Transcript Potential Energy

Unit 2: Heat and Energy
What is energy?
• Energy is defined as “the ability to do
work”, or, in other words, to make things
MOVE.
• Stored energy is called potential energy.
• Moving energy is called kinetic energy.
Can you think of some examples of each
around you? Which kind is more common?
KE-PE Conversions
• Kinetic Energy (KE) is the name for energy
associated with moving objects.
• Potential Energy (PE) is the name for energy
stored in an object--or, the "potential" of an object to
do work.
• The picture below of Horseshoe Fall at Niagara
Falls illustrates the difference between kinetic and
potential energy. In this case, the two forms of
energy are experienced by molecules of water.
Potential energy (due to gravity) at the top of the
waterfall is converted into kinetic energy as is
moves down the waterfall.
Elastic
Potential
Energy
• One form of potential energy is elastic potential
energy. This is the energy stored in elastic
materials as the result of their stretching or
compressing. (Ex: rubber bands, bungee chords,
trampolines, springs, an arrow drawn into a bow,
etc).
• The amount of elastic potential energy stored in
such a device is related to the amount of stretch of
the device - the more stretch, the more stored
energy.
Gravitational
Potential
Energy
• Gravitational potential energy is the energy pendulum
stored in an object as the result of its height
(vertical position).
• The energy is stored as the result of the
gravitational attraction of the Earth for the object.
The gravitational potential energy of an object is
dependent on two variables - the mass of the
object and the height to which it is raised.
• The TOTAL energy of the pendulum remains the
same (sum of KE and PE), although the
pendulum will eventually slow down due to
friction with the air.
Chemical Potential Energy
• Chemical potential energy is stored in the
bonds between atoms in a molecule.
• This energy is released when the bonds
are broken in a chemical reaction (Ex:
combustion, digestion, etc… )
What happens when energy is
added to matter?
• When energy is added to matter it can..
-Make matter move.
-Heat matter up.
Adding energy to matter causes the atoms and
molecules that make up the matter to move
around. This creates heat (and sometimes light).
Adding energy can also break apart the bonds
between atoms and molecules, causing a phase
change (solid↔liquid↔gas).
Thermal Energy and Temperature
Heat is the movement of thermal energy from
warm objects to cooler objects.
Atoms and molecules will vibrate and move
as more and more energy is absorbed.
Temperature is a measure of the average
kinetic energy of the moving particles.
How is thermal energy transferred?
• Heat always moves from a warmer place to
a cooler place.
This means that…
• Hot objects in a cooler room will cool to
room temperature.
• Cold objects in a warmer room will heat up
to room temperature.
Practice Questions
• If a cup of coffee and a red popsicle were
left on the table in this room what would
happen to them? Why?
• ANS: The cup of coffee will cool until it
reaches room temperature. The popsicle will
melt and then the liquid will warm to room
temperature.
Heat transfer occurs in three ways:
• Conduction
• Convection
• Radiation
Conduction
When you heat a metal strip at one end, the heat
travels to the other end.
Heat transfer
Heat added
As you heat the metal, the particles vibrate. These
vibrations make the adjacent particles vibrate, and so on
and so on… the collisions between atoms cause the
vibrations to pass along the metal causing heat transfer.
This form of heat transfer is conduction.
Why are metals such GOOD CONDUCTORS?
Metals have enhanced conductivity!
The outer electrons of metal atoms
drift, and are free to move.
When the metal is
heated, this ‘sea of
electrons’ gain kinetic
energy and transfer it
quickly throughout the
metal.
Insulators, such as wood and plastic, do not
have this ‘sea of electrons’ which is why they
do not conduct heat as well as metals.
Why does metal feel colder than wood, if they
are both at the same temperature?
Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.
Convection
What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
This causes fluid to move.
What is a fluid?
A liquid or gas.
Fluid movement
Cooler, more dense fluids
sink through warmer, less
dense fluids.
This means that warmer liquids
and gases rise up and cooler
fluids sink.
Water movement in a convection current
Cools at the
surface
Cooler
water sinks
Convection
current
Hot water
rises
Why is it windy at the seaside?
Cold air sinks
Where is the
freezer
compartment
put in a fridge?
It is put at the
top, because
cool air sinks,
so it cools the
food on the
way down.
Freezer
compartment
It is warmer
at the
bottom, so
this warmer
air rises and
a convection
current is
set up.
The third method of heat transfer
How does heat energy get
from the Sun to the Earth?
?
There are very few
particles between the
Sun and the Earth so it
CANNOT travel by
conduction or by
convection.
RADIATION
Practice Questions
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation is energy in the form of a wave
True/False
Solar Cookers and Heat Absorption Experiments
Four containers full of water were placed equidistant from a heat
source (ex. the sun). Which container would have the warmest
water after ten minutes?
Dull metal
Shiny metal
Shiny black
Dull black
dull black container would be the warmest after ten
The __________
minutes because its surface absorbs heat radiation the best.
shiny metal container would be the coolest because it is
The _________
the poorest at absorbing heat radiation.
Practice questions
Why are houses painted white in hot countries?
White reflects heat radiation and keeps the house cooler.
Why can shiny foil blankets be used as emergency
blankets to avoid hypothermia?
The shiny metal reflects the heat radiation from the runner
back in, this stops the runner getting cold.
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the
cool air ‘falls through’ the warm air.
What is electromagnetic radiation?
• Gamma rays
• Radio waves
Electromagnetic radiation is energy that travels as
electromagnetic waves. The electromagnetic
spectrum is the full range of energy waves, from high
energy gamma rays to low energy radio waves.
Wavelengths in the Electromagnetic
Spectrum
According to the diagram, what type of
EM radiation passes through Earth’s
atmosphere? What type gets blocked?
What is the greenhouse effect?
Shortwave visible sunlight
passes through the
atmosphere to the earth
below, but longwave
infrared (heat) from the
earth does NOT pass
through.
Adding CO2 to
the atmosphere
increases
this arrow! So
more heat is
trapped by the
CO2 that would
NORMALLY be
allowed to
escape. In a
way, the CO2
acts as a giant
blanket!
How does CO2 cause climate
change?
CO2 and other greenhouse gases act like the glass in a greenhouse, allowing
short-wave energy to pass through, but trapping long-wave radiation inside.
Over time, temperature may rise even further due to positive feedback cycles.
How is energy transferred within
the earth?
• Energy from the earth’s core heats the
surrounding rock in the mantle.
• Mantle rock melts and rises towards the surface.
• This causes volcanoes and results in igneous
rock formation.
• Movement of mantle rock also causes tectonic
plate movement and mountain building.
• Link to convection demo:
http://www.edumedia-sciences.com/a399_l2mantle-convection.html
The Rock Cycle
The internal energy of the Earth causes
matter to cycle through the solid earth.
The rock cycle involves recycling of rock
material in convection currents within the
mantle layer of the earth. Hot, low density
molten rock called magma rises to the surface.
Near the surface, magma cools and becomes denser. This
denser, cooler rock sinks below once again to complete
the cycle.
Review: What is a phase change?
Matter undergoes a phase change when it
moves between the solid, liquid or gas
states.
There are four examples of phase changes:
1. melting
2. freezing
3. evaporating
4. condensing
NEXT LAB: What is a heating curve?
Evaporating
melting
Warming up
• A heating curve shows phase changes for
a particular substance.
How does the water cycle result from solar energy?
The sun causes water to cycle through earth’s major reservoirs through a series of
processes involving evaporation, wind, condensation, runoff and groundwater.
Renewable or nonrenewable???
Where does energy come from?
• We get energy to power our home
appliances from electrical power plants….
but this energy has been converted from
other sources.
• Our energy sources are classified as either
renewable or non-renewable, depending on
how quickly the source can be replaced.
What are non-renewable energy
sources?
• Petroleum, natural gas, and coal are the main
sources of energy in use today. All of these fuels
are classified as fossil fuels.
Why are they called fossil fuels?
• They are all made from decayed plants and
animals that have been preserved in the earth's
crust by pressure, bacterial processes and heat. It
takes millions of years for these organisms to
chemically change into fossil fuels.
• Although these processes are still occurring today,
we are using up the fossil fuels MUCH FASTER
than they can be replaced.
What is Petroleum?
• Liquid fossil fuels, like petroleum, are formed in
areas that geologists believe were once covered
by oceans or seas. These fuels were formed
when dead plants and animals sank to the bottom
of the ocean and were covered by sediments.
Over long periods of time (millions of years), the
sediments were changed into sedimentary rocks
and the plant and animal remains into oil.
• This is an example of a folded-rock
layer oil trap being drilled for
petroleum.
Other fossil fuels
• Another type of fossil fuel is natural gas (methane).
It is found sometimes with petroleum, with coal, or by
itself. Since it is less dense, it is most often found on
top of oil pools. Natural gas is valuable because it
burns cleanly, releases energy, and can be easily
transported in underground pipelines. We use
natural gas in many ways including heating our
homes and cooking our food.
• Coal is solid and is formed from pressurized peat, or
partially decomposed plant material. There are
different varieties of coal which are the result of
geologic forces having altered plant material in
different ways. Some forms of coal contain a lot of
sulfur, which contributes to acid rain.
Fossil Fuels
• Burning coal, natural gas, and petroleum releases chemical energy
stored in the fuel. This energy is released as heat when the fuels
are burned. The energy contained by the fuels originally comes
from the energy of the sun.
•
•
•
•
•
Advantages
Depending on the fuel, relatively easy to obtain
Simple combustion process can directly heat or generate electricity
Inexpensive
Easily distributed to many different areas
• Disadvantages
• Produces carbon dioxide and contributes to global warming
• Questionable availability of some fuels...major price swings based on
politics of oil regions
• Cause of acid rain
Nuclear Fission
Chernobyl
Nuclear fission involves the splitting of a heavy element into lighter
elements. The reaction is set off by a neutron shooting into an atom’s
nucleus. In this reaction, matter is transformed into energy in a
process that is several million times as energetic as combustion
(chemical burning).
•
•
•
•
•
•
•
Advantages
Relatively little fuel is needed and the fuel is relatively inexpensive and
available in trace amounts around the world.
Fission is not believed to contribute to global warming or other pollution
effects associated with fossil fuel combustion
Disadvantages
Possibility of nuclear meltdown from uncontrolled reaction--leads to nuclear
fallout with potentially harmful effects on civilians
Waste products can be used to manufacture weapons
High building cost because plant requires containment safeguards
Hydroelectric
• Hydroelectric systems make use of the energy in running water to create
electricity. In coal and natural gas systems, a fossil fuel is burned to heat
water. The steam pressure from the boiling water turns "propellors" called
turbines. These turbines spin coils of wire between magnets to produce
electricity. Hydropowered systems also make use of turbines to generate
electrical power; however, they do so by using the energy in moving water
to spin the turbines.
•
•
•
•
•
Advantages
Unlimited fuel source
Minimal environmental impact
Produces high levels of energy
Can be used throughout the world
• Disadvantages
• Smaller models depend on availability of fast flowing streams or rivers
• Run-of-the-River plants can impact the mobility of fish and other riverlife.
NOTE: Building a fish ladder can lessen this negative aspect of
hydroelectric power
Biomass
• Biomass is energy produced from organic substances.
The key to the power of biomass lies in the energy of the
sun. All plants undergo a process called photosynthesis,
whereby the plants use chlorophyll to convert the energy
in the sun's rays into stored energy in the plants. This is
released when the organic substances are burned.
• Advantages
• (Theoretically) unlimited fuel source
• Alcohols and other fuels produced by biomass are efficient,
viable, and relatively clean-burning
• Available throughout the world
• Disadvantages
• Could contribute a great deal to global warming and
particulate pollution if directly burned
• Still an expensive source, both in terms of producing the
biomass and converting it to alcohols
Solar Power
• The energy of the sun can be used in many ways. When plants grow, they
store the energy of the sun. Then, when we burn those plants, the energy is
released in the form of heat. This is an example of indirect use of solar
energy.
• The form we are interested in is directly converting the sun's rays into a
usable energy source: electricity. This is accomplished through the use of
"solar collectors," or, as they are more commonly known as, "solar panels."
•
•
•
•
•
Advantages
Unlimited fuel source
No pollution
An excellent supplement to other renewable sources
Versatile--is used for powering items as diverse as solar cars and satellites
• Disadvantages
• Low energy production--large numbers of solar panels (and thus large land
areas) are required to produce useful amounts of heat or electricity
• Only areas of the world with lots of sunlight are suitable for solar power
generation
Wind
• Differences in air pressure due to differences in temperature are the main
cause of wind. Because warm air rises, when air masses of different
temperatures come in contact, the warmer air rises over the colder air,
causing the wind to blow.
• Wind generators take advantage of the power of wind. Long blades, or
"rotors", catch the wind and spin. Like in hydroelectric systems, the spinning
movement is transformed into electrical energy by a generator.
• Advantages
• Unlimited fuel source
• No pollution
• An excellent supplement to other renewable sources
• Disadvantages
• Low energy production--large numbers of wind generators (and therefore
large land areas) are required to produce useful amounts of heat or
electricity
• Only areas of the world with lots of wind are suitable for wind power
generation
• Relatively expensive to maintain; it does not generate very much energy for
the price.
Geothermal
The earth's crust is heated by the decay of radioactive elements. The heat is
carried by magma or water beneath the earth's surface. Some of the heat
reaches the surface and manifests itself in geysers and hot springs
throughout the world.
Geothermal power can be used to directly heat buildings. The pressurized
steam from superhot water beneath the earth's surface can be used to power
turbines and generate electricity.
•Advantages
•Unlimited energy source
•No pollution
•Can be directly used to heat or produce electricity (very cheap)
•An excellent supplement to other renewable sources
•Disadvantages
•Not available in many locations
•Not much power per vent
World energy use
• This graph indicates the role of
various energy sources in world
electricity generation. Which
is the greatest source of energy?
• Note that hydroelectric has a slightly larger share
of world electricity generation than nuclear,
whereas in the United States it is opposite.
• This is due to the fact that many other countries
can use hydroelectric power more easily than the
US because of less populated cities.
How is heat used to generate
electricity?
• Most of the electricity in the united
states is produced using steam
turbines.
• A turbine converts kinetic energy from a
moving fluid (liquid or gas) to
mechanical energy.
• Heat from a fuel source such as nuclear
power, coal burning, is used to create
steam, which is then used to spin the
blades on a turbine.
• A turbine that spins in a magnetic field
will generate electricity.
What are the different forms of
Energy?
Energy appears in many forms. There are five
main forms of energy.
• Mechanical –energy associated with motion
• Heat (thermal) –energy of vibrating molecules
• Chemical –energy of chemical bonds
• Electromagnetic (radiant) –energy that travels
in waves (such as electricity, light, UV, radio)
• Nuclear –energy from the nucleus of an atom
If we get most of our energy from
the sun, then where does the sun
get IT’S energy?
• Nuclear reactions (fusion) occurs in the sun.
This cannot be easily reproduced on Earth
because VERY high temperatures and
pressures are required.
Energy cannot be created or
destroyed… however, energy can
be converted from one form to
another.
• Energy enters the Earth system primarily
as solar radiation, it is captured by
materials and photosynthetic processes,
and is eventually transformed into heat…
How does this work?
Energy transformations
•
Some forms of energy can be converted to other forms.
• For example, solar panels are used to capture light energy
from the sun and convert it to electricity.
• Green plants undergo photosynthesis
to convert light energy into chemical
energy in the form of food.
• The mechanical energy of a waterfall
can be converted to electromagnetic
energy in a generator. Generators
convert other types of mechanical
energy into electromagnetic energy.
Examples of Energy
Conversions
• Photosynthesis is the process by which plants are able to
capture light energy and use that energy to make glucose. In
this process sunlight provides the energy needed by
chlorophyll to change molecules of carbon dioxide and water
into glucose (a type of sugar). Chemical energy is stored in
the plants.
• FOOD: When a cow eats a plant, the chemical energy stored
in the plant is transferred to the cow, which can be converted
into mechanical energy. The cow also uses the energy to
make muscle (beef). This energy is then transferred to you as
you eat your hamburger.
• COMBUSTION: Plants, such as wood, can also be burned to
convert the stored chemical energy into heat and light
(electromagnetic) energy).
Energy Conversions
• It usually takes a whole series of energy conversions to
do a certain job. For example, just to get the energy to
make a piece of toast, there are several energy
conversions involved. What are they?
chemical → electrical → heat
• How does this work?
• Chemical energy stored in coal is released as heat and
light energy when the coal is burned. The heat energy is
used to produce steam and is changed into mechanical
energy in a generator. The generator converts
mechanical energy into electric energy that travels
through power lines into your home. When you use your
toaster, that electric energy is again changed into heat
energy.
More Examples of Energy
Conversions
• Rubbing your hands together…
chemical → mechanical → heat
• Coal burning in a grill
chemical → heat + radiant
• Power generated at a hydroelectric plant
mechanical → electromagnetic
• Baking cookies
electromagnetic → chemical + heat
Conservation of Energy
• Have you ever touched a lightbulb when it was on?
Or tried to change it when it burnt out after being on?
What you have discovered is the conversion process
of energy!
• When you turn on a light, not all of the electricity is
converted to light energy. Some of the energy is
converted to heat.
• Although some energy is changed to heat and does
not help do the work, the energy is not lost. In fact,
energy is never lost; it is only converted from one
form to another. This is stated in the law of
conservation of energy, energy can not be
created nor destroyed by ordinary means.
THE END
Hydrothermal Vents
Angler fish
Some Review Topics
•
•
•
•
What happens when heat is added to matter?
What is a “phase change”? What does it mean?
How is heat energy transferred?
Describe how is energy transformed (or
converted) from one form to another using heat,
light, electricity and motion as examples.
• Explain how solar energy causes water to cycle
through major earth reservoirs.
• Explain how internal energy of the earth causes
matter to cycle through the solid earth.
Some Review Topics
• How is heat used to generate electricity?
• What are some advantages and disadvantages
to using fossil fuels and nuclear fuel? What
environmental issues are associated with these
fuels?
• What are some advantages and disadvantages
to using wind, solar and hydroelectric energy?
What environmental issues are associated with
these sources?
• Discuss why alternative energy sources being
explored and used to address the disadvantages
of using fossil and nuclear fuels.