Energy

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Energy

• Module 5

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The Nature of Energy What is energy?
Energy is the ability to cause a change or the
ability to do work What are the different forms
of energy?
Electrical Energy
Mechanical Energy
Solar Energy
Heat Energy
Nuclear Energy
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The energy of different objects may fall into two
main categories Kinetic Energy and Potential
Energy Kinetic Energy is the energy of a moving
object Potential Energy is stored energy. This
can be thought of in two ways. Stored energy in
our bodies or in a battery is called chemical
potential energy. Stored energy due to the
position of an object above the surface of the
earth is gravitational potential energy.
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Calculating Kinetic Energy
Kinetic Energy depends upon two factors the mass
of the object and its velocity. Mass is measured
in kg and velocity is measured in m/s When the
units are multiplied, the result is kg
m2/s2 Since a Newton (N) is a kg m/s2, therefore
kg m2/s2 is the same as a N-m, which is the same
as a joule (J). Hence, the unit of kinetic energy
is the joule.
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Practice Problems
How much energy does a soccer player with a mass
of 75 kg and a velocity of 5.5 m/s possess?
KE ½mv2 KE ½(75 kg)(5.5 m/s)2 KE 1.1 X
103 kgm2/s2
1 kgm/s2 1N
KE 1.1 X 103 Nm
1 Nm 1 Joule (J)
KE 1.1 X 103 Joules (J)
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Solve the following
What is the velocity of a car with a mass of 675
kg and a kinetic energy of 1690 joules?
2KE v2
KE ½mv2
2KE mv2
2( ) 2( )
m
2KE
v
2(1690J)
v
m
675 kg
v 2.24 m/s
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Calculating Gravitational Potential
Energy Gravitational Potential Energy (GPE)
depends upon three factors the mass of the
object, the height of the object from the surface
of the earth, and the gravitational acceleration
constant (on earth 9.8 m/s2)
Notice how the units multiply mass (kg) x m/s2
x height (m). Hence the unit is kg m2/s2 which is
the joule (J). Lets do a practice problem
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Potential Energy
Calculate the energy possessed by a rock on the
side of a cliff. The rock has a mass of 237 kg
at an elevation of 45 meters.
PE mgh PE (237 kg)(9.8 m/s2)(45m)
PE 1.0 X 105 kgm2/s2
PE 1.0 X 105 Nm
PE 1.0 X 105 Joules
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What is the Law of Conservation of Energy? Energy
cannot be created nor destroyed.but. Energy can
be transformed from one type to another.
In this diagram, electrical energy is converted
into thermal (heat) energy and light energy.
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There are many energy transformations in an
automobile. The explosion of the gasoline
involves the release of chemical energy. The
chemical energy causes the pistons in the engine
to move (mechanical energy) The moving pistons
affect many moving parts including a generator
which produces electrical energy which keeps the
battery charged. The moving pistons and other
moving parts also produce heat energy.
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Objects can convert potential energy to kinetic
energy or kinetic energy to potential energy. The
diagram below illustrates this relationship.
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In nuclear reactions some mass is converted into
energy.
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The major source of energy is the SUN. In
photosynthesis plants convert sunlight into
stored energy in the plants. Animals and humans
eat the plants to get energy. The human body
burns this stored energy in units called
Calories. One Calorie is equal to 4.18 J. The
human body stores energy in Calories and will
utilize burn this energy as needed.
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What is Thermal Energy?
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Particles of matter are in constant motion. This
motion relates directly to the state of matter of
the object (solids, liquids, or
gases). Temperature affects how fast these
particles move. The higher the temperature the
faster the particles move. Moving particles
possess kinetic energy. Temperature is defined at
the average kinetic energy of the particles of an
object.
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Thermal Energy is the sum total of all of the
energy of the particles of an object. Thermal
energy and temperature are related though
DIFFERENT. Temperature is the average kinetic
energy of the particles of an object. Thermal
energy is the total amount of energy of the
particles of an object. A bathtub full of water
at 100oF has more thermal energy than a thimble
of water at 100oF. The temperature is the same
but the total amount of energy is different. The
bathtub has more energy.
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What is heat? Heat is thermal energy that flows
from something at a higher temperature to
something at a lower temperature. What is
specific heat? Specific heat is a property of
matter which determines how readily a material is
to absorb heat and change temperature. Specific
Heat is defined as the amount of heat energy
needed to raise the temperature of 1 kg of a
substance by 1 oC or 1 K.
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Did you notice that water has a very high
specific heat, whereas iron has a low specific
heat? Water requires a lot of heat energy to
raise its temperature.
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Metals require little energy to raise its
temperature. Thermal energy can be calculated
using the following formula
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Try this one!!
Answer 376 J/kg K Did you do this problem?
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Transferring Thermal Energy How is Thermal Energy
Transferred? YOU MUST KNOW THIS!!!!! Conduction
direct contact Convection through a
fluid Radiation by electromagnetic waves
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What is heat transfer by conduction? Heat
transfers as particles of an object increase
their collisions as heated. These collisions
transfer the heat energy through the object by
colliding with adjacent particles.
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What is heat transfer by convection? Heat is
transferred through a substance through currents.
This occurs in fluids (liquids AND
gases) Convection currents are caused by heating
of a liquid or gas, the liquid or gas rises, then
cools and falls. This occurs in the mantle of the
earth And in the atmosphere. Most of our weather
patterns are the result of convection currents in
the atmosphere.
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What is heat transfer by radiation? Radiation is
heat transfer by electromagnetic waves. These
wave may pass through all states of matter and
also through NO matter such as the vacuum of
space. This energy is often called radiant
energy. Radiant energy from the sun travels
through the vacuum of until it reaches the earth.
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How is heat flow controlled? Insulators a
material which does not allow heat to pass
through it easily.
Some animals have good insulation to survive
severe winters.
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Buildings and houses are insulated so that heat
does not pass out of (winter time) and into
(summer time).
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What are some other uses of insulation?
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Using heat How do we use heat in our
lives? Forced-Air Systems a fuel is burned in a
furnace and a fan circulates the heat in the house
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Radiator Systems closed metal containers that
contain hot water or steam. The thermal heat is
transferred to the air and circulated by
convection currents. Electric radiators heat
metal coils which transfer the thermal heat to
the surrounding air.
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Using heat to do work the internal combustion
engine
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Automobile engines are usually four-stroke
engines since each four-stroke cycle converts
chemical/thermal energy to mechanical energy.
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Refrigerators, air conditioners, and heat pumps
how do they work? A coolant is circulated through
pipes by a compressor. When the pipes get small
and narrow the coolant compresses and changes to
a liquid giving off heat energy. When the pipes
get larger, the coolant changes from a liquid to
a gas (evaporates) causing the area to absorb
heat and get cooler
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The human coolant - In-built cooling system The
human body has a cooling system which function
automatically perspiration sweat.
Like a refrigeration system, the evaporation of
the sweat from the surface of the body causes
energy to be absorbed from the skin, causing the
skin to feel cooler.
No sweat!!!
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• This concludes module 5