chapter seven: energy
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Chapter Seven: Energy. 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations. 7.1 What is energy?. Energy measures the ability for things to change themselves or to cause change in other things. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter Seven: Energy7.1 Energy and Systems
7.2 Conservation of Energy
7.3 Energy Transformations
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7.1 What is energy?
Energy measures the ability for things to change themselves or to cause change in other things.
Some examples are changes in temperature, speed, position, pressure, or any other physical variable.
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7.1 Units of energy Pushing a 1-kilogram object with a
force of one newton for a distance of one meter uses one joule of energy.
A joule (J) is the S.I. unit of measurement for energy.
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7.1 Joules One joule is a pretty small amount of
energy.
An ordinary 100 watt electric light bulb uses 100 joules of energy every second!
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7.1 Some forms of energy Mechanical energy is the energy
possessed by an object due to its motion or its position.
Potential energy and kinetic energy are both forms of mechanical energy.
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Energy associated with motion:
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Mechanical Energy
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7.1 Some forms of energy Chemical energy is a form of energy
stored in molecules. Example: Batteries are storage devices
for chemical energy.
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Energy released or stored when atoms are bonded together.
Bonds can be broken to release this energy.
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Chemical Energy
The chemical bonds in a matchstick store energy that is transformed into
thermal energy when the match is struck.
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7.1 Some forms of energy Electrical energy comes from electric
charge. Caused by the flow of electrons
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Electric Energy
Example: Power lines: Electric energy is easily transported through power lines and converted into other forms of energy
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7.1 More forms of energy
Nuclear energy is a form of energy stored in the nuclei of atoms.
In the Sun, nuclear energy is transformed to heat that eventually escapes the sun as radiant energy.
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7.1 More forms of energy Radiant energy is energy that is
carried by electromagnetic waves.
Light is one form of radiant energy.
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7.1 More forms of energy The electromagnetic spectrum
includes visible light, infrared radiation (heat), and ultraviolet light.
Light energy and heat energy are included in the electromagnetic spectrum.
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QUIZ TIME!
What type of energy cooks food in a microwave oven?
ELECTROMAGNETIC ENERGY
What type of energy is the spinning plate inside of a microwave oven?
MECHANICAL ENERGY
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QUIZ TIME!
Electrical energy is transported to your house through power lines.
When you plug an electric fan to a power outlet, electrical energy is transform into what type of energy?
MECHANICAL ENERGY
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QUIZ TIME!
What energy transformation occurs when an electric
lamp is turned on?
ELECTRICAL ENERGY
ELECTROMAGNETIC ENERGY
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What type of energy is shown below?
Chemical Energy
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What type of energy is shown below?
Chemical Energy (yummy)
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What type of energy is shown below?
Thermal Energy
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7.1 Sources of energy Without the Sun’s
energy, Earth would be a cold icy place with a temperature of -273 C.
As well as warming the planet, the Sun’s energy drives the entire food chain.
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7.1 Sources of energy All objects with mass feel forces in
the presence of Earth’s gravity.
These forces are a source of energy for objects or moving matter such as falling rocks and falling water.
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7.1 Energy and work In physics, the word work
has a very specific meaning.
A force is said to do work when it acts on an object & there is a displacement in the direction of the force.
Work is the transfer of energy that results from applying a force over a distance.
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Work and Potential Energy The work done on the ball gives the
ball gravitational potential energy.
Gravitational potential energy = mgh
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Energy stored in an object due to its position.
Examples:
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Potential Energy
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Depends on the weight of the object and its height above some reference point.
Formula:
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Gravitational potential energy
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Gravitational Potential Energy Example:Both blocks acquire the same gravitational potential energy, mgh. The same work is done on each block. What matters is the final elevation, not the path followed.
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Elastic Potential Energy
The potential energy due to the physical distortion (stretched or compressed) of an object.
Depends on:Type of material
Amount of distortion
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Elastic Potential EnergyExample:Work is done on the bow.The work done is storedin the bow and string aselastic potential energy.After release, the arrow issaid to have kinetic energy,1/2 mv2. Energy is measured in the same units (joules) as work.
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Potential Energy Systems or objects with potential
energy are able to exert forces (exchange energy) as they change.
Potential energy is energy due to position, which can change.
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Potential Energy
EP = mgh
height object raised (m)
gravity (9.8 m/sec2)
PE (joules)
mass of object (g)
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Kinetic energy Energy of motion is called kinetic
energy. A moving cart has kinetic energy
because it can hit another object (like clay) and cause change.
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Energy of motion
All moving objects possess kinetic energy
Can change from one type to another
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Kinetic Energy
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Factors Which Affect Kinetic Energy:
Mass & Velocity
KE = ½ mv2
This makes the velocity much more important than the mass.
Doubling the velocity will quadruple the kinetic energy.
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Kinetic Energy
EK = ½ mv2
mass of object (kg)
velocity (m/sec)
KE (joules)
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Solving Problems
A 2 kg rock is at the edge of a cliff 20 meters above a lake.
It becomes loose and falls toward the water below.
Calculate its potential and kinetic energy when it is at the top and when it is halfway down.
Its speed is 14 m/s at the halfway point.
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Solving Problems1. Looking for:
…initial EK, EP and EK, EP half way down.
2. Given: mass = 2.0 kg; h = 20 m
v = 14 m/s (half way)
3. Relationships:
EP =mgh
EK = ½ mv2
Assume rock starts from rest.
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Solving Problems
EK = 0 J
EP = mgh
EK = ½ mv2
EP = mghh = 10 m
h = 20 m
4. Solution Draw a free body
diagram. EP = (2 kg)(9.8 N/kg)(20 m) = 392 J at top
EP = (2 kg)(9.8 N/kg)(10 m) = 196 J half way EK = 0 J, rock is at rest
EK = (1/2)(2 kg)(14 m/s)2
= 196 J half way
m = 20 kg
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Chapter Seven: Energy
7.1 Energy and Systems
7.2 Conservation of Energy
7.3 Energy Transformations
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7.2 Conservation of Energy
Systems change as energy flows and changes from one part of the system to another.
Each change transfers energy or transforms energy from one form to another.
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7.2 Energy flow
How can we predict how energy will flow?
One thing we can always be sure of is that systems tend to move from higher to lower energy.
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7.2 Sources of energy The chemical potential energy stored in
the food you eat is converted into simple sugars that are burned as your muscles work against gravity as you climb the hill.
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Bell Work!Energy Transformation
1. What is the formula for Gravitational potential energy?
2. What is the formula for Kinetic energy?
3. How does the Pe at the top of a hill compare to the Ke at the bottom?
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Energy Transformation
The work done in lifting the mass gave the mass gravitational potential energy.
Potential energy then becomes kinetic energy.
Kinetic energy then does work to push stake into ground.
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7.2 Units of energy Some units of energy that are more
appropriate for everyday use are the kilowatt hour (kWh), food Calorie, and British thermal unit.
oF
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Chapter Seven: Energy
7.1 Energy and Systems
7.2 Conservation of Energy
7.3 Energy Transformations
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7.3 Conservation of Energy
The idea that energy tranforms from one form into another without a change in the total amount is called the law of conservation of energy.
The law of energy conservation says the total energy before the change equals the total energy after it.
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7.3 Conservation of Energy
When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic.
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EnergyConservation
Total energy is the sum of both types of energy.
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Energy Conversion in Pendulums
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Energy conversions
All forms of energy can change from one form to another.
Examples:
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Solving Problems
A 2 kg car moving with a speed of 2 m/sec starts up a hill.
How high does the car roll before it stops?
mgh = ½ mv2
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A 2 kg car moving with a speed of 2 m/sec starts up a hill.
How high does the car roll before it stops?
EK= ½ 2kg (2 m/s)2EK= 4 J
Ep= 2kg (9.8 m/s2) X
mgh = ½ mv2
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Solving Problems
1. Looking for: …height of hill
2. Given … mass = 2 kg, v = 2 m/s
3. Relationships:
Energy transformed from EK to EP
EK = ½ mv2
EP =mgh
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Solving Problems
1. Solution Find beginning EK
EK = ½ (2 kg) (2 m/s)2 = 4 Joules
Assume energy before = energy after EK = EP
EP =mgh4 J = mgh
h = (4 Nm)/(2 kg)(9.8 N/kg) = .2 m
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7.3 Conservation of Energy Many people are concerned about
“running out” of energy. What they worry about is running
out of certain forms of energy that are easy to use, such as fossil fuels like oil and gas.
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7.3 Conservation of Energy It took millions of years
to accumulate these fuels because they are derived from decaying, ancient plants that obtained their energy from the Sun when they were alive.
Because it took a long time for these plants to grow, decay, and become oil and gas, fossil fuels are a limited resource.
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7.3 Conservation of Energy Regular
(incandescent) light bulbs convert only 10% of electrical energy to light.
That means 90% of the energy is released as wasted heat.
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7.3 Conservation of Energy
Other forms of energy, such as thermal energy, flowing water, wind, and solar energy are not as limited.
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Conservation of Energy
Energy can be neither created nor destroyed.
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EOC Pages 184-186 Concepts 5-6, 10-12, Problems 1-5, 7-8, Applying your Knowledge # 4
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1.9 cm = 0.019m62 g = 0.062
kg
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Speed
= 0.01 m
0.01 m