Transcript modeling and measuring electricity

Unit D - Electricity
Week 2
MODELING AND MEASURING
ELECTRICITY
I can…
• Measure voltage and amperages in circuits
and calculate resistance using Ohm’s Law
(V=IR)
• Read Paragraph and look at the pictures of the
different waterfalls
• In your notes, write how currents can be
compared to a Waterfall.
• Write how water flow can be used to describe
resistance
Types of resistors
• There are many types of resistors for different things
ex. Electronics in a TV
• Why would we add a resistor (that doesn’t use the
energy usefully) into a current?
• The most common types of resistors are wire-wound
and carbon-composition
• They vary from 0.1 Ω to 200 kΩ
4 factors affecting resistance in wire
• Factor
• 1) Length
Effect
-Resistance
INCREASES with
length
- So: if the length
doubles so does the
resistance
• Factor
• 2) Cross sectional
area
Effect
- Resistance
DECREASES
with area
- So, if the
cross sectional
area doubles
the resistance
is half as great
• Factor
• 3) Temperature
Effect
-As the temperature
of wire increases
the resistance
INCREASES
• Factor
• 4) Material
Effect
- Some metals allow
electrons to move
more freely than
others
Measuring Electricity
• There are 2 ways that we measure electricity:
–
–
–
–
–
–
1) Amperes: how many electrons going by
Units: Amps (A)
Measured using: ammeter
2) Voltage: how much energy each electron has
Units: Volts (V)
Measured using: voltmeter
– Extra tool: Galvanometer- measures volts and amps
More about voltage
• Voltage is also called potential difference
• Named after Alessandro Volta
• Some common voltages
– Flashlights- 6 V or less
– Car Batteries- 12 V
– Wall Sockets- 120 V
– Major Power Lines- 100 kV
Measuring Amps and volts
How Many Volts are there in a…
• D Battery
• AA Battery
Notes- when using a voltmeter you must attach the
red terminal to the positive side of the battery and
attach the black side to the negative side of the
batter
• AND: sometimes you have to change the readings
from mV to V
Ohm’s law
• George Simon Ohm experimented with different
substances, and found a relationship between
voltages, current and resistance
• He found that if the temperature remained the same
then
– 1) The resistance of a conductor stays constant
– 2) Current is directly proportional to the voltage
applied
• So… if you increase the voltage, you increase the
current too and vice versa
The math of ohm’s law
•
•
•
•
V= I x R
V= voltage  volts (V) (voltmeter)
I= current  amps (A) (ammeter)
R= resistance  ohms (Ω) – (ohmmeter)
Steps to solving ohm’s law questions
•
•
•
•
1) Identify the known quantities
2) Identify the unknown quantities
3) Rearrange the formula if necessary
4) Fill in the numbers for the variable and
solve
Steps to solving ohm’s law questions
Example 1
• 1) A motor has an internal resistance of 40 Ω.
The motor is in a circuit with a current of 4.0
A. What is the voltage?
Example 2
• A 30 V battery creates a current through a 15
Ω resistor. How much current is created?
Example 3
• An electric stove is connected to a 240 V
outlet. If the current flowing through the
stove is 20 A, what is the resistance of the
heating element?
Example 4
• A hair dryer takes 1200 mA to run and it gives
a resistance of 40 Ω. What voltage is
necessary?
To do:
• Ohm’s Law Worksheet- To hand in
Assignment Questions
• How much current is created if a 12 V battery passes current
through a 10 Ohm resistor?
• An electric shaver that is plugged into a 110 V wall outlet has a
resistance of 6,000 Ohms. How much current flows through this
device?
• A current of 12 A flows through a vacuum cleaner that is plugged
into a 120 V household circuit. What is the total resistance of the
vacuum cleaner?
• A current of 2.0 Amperes flows through a resistor rated at 30 Ohms
connected to a battery. What is the voltage of the battery?
• An electric garage heater draws 30 A from a special 220 V circuit
designed for heavy duty heaters. What is the heaters resistance?
Review
• What are the four factors that affect
resistance?
ENERGY FORMS AND
TRANSFORMATIONS
I can…
• Describe energy conversions (mechanical,
chemical, thermal (heat), electrical) in
household appliances and energy transport
systems (power generating station to your
home)
Think about it!
• What types of energy do we know of?
4 common forms of energy
• 1) Chemical Energy- the energy stored in
chemicals. This is a form of potential or stored
energy. This energy is released when
chemicals react
• Draw or write 3 things that contain chemical
energy
• 2) Electrical Energy- the energy of charged
particles. Electrons are negatively charged.
Electrical energy is transferred when electrons
travel from place to place
• Draw or write 3 things that contain electrical
energy
• 3) Mechanical Energy- the energy possessed
by an object because of its motion or its
potential to move
• Draw or write 3 things that contain
mechanical energy
• 4) Thermal Energy- the total kinetic energy of
all particles in a substance (its temperature).
The faster a particle moves, the more kinetic
energy it has.
• Draw or write 3 things that contain thermal
energy
Transformations involving chemical
and electrical energy
• There are many ways to convert energy forms into the other forms
• Using this table, come up with devices that you use every day that follow
the same energy transformation
• The Law of Conservation of Energy – Energy can not be created or
destroyed, but can change from one form to another.
(EXAM Question!)
Input Energy
Device (Converter)
Output Energy
Electrical
Thermal
Chemical
Electrical, light and
thermal
Electrical
Mechanical
Chemical
Electrical,
mechanical and
sound
Lets read!
• What is a thermocouple?
• When is a thermocouple useful?
• What energy conversions do thermocouples do?
• What energy conversion to heaters and ovens do?
ENERGY TRANSFORMATIONS
INVOLVING ELECTRICAL AND
MECHANICAL ENERGY
I can…
• Explain the parts in a motor and generator
and suggest how to modify these parts to
increase output.
• Predict the effect of changes in the orientation
and placement of magnets, commutator and
armature in a St.Louis motor or in a
personally-built model of a motor.
Introduction
• In 1820 Hans Christian Oersted discovered
that a wire with a current running through it
created a magnetic field
• Michael Faraday used the connection
between electricity and magnetism to make
the first motor in 1831
• However, he used an open container or
mercury in his design. Why wouldn’t we used
that today?
Electromagnets
• Electromagnets can be created by winding a wire
into a coil
• An electromagnet is only a magnet when electricity
is run through the wire
• It is possible to change the poles of the
electromagnet by running the electricity the other
way
• How is that different from a permanent magnet?
• An electromagnet will line up in a magnetic field
made from permanent magnets
Electric motors
• An electromagnet will line up in a magnetic
field made from permanent magnets
• How can we keep an electromagnet moving
within an electrical field?
• The N and S of the electromagnet needs to be
switched every half turn so that the ends
never align and there is always attraction to
somewhere new– called changing polarity
Parts of an electric motor
• A commutator and brushes are used to
reverse the flow of electricity through the
electromagnet core
• The commutator is a split ring that stops the
flow of electricity for a brief moment so that
magnetic force can flip
• The armature keeps spinning during that time
because of momentum
• What is momentum?
• The commutator keeps spinning and
reconnects to the brushes (usually made of
carbon bars) and therefore the motor will
keep spinning
• There brushes touch the commutator by
brushing against it
Direct and alternating current
• Direct current (DC) only flows in one direction
• Many devices such as cell phones and computers use
DC
• Alternating current (AC) flows back and forth 60
times a second
• AC current flows into your house
• Devices that need DC often come with their own
power supply
Transformers
• Power companies use AC because it can be
transformed into different voltages without
energy loss
• Power comes from generating stations at a
high voltage and is transformed to a lower
voltage through a transformer
• More coils mean more voltage- we can “step
up” to higher voltage or “step down” to a
lower voltage
Generating electricity
• Michael Faraday also discovered that he could
create an electrical current by moving a wire
through a magnetic field (the motion created
the electricity)
• A hand-held generator moves a coil of wire in
permanent magnets and creates a current
• The faster you turn the more current is made
Generating in real life
• Generators are machines that convert energy
from one form to another
• Most electric power stations use generators
– How would hydroelectric dams use a generator?
– How would a wind turbine use a generator?
– How would a coal burning plant use a generator?
Let’s read!
• About Generating AC and DC in the
paragraphs in your book
MEASURING ENERGY INPUT AND
OUTPUT
I Can…
• Apply appropriate units, measures and
devices in determining and describing
quantities of energy transformed by an
electrical device. P=IV, E = Pt
• Compare energy inputs and outputs of a
device, and calculate its efficiency.
Power
• Power is the rate at which a device converts
energy
• Measured in watts which is equal to J/s
•
•
•
•
P=IxV
P= power (watts)
I= current (amps)
V= voltage (volts)
Steps for solving power problems
•
•
•
•
1) Identify known quantities
2) Identify unknown quantities
3) Rearrange the formula if necessary
4) Fill in the numbers and solve the problem
Example 1
• If a blender uses 40 V when there is a current
of 3.4 A, what power did it use?
Example 2
• A hair dryer has a power rating of 1000 W. It is
plugged into a 120 V outlet. What is the
current flowing through the hair dryer?
Example 3
• A remote control car used 345 mV and had a
power rating of 560 W. How much current
runs through it?
Energy
• In order to discover how much energy a
device uses we need to know how long it uses
power for
• The formula for energy is
• E=Pxt
• E= energy (Joules- J)
• P= power (W)
• t= time (seconds)
Steps for solving energy problems
•
•
•
•
1) Identify known quantities
2) Identify unknown quantities
3) Rearrange the formula if necessary
4) Fill in the numbers and solve the problem
Example 1
• A microwave oven has power rating of 800 W.
If you cook a roast in this oven for 30 minutes
on high, how many Joules of electrical energy
are converted by the microwave?
Example 2
• If 6200 kJ of energy are used to run a TV for a
week and the power rating of a TV is 300 W,
how many seconds, minutes and hours was
the TV on for?
Kilowatt hours
• A kilowatt hour is the unit used to measure
how much energy households or businesses
use- usually in a hour and then have to pay for
• To find kilowatt hours you use the same
formula
• E(kWh)= P(kW) x t(h) but use kilowatt instead
of watts and hours instead of seconds
Example 1
• A microwave oven has a power rating of 800
W. If you cook a roast in this oven for 30
minutes on high, how many kilowatt hours did
it use during that time?
Example 2
• A CFL light bulb that uses 25 watts of power is
used for 125 hours. It costs 15 cents/kWh to
run one light bulb. Calculate the kilowatt
hours that the light bulb uses, then calculate
how much it will cost to run the light bulb.
Energy Dissipation
• The Law of Conservation of Energy says:
• However, the output energy of an energy
transformer is always less than its input
• Where does the energy go?
Example
• You create an electric pump with a motor to
pump water from an irrigation ditch to irrigate
a crop. You find that the motor used 100 kJ of
electrical energy for every 75 kJ of work done
raining water to the field. Where did the other
25kJ go?
Understanding efficiency
• The efficiency of a device is a ratio of useful
energy to the total energy that went in
• The more useful energy that comes out the
more efficient the device is
• We use percent efficiency
• Percent efficiency = joules of useful output
joules of input energy
x100
Example:
• Calculate the percent efficiency of an
incandescent light bulb that produces 2 500 J
of light energy from 50 000 J of electrical
energy.
Comparing efficiencies
• We can compare efficiencies of devices which
will tell us about their energy cost and their
environmental impact
• New technologies often help make devices
more environmentally friendly and make them
more efficient
Videos
• https://www.youtube.com/watch?v=ByCOTG2
-mhg 90 seconds!
• https://www.youtube.com/watch?v=H5s1ia50
-aw