen within the
system/but no energy is entering or leaving
Thermal Energy of this system doesnt change
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THERMODYNAMICS
First Law of Thermodynamics States The INCREASE
in THERMAL ENERGY of a system the WORK DONE
plus the HEAT TRANSFERRED to the
system or The increase in energy of a system
the energy added to the system
90 J of energy added to the system
Increase of 90 J within the system
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1st law explains the following -When Heat flows
from a warm object to a cool object, the Thermal
Energy of the Warm object decreases the
Increase in Thermal Energy of the Cool
Object So Can heat flow spontaneously from a
cool object to a warm object??
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THERMODYNAMICS
Second Law of Thermodynamics It is IMPOSSIBLE
for HEAT to FLOW from a COLD OBJECT to a WARM
OBJECT unless WORK IS DONE Also explains that it
is impossible to build a device that is 100
efficient
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Friction between objects can completely convert
work into heat. -Is it possible to completely
convert heat into work?
2nd Law of Thermodynamics FORBIDS this from
happening. 2nd Law makes it IMPOSSIBLE to build a
device that converts heat completely into work!
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HEAT ENGINE A device that converts heat into
work.
Ex Cars Engine - converts chemical energy (gas)
into heat, then transfers some of the thermal
enegy into work by rotating the wheels. Only 25
of the heat is converted to work. Where does the
rest go??