Transcript energy - zietlow

Energy: Transformation
and Transfer
PSc.3.1 OBJECTIVE: Understand types
of energy, conservation and energy
transfer.
Energy
THERMAL
The ability to
cause change.
internal motion of
particles
MECHANICAL
NUCLEAR
ENERGY
motion of objects
changes in the
nucleus
ELECTRICAL
CHEMICAL
bonding of atoms
joules (J)
motion of electric
charges
Objectives
PSc.3.1.1
• Explain thermal energy and its
transfer.
Temperature
 Temperature
is a
measure of the
average kinetic
energy of the
particles in a
sample of matter.
Thermal Energy
 Thermal
Energy is the total energy of
the particles in a material.
 It is the sum of the kinetic energy
(due to the movement of particles)
and the potential energy (due to the
forces within or between particles
due to position).
Thermal Energy
 Thermal
Energy depends on the
temperature, mass, and type of
substance.
Thermal Energy
Thermal Energy
• As temperature increases, so does
thermal energy (because the kinetic
energy of the particles increased).
• Even if the temperature doesn’t
change, the thermal energy in a
more massive substance is higher
(because it is a total measure of
energy).
Thermal Energy
 Which
beaker of water has more
thermal energy?
• B - same temperature, more mass
80ºC
A
80ºC
B
200 mL
400 mL
Heat
Heat involves a transfer of
energy (a flow of thermal
energy) between 2 objects due
to a temperature difference.
Cup gets cooler while hand
gets warmer
Ice gets warmer while hand
gets cooler
Heat flows
from “hot to
cold.”
Heat Transfer
 Why
does A feel hot and B feel cold?
• Heat flows from A to your hand = hot.
• Heat flows from your hand to B = cold.
80ºC
A
10ºC
B
Law of Conservation of Energy
 When
the warmer object loses heat,
its temperature decreases and q is
negative.
 When the cooler object absorbs
heat, its temperature rises and q is
positive.
Specific Heat
Some things heat up or cool down
faster than others.
Land heats up and cools down faster
than water.
Specific Heat
 The
specific heat of any substance is
the amount of heat required to raise
the temperature of one gram of that
substance by one degree Celsius.
C water = 4184 J / kg °C
C sand = 664 J / kg ° C
This is why land heats up quickly
during the day and cools quickly at
night and why water takes longer.
Specific Heat
 Because
different
substances have
different
compositions,
each substance
has its own
specific heat.
Specific Heat Values
(J/(kg·K))
Water
4184
Alcohol
2450
Aluminum
920
Carbon (graphite) 710
Sand
664
Iron
450
Copper
380
Silver
235
Why does water have such a high
specific heat?
water
metal
Water molecules form strong bonds
with each other; therefore it takes more
heat energy to break them. Metals
have weak bonds and do not need as
much energy to break them.
Heat Transfer
 Which
sample will take longer to
heat to 100°C?
Specific Heat
Values (J/(kg·K))
50 g Al
50 g Cu
Aluminum
Copper
920
380
• Al - It has a higher specific heat.
• Al will also take longer to cool down.
Specific Heat
q = m Cp ΔT
q = heat (J)
Cp = specific heat (J/(g.°C)
m = mass (g)
ΔT = change in temperature = Tf
– Ti
(°C)
Exothermic and Endothermic
Exothermic: Heat flows out of
the system (to the surroundings).
The value of ‘q’ is negative.
Endothermic: Heat flows into the
system (from the surroundings).
The value of ‘q’ is positive.
Example
 The
temperature of a sample of iron
with a mass of 10.0 g changed from
50.4°C to 25.0°C with the release of
114 J heat. What is the specific heat
of iron?
-114
q = 10.0
m Ciron (25.0
∆T – 50.4)
Ciron = 0.449 J/g°C
Problem #1
A
piece of metal absorbs 256 J of heat
when its temperature increases by
182°C. If the specific heat of the metal
is 0.301 J/g°C, determine the mass of
the metal.
m = 4.67 g
Problem #2
 If
335 g water at 65.5°C loses 9750 J of
heat, what is the final temperature of
the water? (Cp = 4.18 J/g*C)
Tf = 58.5 °C
Measuring Heat
 Heat
changes that occur during
chemical and physical processes can
be measured accurately and precisely
using a calorimeter.
 A calorimeter is an insulated device
used for measuring the amount of
heat absorbed or released during a
chemical or physical process.
A coffee-cup
calorimeter
made of two
Styrofoam cups.
Phase Changes
Melting
Solid
Vaporization
Liquid
Freezing
Gas
Condensation
Sublimation
Melting
Vaporization
Solid
Liquid
Freezing
Gas
Condensation
Deposition
Heating Curve for Water
120
boiling
Water and
100
Steam
Steam
80
Temperature is
constant
Water
during a phase
change!
60
40
20
melting
0
Ice
Water
and Ice
-20
0
40
120
220
760
800
Energy and Phase Change
 Latent
heat is the energy released or
absorbed by a substance in order for
a phase change to occur.
 Latent heat relates to potential
energy, NOT the average kinetic
energy of the particles because the
temperature remains the same.
Energy and Phase Change
 If
heat is used to change state, then
that energy is used for that purpose
and the substance does not get any
hotter.
 It gives the particles in the substance
more ‘freedom’ rather than increasing
their kinetic energy.
 It is increasing their potential energy.
Heat Transfer
Method
Conduction
Convection
Radiation
Notes
Heat Transfer
Thermal energy flows from higher
temperature to lower temperature.
This process is called heat transfer.
 There are three ways heat flows:
• heat conduction,
• convection, and
• thermal radiation.

Heat Transfer
 Heat
conduction
is the transfer of
heat by the
direct contact of
particles of
matter.
Heat Transfer
 Conduction
occurs
between two
materials at different
temperatures when
they are touching
each other.
Heat Transfer
 Thermal
equilibrium occurs when two
bodies have the same temperature.
 No
heat flows in thermal equilibrium
because the temperature is the same
in the two materials.
Thermal Conductors and
Insulators
 Materials
that conduct
heat easily are called
thermal conductors
and those that conduct
heat poorly are called
thermal insulators.
Convection
 Convection
is
the transfer of
heat through the
motion of matter
such as air and
water.
Convection
 The
hot water at the bottom of the
pot rises to the top and replaces the
cold water.
Convection
 Convection
is mainly what
distributes heat throughout a room.
Thermal Radiation
 Heat
from the Sun is transferred to
Earth by thermal radiation.
 All the energy the Earth receives
from the Sun comes from thermal
radiation.
Thermal Radiation
 The
higher the temperature of an
object, the more thermal radiation
it emits.
Thermal Radiation
 Thermal
radiation is also absorbed
by objects.
 The amount of thermal radiation
absorbed depends on the surface of
a material.
Thermal Radiation
 Dark
surfaces
absorb most of
the thermal
radiation they
receive.
 Silver or mirrored
surfaces reflect
thermal radiation.
Objectives
PSc.3.1.2
• Explain the law of conservation
of energy in a mechanical
system in terms of kinetic
energy, potential energy and
heat.
The Nature of Energy
 Energy
is the ability to do work or
produce heat.
 It exists in two basic forms, potential
energy and kinetic energy.
The Nature of Energy
 Potential
energy is energy due to the
composition or position of an object.
 Gravitational potential energy is
energy stored by objects due to their
position above the Earth’s surface.
Potential Energy
♦
Which boulder has greater
gravitational potential energy?
It has a greater height
from the ground.
The Nature of Energy
 Kinetic
energy is energy of motion.
 The kinetic energy of a moving object
depends on the object’s mass and its
velocity.
Kinetic Energy
♦ Which has the most kinetic energy?
80 km/h truck

Which has the
least kinetic
energy?
80 km/h
50 km/h
50 km/h motorcycle
80 km/h
Law of Conservation of Energy
 The
law of conservation of energy
states that in any chemical reaction
or physical process, energy can be
converted from one form to another,
but it is neither created nor
destroyed.
The Nature of Energy
 The
potential energy of dammed
water is converted to kinetic energy
as the dam gates are opened and the
water flows out.
Law of Conservation of Energy
 To
better understand the
conservation of energy, suppose you
have money in two accounts at a
bank and you transfer funds from one
account to the other.
Law of Conservation of Energy
 Although
the amount of money in
each account has changed, the total
amount of your money in the bank
remains the same.
Law of Conservation of Energy
• Mechanical energy is the total
amount of potential and kinetic
energy in a system and can be
expressed by this equation.
mechanical energy = potential energy +
kinetic energy
Conservation of Energy
Potential Energy  Kinetic Energy
Conservation of Energy
Potential Energy  Kinetic Energy
Conservation of Energy
Potential Energy  Kinetic Energy
Is Energy Always Conserved?
♦ While coasting along a flat road on a
bicycle, you know that you will
eventually stop if you don’t pedal.
♦ If energy is
conserved, why
wouldn’t your
kinetic energy stay
constant so that
you would coast
forever?
The Effect of Friction
♦ You know from
experience that if
you don’t continue
to pump a swing or
be pushed by
somebody else,
your arcs will
become lower and
you eventually will
stop swinging.
The Effect of Friction
♦ In other words, the mechanical
(kinetic and potential) energy of the
swing seems to decrease, as if the
energy were being destroyed. Is this
a violation of the law of conservation
of energy?
The Effect of Friction
♦ With every movement, the swing’s
ropes or chains rub on their hooks
and air pushes on the rider.
♦ Friction and air
resistance cause
some of the
mechanical energy
of the swing to
change to thermal
energy.
The Effect of Friction
♦ With every pass of the swing, the
temperature of the hooks and the air
increases a little, so the mechanical
energy of the swing is not destroyed.
♦ Rather, it is transformed into thermal
energy (heat).
The Effect of Friction
 Remember
heat, which is represented
by the symbol q, is energy that is in
the process of flowing from a warmer
object to a cooler object.
 The SI unit of heat and energy is the
joule (J).
The Effect of Friction
Conservation of Energy
Mechanical Energy  Thermal Energy
Near the end of the run, the skier
encounters the force of friction.
Objectives
PSc.3.1.3
• Explain work in terms of the
relationship among the applied
force to an object, the resulting
displacement of the object, and
the energy transferred to an
object.
Work
 Work
is the transfer of energy
through motion.
 It is also a force exerted through a
distance.
W = Fd
W:
F:
d:
work (J)
force (N)
distance (m)
1 J = 1 N·m
Distance must be in direction of force!
Work
Work
♦ When you lift a stack
of books, your arms
apply a force upward
and the books move
upward. Because the
force and distance
are in the same
direction, your arms
have done work on
the books.
Work
♦ When you carry
books while walking,
you might think that
your arms are doing
work.
♦ However, in this
case, the force
exerted by your
arms does no work
on the books.
Work
 Brett’s
backpack weighs 30 N. How much
work is done on the backpack when he
lifts it 1.5 m from the floor to his back?
GIVEN:
F = 30 N
d = 1.5 m
W=?
WORK:
W = F·d
W = (30 N) (1.5 m)
W = 45 J
W
F d
Work
A forklift does 12300 J of work while raising a
pallet with 4900 N of physics textbooks from
the ground to an unknown height. Calculate
the distance the books were raised.
GIVEN:
F = 4900 N
d=?
W = 12300 J
W
F d
WORK:
d=WF
d = (12300 J)  (4900 N)
d = 2.5 m
Work
Two men do 235440 J of work to push a
car 218 m to the nearest fuel station.
Determine the force applied to the car.
GIVEN:
W = 2354400 J
d = 218 m
F=?
W
F d
WORK:
F=Wd
F = (235440 J)  (218 m)
F = 1080 N
Work

A dancer lifts a 40 kg ballerina 1.4 m in the air
and walks forward 2.2 m. How much work is
done on the ballerina during the lift?
GIVEN:
m = 40 kg
d = 1.4 m - during
d = 2.2 m - after
W=?
W
F d
WORK:
F = m·a
F =(40 kg)(9.8 m/s2) = 392 N
W = F·d
W = (392 N)(1.4 m)
W = 549 J during lift
Work

A dancer lifts a 40 kg ballerina 1.4 m in the air
and walks forward 2.2 m. How much work is
done on the ballerina after the lift?
GIVEN:
m = 40 kg
d = 1.4 m - during
d = 2.2 m - after
W=?
W
F d
WORK:
W = F·d
No work after lift. “d” is not
in the direction of the force.
Objectives
PSc.3.1.4
• Explain the relationship among
work, power and simple
machines both qualitatively and
quantitatively.
Power
♦ Power is the
amount of
work done
in one
second. It
is a rate —
the rate at
which work
is done.
Power
W Fd
P

 Fv
t
t
P
= Power (Watts - W)
 v = average velocity (m/s)
Power
WW
Fd P
P P t 
FFvv
t
t
Fd
P t
Power
A 5 kg cart is pushed by a 30 N force against
friction for a distance of 10 m in 5 seconds.
Determine the power needed to move the cart.
GIVEN:
m = 5 kg
F = 30 N
d = 10 m
t=5s
P=?
WORK:
P = Fd  t
P = (30 N) (10 m)  5
P = 60 W
Fd
P t
Power
Jerome
does 2289 J of work while running up
a flight of stairs. If his power is 1730 W, how
long does it take him to climb the stairs?
GIVEN:
W = 2289 J
P = 1730 J
t=?
W
P t
WORK:
t=WP
t = 2289 J  1730 W
t = 1.32 s
Power
A 245 N crate it lifted on to a ledge by a worker
that exerts 325 W of power. With what speed
was the crate lifted to the ledge?
GIVEN:
F = 245 N
P = 325 W
v= ?
P
F v
WORK:
v=PF
v = 325 W  245 N
v = 1.33 m/s
Simple Machines
• A machine is a device
that makes doing
work easier.
• Machines can be simple.
Simple Machines
• Some, like knives, scissors, and
doorknobs, are used everyday to
make doing work easier.
• Machines can make work easier by
increasing the force that can be
applied to an object.
Simple Machines
• A second way that machines can make
work easier is by increasing the
distance over which a force can be
applied.
• Machines can also make work easier
by changing the direction of an
applied force.
Increasing Force
• A car jack is an
example of a machine
that increases an
applied force.
• The upward force
exerted by the jack is
greater than the
downward force you
exert on the handle.
Increasing Force
• However, the distance you push the
handle downward is greater than the
distance the car is pushed upward.
• The jack increases the applied force,
but doesn't increase the work done.
Force and Distance
• The work done in lifting an object
depends on the change in height of
the object.
Force and Distance
• The same amount of work is done
whether the mover pushed the
container up the long ramp or lifts it
straight up.
• If work stays the same and the
distance is increased, then less force
will be needed to do the work.
Changing Direction
• Some machines
change the
direction of the
force you apply.
• The wedge-shaped
blade of an axe is
one example.
The Work Done by Machines
• When you use an
axe to split wood,
you exert a
downward force
as you swing the
axe toward the
wood.
The Work Done by Machines
• The blade
changes the
downward force
into a horizontal
force that splits
the wood apart.
The Work Done by Machines
• When you use a machine such as a
crowbar, you are trying to move
something that resists being moved.
The Work Done by Machines
• If you use a
crowbar to pry
the lid off a crate,
you are working
against the
friction between
the nails in the lid
and the crate.
The Work Done by Machines
• You also could use a crowbar to
move a large rock.
• In this case, you would be working
against gravity—the weight of the
rock.
Input and Output Forces
• Two forces are involved when a
machine is used to do work.
• The force that is applied to the
machine is called the input force.
• The input force is also referred to as
the effort force.
• FE stands for the effort force.
Input and Output Forces
• The force applied by the machine is
called the output force.
• The output force is also called the
resistance force, symbolized by FR.
Work
• Two kinds of work need to be
considered when you use a
machine—the work done by you on
the machine and the work done by
the machine.
• The work done by you on a machine is
called the input work and is
symbolized by Win.
Work
• The work done by the machine is
called the output work and is
abbreviated Wout.
Conserving Energy
• When you do work on the machine,
you transfer energy to the machine.
• When the machine does work on an
object, energy is transferred from
the machine to the object.
Conserving Energy
• The amount of energy the machine
transfers to the object cannot be
greater than the amount of energy
you transfer to the machine.
• A machine cannot create energy,
so Wout is never greater than Win.
Conserving Energy
• When a machine is used, some of the
energy transferred changes to heat
due to friction.
• The energy that changes to heat
cannot be used to do work, so Wout is
always smaller than Win.
Ideal Machines
• Suppose a perfect machine could be
built in which there was no friction.
• None of the input work or output work
would be converted to heat.
Ideal Machines
• For such an ideal machine, the input
work equals the output work.
Ideal Machines
• Suppose the ideal machine increases
the force applied to it.
• This means that the output force is
greater than the input force.
• Recall that work is equal to force
times distance.
Ideal Machines
• If the output force is greater than
input force, then Win and Wout can
be equal only if the input force is
applied over a greater distance
than the output force is exerted
over.
Actual Mechanical Advantage
• The ratio of the resistance force
(output force) to the effort force (input
force) is the actual mechanical
advantage (AMA) of a machine.
Actual Mechanical Advantage
• The actual mechanical advantage of
a machine can be calculated from
the following equation.
FR
AMA 
FE
• FR = resistance force
• FE = effort force
Mechanical Advantage
• Window blinds are a machine that
changes the direction of an input
force.
• A downward
pull on the
cord is
changed to
an upward
force on the
blinds.
Mechanical Advantage
• The input and output forces are
equal, so the MA is 1.
Ideal Mechanical Advantage
• The mechanical advantage of a
machine without friction is called the
ideal mechanical advantage, or IMA.
• The IMA can be calculated by
dividing the effort (input) distance
by the resistance (output) distance.
Ideal Mechanical Advantage
dE
IMA 
dR
• dR = resistance distance
• dE = effort distance
Efficiency
• Efficiency is a measure of how much
of the work put into a machine is
changed into useful output work by
the machine.
• A machine with high efficiency
produces less heat from friction so
more of the input work is changed to
useful output work.
Calculating Efficiency
• To calculate the efficiency of a
machine, the output work is divided by
the input work.
• Efficiency is usually expressed as a
percentage by this equation:
Wout
efficiency 
x100
Win
Calculating Efficiency
• In an ideal machine there is no friction
and the output work equals the input
work. So the efficiency of an ideal
machine is 100 percent.
• The efficiency of a real machine is
always less than 100 percent.
Increasing Efficiency
• Machines can be made more efficient
by reducing friction. This usually is
done by adding a lubricant, such as
oil or grease, to surfaces that rub
together.
Increasing Efficiency
• A lubricant fills in the gaps between
the surfaces, enabling the surfaces
to slide past each other more easily.
Machines and Work – Problem #1
You are using a lever to lift the edge of a
crate in order to slide a roller under it. The
crate weighs 5250 N. You are able to exert
a force of 400 N and move the handle of
the lever 1.20 meters. The crate is lifted a
distance of 0.0800 meters.
(a) What is the input work done?
(a) Win = 480 J
(b) What is the output work done?
(b) Wout = 420 J
Machines and Work – Problem #1
You are using a lever to lift the edge of a
crate in order to slide a roller under it. The
crate weighs 5250 N. You are able to exert
a force of 400 N and move the handle of
the lever 1.20 meters. The crates is lifted a
distance of 0.0800 meters.
(c) What is the efficiency of the lever?
(c) eff = 87.5%
Machines and Work – Problem #1
You are using a lever to lift the edge of a
crate in order to slide a roller under it. The
crate weighs 5250 N. You are able to exert a
force of 400 N and move the handle of the
lever 1.20 meters. The crate is lifted a
distance of 0.0800 meters.
(d) What is the actual mechanical advantage
of the lever?
(d) AMA = 13.1
(e) What is the ideal mechanical advantage?
(e) IMA = 15
Machines and Work – Problem #2
A ramp is used to raise barrels that weigh
824 N up onto a 1.50 meter high loading
dock. The ramp is 4.00 m long. The ramp’s
actual mechanical advantage is 2.20.
(a) How much effort must be exerted to roll
the barrels up the ramp?
(a) FE = 375 N
(b) What is the input work?
(b) Win = 1500 J
Machines and Work – Problem #2
A ramp is used to raise barrels that weigh
824 N up onto a 1.50 meter high loading
dock. The ramp is 4.00 m long. The ramp’s
actual mechanical advantage is 2.20.
(c) What is the output work?
(c) Wout = 1236 J
(d) What is the efficiency of the lever?
(d) eff = 82.4%
Machines and Work – Problem #2
A ramp is used to raise barrels that weigh
824 N up onto a 1.50 meter high loading
dock. The ramp is 4.00 m long. The ramp’s
actual mechanical advantage is 2.20.
(e) What is the lever’s ideal mechanical
advantage?
(e) IMA = 2.67
Machines and Work – Problem #3
A pulley’s actual mechanical advantage is
8.00 and you are using it to raise containers
weighing 12,400 N to a height of 14.0 m. You
need to pull out 122 m of cable to lift the
crates.
(a) What amount of effort is required to lift
the containers with this pulley?
(a) FE = 1550 N
(b) What is the input work done?
(b) Win = 189100 J
Machines and Work – Problem #3
A pulley’s actual mechanical advantage is
8.00 and you are using it to raise containers
weighing 12,400 N to a height of 14.0 m. You
need to pull out 122 m of cable to lift the
crates.
(c) What is the output work done?
(c) Wout = 173600 J
(d) What is the pulley’s efficiency?
(d) eff = 91.8%
Types of Simple Machines
• A simple machine is a machine that
does work with only one movement
of the machine.
• There are six types of simple
machines: lever, pulley, wheel and
axle, inclined plane, screw and
wedge.
Types of Simple Machines
Levers
• A lever is a bar that is free to pivot
or turn around a fixed point.
• The fixed point the lever pivots on
is called the fulcrum.
Levers
• The input arm of the lever is the
distance from the fulcrum to the
point where the input force is
applied.
• The output arm is the distance
from the fulcrum to the point
where the output force is exerted
by the lever.
Levers
• If the output arm is shorter than the
input arm, then the output force is
greater than the input force.
Ideal Mechanical Advantage of
a Lever
• The IMA of a lever can be calculated
from this equation:
Pulleys
• A pulley is a grooved wheel with a
rope, chain, or cable running along
the groove.
• A fixed pulley is a
modified first-class
lever.
• The axle of the pulley
acts as the fulcrum.
Fixed Pulleys
• A fixed pulley is
attached to something
that doesn't move,
such as a ceiling or
wall.
• Because a fixed pulley
changes only the
direction of force, the
IMA is 1.
Wheel and Axle
• A wheel and axle
is a simple
machine
consisting of a
shaft or axle
attached to the
center of a larger
wheel, so that the
wheel and axle
rotate together.
Wheel and Axle
• A wheel and axle is a also a variation
of the lever.
Mechanical Advantage of the
Wheel and Axle
• The output force is exerted at the rim
of the axle.
• So the length of the output arm is the
radius of the axle.
Mechanical Advantage of the
Wheel and Axle
• The IMA of a wheel and axle is given
by this equation:
Inclined Planes
• A sloping
surface, such
as a ramp that
reduces the
amount of
force required
to do work, is
an inclined
plane.
Mechanical Advantage of an
Inclined Plane
• By pushing a box up an inclined
plane, the input force is exerted over
a longer distance compared to lifting
the box straight up.
Mechanical Advantage of an
Inclined Plane
• The IMA of an inclined plane can be
calculated from this equation.
The Screw
• A screw is an inclined plane wrapped
in a spiral around a cylindrical post.
• You apply the input
force by turning
the screw.
• The output force is
exerted along the
threads of the
screw.
The Wedge
• The wedge is also a simple machine
where the inclined plane moves
through an object or material.
• A wedge is an
inclined plane with
one or two sloping
sides. It changes the
direction of the input
force.