Transcript S30UnitC1 - MrsSteinbrenner

Unit C: Electromagnetic
Energy
Chapter 1: Electric and Magnetic fields
1.1: Field Lines
Lightning occurs when warm air floats above cool air;
starts to condense and release energy.
As warm air rises, electrons are transferred to rain drops,
creates charge in cloud.
Bottom = Top = +
Lightning
Electrons move away from cloud, cause charge
separation.
Surface becomes positively charged and below the
surface becomes negatively charged.
Lightning strikes when electrons from the cloud are
attracted to the positively charged surface.
Thunderstorms
Why is lightning
dangerous?
A huge amount of electrons are transferred (1018
electrons).
A coulomb of energy = 6.25 x 1018 electrons.
If an object transfers electrons, the charge is
determined in coulombs.
q = -1.00 or +1.00
Lightning video
a) Voltage
Describes the electric potential difference in 2
substances (e.g. A static shock when you walk on
carpet).
Potential energy stored in your finger is
converted to light, sound and energy (why it
hurts).
Determined by:
V = ∆E/q
Where V = voltage. (V)
∆E = change in energy (J)
q = charge (C)
Electricity
Energy and charge
What does 1.5V mean?
1.5J of energy per coulomb.
Voltage is related to energy transferred per coulomb;
if there is a lot of energy, the voltage is high.
Lightning is dangerous because voltage and charge
(coulombs) are large values.
John Travoltage
b) Forces and fields
Basic Principles accepted as fact:
All matter has mass
Energy exists in various forms
Everything in the universe is in motion
Changes in motion are due to forces
Forces act by means of fields
Fields
A field explains why 2 objects affect each other if not
touching.
Fields are spheres of influence.
Are scalar (sound, heat) – no direction.
Campfire… is it the same at all points around the fire?
Why or why not?
Are vectors (gravitational) – has direction.
* Force fields anyone? Trekkies?
1) Electric Fields
A region of space around a charged object
within which any charged object will
experience a force.
Any charged object surrounds itself with a
field.
Forces experienced may be attractive or
repulsive forces.
Unlike charges attract
Like charges repel
Electric Field Applet
2) Magnetic Field
Magnets also create fields.
The forces experienced by magnets are both
attractive and repulsive.
Magnets do NOT create electric fields. They
create magnetic fields. Magnets are not
charged. They have poles!
All magnets have a NORTH and a SOUTH
POLE.
Two like poles will repel each other.
Two unlike poles will be attracted to each other.
What makes some metals magnetic?
Some metals called “Ferromagnetic substances” have
strong magnetic properties. Examples: Iron, Cobalt, Nickel
Inside these magnetic substances are “tiny magnets” called
DOMAINS.
If the domains are aligned the material is magnetized.
If the domains are unaligned the material is not magnetized.
Drawing Magnetic Fields
The electric field created by a magnet is away
from the north pole and towards the south pole
as below.
fields
Uses of Magnets:
Navigation
generating electric currents
motors
3) Gravitational Fields
The modified space around a mass is a
gravitational field
2 masses exert gravitational forces on each other
Isolated masses also modify the space around
themselves
Any object having mass creates a gravitational
field around itself
Field always points towards the most massive
object!
The Gravitational Field
c) Field lines
Describe the direction of a field (arrow towards
centre) and amount (more arrows = stronger field).
Can be used for all types of fields:
Magnetic- always N to S.
Gravitational– always toward object.
Electric- towards (-) and away from (+).
Which is (+)? Which is (-)?
Compass
1.2) Equations for fields
Fields can be calculated using equations when an
exact value is needed.
A field will usually be calculated using a test body (an
object that is put in the field).
a) Gravitational Field (g)
The force per unit mass that an object would experience if
placed in a gravitational field.
As distance from source increases, field strength decreases.
Calculated using:
Gm1
g 2
r
where:
g = gravitational field (N/kg)
G = gravitational constant (6.67 x 10-11 Nm2/kg2)
m = mass of source (kg)
r = distance from centre of source (m)
Example
1.
What is the gravitational field strength of Earth 20
km above the surface of the Earth?
Gravitational Force (Fg)
The mass on an object created by the field.
Calculate using:
Fg = mg
Fg = force of gravity (N)
m = mass of object (kg)
g = acceleration due to gravity (m/s2).
Example
The moon has an average radius of 1740 km and a
mass of 7.35 x 1022 kg.
Calculate the gravitational field strength of the moon.
If an astronaut with a mass of 100kg was on the moon,
what is the force of gravity on the astronaut?
b) Electrical Field
Strength (E)
The force per unit charge that an object would
experience if placed in an electric field.
How strong the electric field is of ONE object with an
electrical charge.
Determined using:
Where: E = electric field strength (N/C)
k = constant ( 8.99 x 109 Nm2/C2)
q = charge on source (C)
Example
1. Electrons are scraped from your hair when you comb it, and
your comb develops a charge of -4.0 x 10-6C. Find the
electrical field strength 0.55 m from the comb.
Electrical Force (Fe)
The force on an object created by an electrical field.
Calculated using:
Fe = Eq
Where: Fe = electrical force (N)
E = electric field (N/C)
q = charge on object (C)
Example
1.
The centre of one object (charge = +0.0200C) is
20.0m away from the next. Calculate the force of
the 1st object on the second (the Electrical field on
the planet is 449 500 N/C).
c) Moving Charges and magnetic
fields
Moving charges (a current of electricity) create a
magnetic field.
Created using a coil of wire; 1 end is considered to be
North and 1 South.
Electrons move from N to S; causes spinning motion.
Lots of electrons moving and lining up = magnets!
magnets
Electromagnets
Deflection currents
An electron can be deflected
around an object using a
magnetic field.
This is the basis for the NASA
design to protect the moon
base.
Electromagnetic Induction
Astronauts in Space
Solar wind: ionized particles (electrons, protons,
helium nuclei) ejected from the sun at high speeds
Cosmic rays: ionized particles (atomic nuclei) ejected
from objects beyond the solar system at high speeds
Can break both strands of DNA
Northern Lights
Solar wind particles
collide with Earth’s
atmosphere
Nitrogen - purple
Atomic 02 - red and
green
1.3) Motors and
Generators
How do we use electrical energy? Does it need to be
converted first?
Electrical energy can not be seen; it is not easily
studied due to this.
Electrical energy is converted into mechanical energy
(moving energy) and then studied.
a) Motors
Invented by Oersted
Input Energy = Electric
Output Energy = Mechanical
How it works:
An electric current flows through a
wire
This creates a magnet
The magnet is attracted to another
part of the machine and it moves
Example: Doorbell
Parts of a Motor
Armature: spinning part of motor (shaft, wire coil,
commutator).
Commutator: provides electrical contact; current
flows to coil.
Shaft: supports coil, provides axis of rotation.
Brush: stationary part that connects with rotating
commutator.
1.
How a motor works
An electrical current creates a
magnetic field; the N of the wire is
attracted to the S of the magnet =
rotation (half-turn).
2. Brushes and commutator touch, reestablish current. Current reverses
and coil continues to spin (N of
magnet and N of coil are at top).
3. Reaches beginning and repeats.
b) Generators
Invented by: Faraday
Input Energy = Mechanical
You have to move something!
Move a magnet through a coil of wire OR Move a coil
of wire in a magnetic field
Result: an electric current is created
Ie. Output Energy = Electric
generators
From motor to
generator
Problem: current only
flows when
commutator connected
to brushes.
Solutions:
Wrap more wire
around core (increase
current).
Split-ring design
(increase contact
time).
AC Vs. DC
DC = Direct Current
electrons flow in a continuous loop in ONE DIRECTION only
Example: from a battery
Can NOT be used with transformers
AC = Alternating Current
Electrons flow in one direction in the circuit and then they flow
the other way
Example from a generator
Can be used with transformers
AC
DC
AC generators
DC generators
Why AC?
AC generators are used to power all household
appliances.
To maximize the energy output:
1.
2.
3.
4.
Increase # of turns in wire.
Spin armature as fast as possible.
Use strong magnets.
Use iron core inside wire.
1.4- Electric Circuits
Circuits are used to transport electricity to an object.
You can measure Current (I) using an ammeter; Voltage
(V) using a voltmeter.
There are 2 types of circuits:
Series- only one path for electricity to flow.
Parallel- more than 1 path for electricity to flow.
Multimeters and
circuits
Electric Circuits
Generating current in
circuits
Electric fields exist around any charged object and between
charged objects
Conductors permit the movement of electrons
If you have electric conductors in contact with oppositely
charged poles of a battery, electrons will move from the
negative pole to the positive pole.
The movement of electrons is called an electric current.
Current is the amount of electrical charge moving past a
point in a specific time.
Any continuous set of conductors or a network of
conductors is called a Circuit.
a) Resistance
A resistor is anything that reduces the current
flow in a circuit.
Used to regulate the speed of a motor or ensure
safe limits are maintained.
Resistance is the ratio of voltage to current.
R = V/I
Where: R = resistance (ohms) (Ω)
V = voltage (Volts) (V)
I = current (Ampere) (I)
Examples
A car headlight uses a current of 5.0A from the car’s
12.0 V battery.
Is this a DC or AC source?
What is the resistance of the headlight when it is on?
Resistance
Factors that affect
resistance
cross-sectional area of a conductor
bigger cross section = less resistance
the longer the conductor the greater the
resistance
temperature: temperature increases as
resistance increases
Measuring resistance
Use a multimeter, set dial to ohmmeter.
To use:
Set dial to ohmmeter.
Attach leads (red = +, black = - ).
Switch off power to circuit.
Connect meter and measure value.
b) Creating circuits
Schematic diagrams are used to
represent circuits; key components
are indicated using symbols.
You must memorize these!
Pg. 374
1) Series circuits
When cells are arranged in series, energy output is
increased.
If 1 bulb fails then the whole circuit fails (eg.
Christmas lights).
Total voltage is determined by:
VT = V1 + V2 + V3 …
Total resistance is determined by:
RT = R1 + R2 + R3 …
2) Parallel circuits
Cells (batteries) last longer than in series.
Connected with more than 1 path for the
electrons to flow; can turn on/off single parts of
the circuit.
Current increases as more paths added;
resistance is halved.
Total voltage is identical:
VT = V1 = V2 = V3 …
Total resistance determined by:
1 1 1 1
RT = R1 + R2 + R3 …
Electric
Circuits
Circuit Builder
Voltage and current
c) Ohm’s law
V  IR
V = Potential Difference / Voltage (V)
I = Current (A)
R = Resistance ()
Parallel Example
Two different
Multiloop Circuits
Series Example
Please see example problems on pages 379-381.
d) Combined circuits
In all practical circuits, both
parallel and series circuits
are used.
Kitchen devices are
connected in parallel- (you
can turn off the blender but
not the fridge).
Each device has a series
circuit used to switch it on
or off.
circuits
1.5) Transmitting electrical energy
All electrical devices use energy; that energy needs
to be transported to the device.
Every device has a power rating = energy consumed
per second.
Determined using:
Power (W) = Energy (J) /time (s)
Power in electrical
systems
Power rating describes the maximum safe input of
electricity to a device.
Equation can be reworked so that:
P=IV
Where:
P = power (Watts)
I = Current (Amps)
V = Voltage (Volts)
Speakers
An AC device that works only if the
current changes directions.
Coil supplies the resistance to the circuit.
Example:
Calculate the voltage applies across the
speaker. ( R = 4.0Ω, I = 2.00A).
Calculate the Power consumed by the
speaker.
a) Power
In many cases Power needs to be calculated when
Voltage is unknown.
Use the following formula:
P = IV and V = IR so….
P = I2R
Where:
P = Power (Watts)
I = Current (Amps)
R = Resistance (Ohms)
Example
The volume is turned up in a car with a 4.0Ω resistor
so that 4.50A of alternating current flows to the
speaker. Calculate the Power consumed by the
speaker.
b) Billing energy
Energy companies bill for the electricity used
through meters.
Unit of energy used by companies = kilowatt hour
(kWh).
Formula used to determine:
E = Pt
Where: E = Energy (kWh)
P = Power (kW)
t = time (h)
Las Vegas uses 400,000
kWh per year =
$40,000,000.
kilowatt Hour
calculations
A lava lamp rated at 1000W operates for 60.0min in a typical
week.
Calculate the energy consumed in kWh.
Calculate the energy in Joules (hint, put time in seconds).
If the price of electricity is 9.3¢ kWh, how much does it cost
per year?
c) Power Transmission
P = IV
To transmit lots of power we need either a high
voltage or a high current
High voltage is dangerous but . . .
If current is high in power lines lots of electrical
energy is lost as heat.
To transmit power efficiently we need to use
unsafe high voltages.
Solution: Use AC current and Transformers!
What is one advantage of AC over DC current?
Summary of Power
Transmission
Power Generation
Transformers
step up voltage
Transport of
electricity can use
a low current and
a high voltage
which is efficient
Transformers
step down voltage to a safe level before the
electricity enters homes
d) Transformers
2 coils of wire:
Primary:
receives input voltage from source.
Current and voltage remain in coil because it is insolated.
Secondary:
primary coil creates current and voltage.
supplies output voltage.
Primary and secondary coils are not connected;
secondary coil is affected only if current is
changing (AC).
Changing voltage
Transformers can change the voltage of an
electrical supply by:
stepping it up (increasing it)
stepping it down (decreasing it)
step up transformers: more secondary coils than
primary coils
step down transformers: fewer secondary coils
than primary coils
Np = Vp
Ns
Vs
* See page 3 in data
book
Which transformer steps
up voltage?
What happens to current when voltage is stepped up?
Hint: Power stays the same!
Transformer videos
Transformer Calculations
Find the formulas to use with transformer problems in
your data booklet
Example: A transformer has 50 primary coils and 72 turns
in the secondary coil. If the voltage going to the
transformer is 2250 V, what is the voltage leaving the
transformer?
If the current leaving the transformer above is 50 A, what
was the current as it entered the transformer?
e) Ideal Transformer
An ideal transformer is one that does not lose
energy.
Uses the formula: