Transcript Section 3.2 - Lamont High

Section 3.1
Energy Forms and Transformations
Nature of Energy
Energy is all around you.
• You hear energy as sound, you see energy
as light, you can feel energy in wind.
• Living organisms need energy for growth
and movement.
• You use energy when you hit a tennis ball,
compress a spring, or lift a grocery bag.
• Energy is the ability to do work.
Energy
The energy released by
a supernova is capable
of destroying a nearby
solar system in just a
few hours. A supernova
is one of the greatest
concentrations of
energy in the universe.
• It is estimated that it would take
approx. 2800 h of strenuous manual
labor to produce as much energy as one
average Canadian uses in one day.
• In other words: A team of 350 strong
people working 8 h straight!
James Joule (1818-1889)
• Studied energy
• He proved that mechanical work and
electricity can produce heat and viceversa
• Unit of measure for energy = the Joule
(J)
There are Four Common forms of energy:
1. Chemical Energy
• Energy stored in chemicals
and released when
chemicals react
• It is potential or stored
energy in foods, fuels,
batteries, and explosives
• Eg. Digesting food…bonds are
broken to release energy for
your body to store and use.
2. Electric Energy
The energy of charged
particles. Electrical
energy is transferred
when electrons travel
from place to place
Examples:
• Power lines carry
electricity
• Electric motors
• electric appliances
3. Mechanical Energy
-Energy of motion.
-The energy
possessed by an
object because of
its motion or its
potential to move.
Examples:
Water in a waterfall, Wind, Moving vehicles,
Sound, hammer hitting nail
4. Thermal Energy
• Total kinetic energy of all
the particles in a
substance.
• The faster a particle moves
the more kinetic energy it
has.
Examples:
Friction
Changes in state of matter
Thermal Energy
• TWO CUP ANALOGY: Compare two
cups holding equal amounts of water:
the one containing more thermal
energy will feel warmer
Energy Conversions
• All forms of energy
can be converted to
other forms.
• Law of Conservation
of Energy: Energy
cannot be created
or destroyed.
TRANSFORMATIONS OF
ENERGY
What energy transformations are taking place?
EXAMPLE 1:
• place in this picture?
•Chemical energy in the baby’s muscles are
being converted into mechanical energy
(motion) of the lawn mower
•Chemical energy is also being converted
into thermal energy as her muscles strain
to push the lawnmower.
•
Example 2
What energy
transformations are
taking place with
this electric kettle?
Electric energy to Thermal energy
Example 3
• What energy
transformations
occur when
listening to music
with a discman?
Chemical Energy Mechanical Energy 
Sound Energy
Transformations Involving Chemical and
Electrical Energy :
Examples of Devices that convert Energy from one
form to another include:
Input Energy
Device
Output Energy
electrical
toaster
thermal
chemical
flashlight
electrical, then light
and thermal
electrical
blender
mechanical
chemical
battery-operated
clock
electrical,
mechanical and
sound
Thermocouple: A device that converts
thermal energy to electricity.
– Made of 2 different metals (bimetal) joined
together that conduct heat at slightly
different rates.
– When heated the difference in conductivity
results in electricity flowing from one metal to
the other.
– The higher the temperature difference
between the two metals, the greater the
amount of electricity produced.
A Thermocouple can be used as a
thermometer in extreme high temps. or
difficult to access places.
OTEC
(Ocean Thermal Energy Conversion)
• Scientists are currently
researching ways to use
the Ocean’s natural
thermal energy
differences to generate
electricity.
• Q: what is the temp
difference between the
surface and the bottom?
(p.323)
OTEC
• Now do Check and Reflect P.323 #1-9
Section 3.2
Energy Transformations Involving
Electrical and Mechanical Energy
1820: The Hans Christian Oersted
Demonstration
• A compass needle is deflected by the
magnetic field of a current carrying wire.
• He proved that a current flowing through
a wire creates a magnetic field around
the wire.
1831: Michael Faraday produces first
electric motor, proving that
electromagnetic forces could produce
continuous motion.
Getting Your Motor Runnin’
How a motor works
Types of Magnets
1. Permanent magnet- a hard steel alloy
which remains magnetized for a long period of
time (like fridge magnets)
2. Electromagnet - a coil of wire (usually with
an iron core ) which when attached to a
current has magnetic effects.
Advantage: you can turn it off and on… like
the ones used at car impounds/ wreckers)
Yes, if you wrap a piece of metal in a currentcarrying wire you get…an ELECTROMAGNET!!!!
1. Wrap a piece of metal in wire
2. Hook the wires up to a battery
3. Reversing the wires, reverses the
current AND reverses the polarity of the
magnet.
A simple Electric Motor requires the
interaction of a permanent magnet and an
electromagnet.
Strong electromagnets are made by winding
wire into a coil around an iron core.
How do you keep an electromagnet spinning in
a magnetic field to make the motor run?
Let’s look at the St. Louis Motor…
St. Louis Motor
brushes
Split-ring commutator
Permanent Magnets
how a motor works
armature
Parts of the motor:
• Armature - the rotating shaft with the coil
wrapped around it
• Commutator - a split ring that breaks the flow
of electricity for a moment and then reverses
the flow in the coil, when the contact is broken,
so is the magnetic field
• Brushes - reverse the flow of electricity
through the electromagnetic coil
– make contact with commutator by “brushing”
against it
• The poles of the armature are attracted to
the opposite poles of the permanent magnet
• They spin toward it.
• At 180˚, the contact is broken and the
poles are reversed.
• The split-ring commutator breaks the flow
of electricity for a moment and then
reverses the current flow in the coil
What does changing the polarity do?
• This keeps the motor spinning in one
direction.
• The armature (the rotating shaft with the
coil wrapped around it) continues to spin
because of momentum, allowing the brushes to
come into contact once again with the
commutator.
• The poles keep being reversed as the current
flow is reversed through the coil thus
continuously turning the motor.
Explain the steering wheel analogy to
show how the split-ring commutator
helps the armature spin continuously.
(p.328)
AC or DC Current
DC = direct current
AC = alternating current
Electrons flow one
way
Electrons move back
and forth 60 times/s
Most battery
operated things
use DC
Wall outlets
AC allows electricity to
travel without much loss
of energy
Transformers
Transformers
No! Not those transformers!
Transformers
Power companies generate AC power
because with AC, transformers can be
used to change the amount of voltage
with minimal energy loss.
Some transmission lines carry 500 000 V.
This needs to be reduced before it can be
used in your home.
Transformers: a current-carrying wire is
wrapped around one side of an iron ring
called a core. This is the primary coil.
- A secondary coil of wire is wrapped
around the other side of the core.
- Current flowing through the primary coil
generates an electromagnetic field which
induces a current in the secondary coil.
1. Step-down Transformers - reduce voltage
• More windings on the primary coil.
• Fewer windings on the secondary coil.
2.Step-up Transformers - increase voltage
• Fewer wraps around primary coil
• More wraps around secondary coil.
Generating Electricity
1831: Michael Faraday discovers
electromagnetic induction.
– Electric current can be generated by moving a
conducting wire through a magnetic field.
Today large generators move massive
coils of wire rotating between powerful
magnets to generate enough electricity to
power entire cities.
1. DC Generator - is structurally the
same as a DC motor.
if electricity is passed through a DC
generator, it will spin like a motor.
2. AC Generators
• As the motor turns one side of the coil
moves up between the magnets and the
other side moves down.
• Current is induced in the coil.
• As the generator continues to rotate
current is induced in the coil in the
opposite direction, due to two slip rings.
• This reversal of current every ½
rotation generates alternating current.
• Generator Animation
The central axle of an AC generator has a loop of wire attached
to two slip rings. The current is switched as the loops move up
and down alternatively through the magnetic field. The slip rings
conduct the alternating current to the circuit through the brushes.
Types of Generators:
• Wind
• Hydro-electric
• Steam driven: nuclear, coal, geothermal
and biomass.
• Now Do Check and Reflect P.331#1-4,7-10
3.3 Measuring Energy Input and Output
A. Power - the rate at which we use energy.
Measured in Watts (W)
1 Watt = 1 J/sec.
• Power is calculated by multiplying
current x voltage
P = IV
P =IxV
I =P/V
V=P/I
P = power in watts (W)
I = current in amperes (A)
V = voltage in volts (V)
Example:
An electric stove runs on 240 V and
draws 30 A of current. How much
power does it use?
Example:
• An electric stove runs on 240 V and draws 30
A of current. How much power does it use?
V = 240 V
I = 30 A
P=?
Example:
• An electric stove runs on 240 V and draws 30
A of current. How much power does it use?
V = 240 V
I = 30 A
P=?
P = IV
Example:
• An electric stove runs on 240 V and draws 30
A of current. How much power does it use?
V = 240 V
I = 30 A
P=?
P = IV
= (30 A)(240V)
= 7200 W
What do you suppose it
would cost to run each of
these appliances for one
hour?
(A) This light bulb is
designed to operate on a
potential difference of 120
volts and will do work at the
rate of 100 W.
(B) The finishing sander
does work at the rate of 1.6
amp x 120 volts or 192 W.
(C) The garden shredder
does work at the rate of 8
amps x 120 volts, or 960 W.
B. Energy: The ability to do work. Measured in
Joules (J).
E = Input power x the time device operates
E = Pt
E=Pxt
P=E/t
t =E/P
E = energy in joules (J)
P = power in watts (W) or (J/s)
t = time in seconds (s)
Example
A 800W microwave runs for 3 minutes.
How much energy does it use?
Example
• A 800W microwave runs for 3 minutes.
How much energy does it use?
E = Pt
P= 800W
t= 3min
E= ?
Example
• A 800W microwave runs for 3 minutes.
How much energy does it use?
E = Pt
P= 800W
t= 3min x 60 s/min = 180s
E= ?
Example
• A 800W microwave runs for 3 minutes.
How much energy does it use?
E = Pt
P= 800W
= (800W)(180s)
t= 3min x 60 s/min = 180s = 144000 J
E= ?
or 144 KJ
Kilowatt Hours
• Joules are a very small unit of measure.
• A more common way to measure energy is to
use kilojoules or kilowatt hours.
• The formula for energy remains the same, but
we use KW for power and hours for time.
E = Pt
J=Ws
KJ = KW h
This meter measures the amount of electric work
done in the circuits, usually over a time period of a
month. The work is measured in kWhr.
Energy Dissipation
• Law of Conservation of Energy:
Energy can not be created or destroyed,
only transformed from one form to
another.
• However, the output energy of a device
is almost always less than the input
energy. Why?
• Some of the energy has dissipated as heat
or other forms of unusable energy (like
sound).
• No mechanical system is 100% percent
efficient. Therefore, the output energy will
always be less than the input energy. This
is due to friction.
C. Efficiency Calculations
% efficiency = output energy (J) x 100
input energy (J)
Example:
• A typical gas powered SUV produces
81 KJ of useful output energy for every
675 KJ of input energy (supplied by the
fuel). Calculate the efficiency of the
SUV.
Example:
• A typical gas powered SUV produces 81
KJ of useful output energy for every 675
KJ of input energy (supplied by the fuel).
Calculate the efficiency of the SUV.
Easiest way to solve is to use a ratio
Eout = 81 J = ____
Ein 675 J
100
Example:
• A typical gas powered SUV produces 81
KJ of useful output energy for every 675
KJ of input energy (supplied by the fuel).
Calculate the efficiency of the SUV.
Easiest way is to use a ratio
Eout = 81 J = 0.12 X 100
Ein 675 J
12% efficient
Comparing Efficiencies
• Florescent lights are about 4x more
efficient than incandescent lights.
• Arc-discharge lights (streetlights)
are even more efficient.
• Hybrid gasoline-electric vehicles
are more efficient than gaspowered vehicles.
• Do Check and Reflect P.338 # 1-9
3.4 Reducing the Energy Wasted by
Devices
Devices that have an energy-efficient
design are an important consideration
for the consumer, because these devices
use less electricity.
Energy costs money and it also affects
the environment, so reducing energy
consumption is a good practice.
Limits to Efficiency
Electric heater come very close to being
100% efficient, but devices which convert
electricity to other forms can never be 100%
efficient.
Some energy is lost, or dissipated in a form
that is not useful output. Friction causes
thermal energy to be lost, or dissipated, in
many devices.
Increasing Efficiency
Increasing the efficiency of a device depends
on its purpose.
The easiest way to increase efficiency in
many devices is to reduce friction, as much as
possible.eg. Bearings, lubricants
Insulating a device from heat loss is also
another practical way to increase efficiency.
Using capacitors (formerly condensers) in
electrical circuits is also another way to
increase efficiency.
• Do Worksheet 3.4
• Check and Reflect P.342 #4,5,6
• and Section Review P.343