Transcript Electrical Energy and Magnetism

Electrical Energy
and
Magnetism
What is a Electricity?
Electricity- the energy associated with
charged particles as they move from place to
place
While these particles are usually electrons,
any charged particles will do
Charges Exert Forces
Electric field- a region surrounding every electric
charge in which a force of attraction or repulsion is
exerted on the other electric charges
Electric field arrows point away from positive
charges and toward negative
Electric Charge
Electric charge- a
property of electrons (-)
and protons (+)
When particles with
these charges come
near one another, a
force of attraction or
repulsion results
Like charges repel and
opposites attract
Electric Charge
Electrons in atoms are
bound more tightly to some
atoms than others
Substances made of atoms
with loosely bound electrons
can transfer the electrons to
substances they come into
contact with
Remember that electrons
are negatively charged
Protons are stuck in the
nucleus
Charge Can Be Transferred By:
1.) Contact: an object with an excess of electrons
touches or rubs a neutral object, electrons are
passed to the neutral object
2.) Induction: the rearrangement of electrons on a
neutral object caused by a nearby charged object
without contact
Contact vs. induction simulation
Transferring Charges
What causes you to be shocked when you rub
your feet across carpet?
The passing of an electrons through the air from
a negatively charged object to a positively
charge object causes current
Transferring Charges
The Van de Graaf
generator deposits
electrons on the ball,
which travel through
the person’s hand
touching it
Body and hair
temporarily get a
negative charge
What up with the hair?
Static Electricity
Static electricity- the
accumulation of excess
stationary electric charge
on an object
Negative charge in cloud
builds up and repels that
of the ground
The actual flash of
lightning you see is the
mass of positive ions from
the ground surging
upward to meet the
negative particles
Lightning video
Static Electricity
An electroscope detects static charges
Electric Current
Electric current- the net
movement of electric charges in
a single direction; represented
by “I” and measured in
amperes
In a metal wire without current,
the electrons are in constant
motion in all directions
Stereos, lights, etc. work when
electric current is through them
A voltage difference is what
causes electric current
Kinetic energymovement!
Static vs. Current Electricity
Static electricity is stationary or collects on the
surface of an object, whereas current electricity
is flowing very rapidly through a conductor
The flow of electricity in current electricity has
electrical pressure or voltage
Voltage Difference
Voltage difference- related to the electrical force that
causes charges to flow; represented by “V” and
measured in volts
The direction of the current is always from higher
voltage (+) to lower voltage (-) but the electrons travel
in the opposite direction
Current and voltage applet
Batteries
Batteries are composed of a
chemical substance which can
generate voltage
There are two kinds of batteries:
dry cell and wet cell batteries
This is an example of a dry cell
and is called a dry cell because
the electrolyte is a moist paste,
and not a liquid
One electrode is the carbon rod,
and the other is the zinc
container
Batteries
Wet cell batteries are most
commonly associated with
automobile batteries
A wet cell contains two
connected plates made of
different metals or metallic
compounds in a conducting
solution
Electric Circuit
Electric circuit- a closed path that electric current
follows
In a battery, there is a voltage difference between
the two terminals (ends)
Negatively charged electrons flow from the negative
terminal of a battery to the positive terminal and the
current goes the opposite way
Resistance
Resistance- the tendency for a material to resist the
flow of electrons to convert electrical energy into
other forms, such as thermal; represented by “R”
and measured in Ohms (Ω)
Almost all materials have some resistance
Resistors in electronics are used to reduce current
through all or part of a circuit
Resistance
The metal which makes
up a light bulb filament
has a high electrical
resistance which causes
light and heat to be given
off
What Influences Resistance?
Material of wire
Thickness – the thicker the wire the lower the
resistance
Length – shorter wire has lower resistance
Temperature – lower temperature has lower
resistance because the atoms of the wire are not
moving as much and interferring with flow
Conductors vs. Insulators
Conductor- a material
through which electrons
move easily because the
electrons are not held
tightly by the atoms
 Metals, skin, solutions
Insulator- a material in
which electrons are not
able to move easily
because the electrons are
held tightly by the atoms
 Plastics and glass
Ohm’s Law
Ohm’s Law states that the voltage difference (V),
current (I), and resistance (R) in a circuit are
related
Current (amps) = voltage difference (volts)
resistance (ohms)
I=V
R
Ohm’s Law Interactive
Check for Understanding
Ohms
4
15
2
9
6
Volts
100
150
30
45
48
Amps
25
10
15
5
8
Types of Current
Alternating current (AC)the kind of current from
household electrical
outlets in which the
current changes
direction (120 times/sec
in the U.S.)
Direct current (DC)- the
kind of current that
battery powered devices
use in which the current
doesn’t change direction
AC
200
150
Voltage (V)
100
50
0
0
0.005
0.01
0.015
0.02
-50
-100
-150
-200
Time (s)
0.025
0.03
0.035
0.04
DC
Electric Circuits
There are two types of circuits:
 Series circuit- the components are lined up
along one path. If the circuit is broken, all
components turn off
Electric Circuits

Parallel Circuit – there are several branching
paths to the components. If the circuit is
broken at any one branch, only the
components on that branch will turn off
Electric Circuits
Electrical energy enters
the home usually at a
breaker box or fuse
box and distributes the
electricity through
multiple circuits
A breaker box or fuse
box is a safety feature
Household Circuits
Most household wiring is logically designed with a
combination of parallel circuits
Electric Power
Electric power- the rate at which electrical energy
is converted into another form of energy
Electrical power is the product of the current (I)
and the voltage (V)
The unit for electrical power is the same as that
for mechanical power – the watt (W)
Electrical power = current (amps) x voltage (V)
P =IV
Check for Understanding
How much power is used in a circuit which is
110 volts and has a current of 1.36 amps?
Check for Understanding
How much power is used in a circuit which is
110 volts and has a current of 1.36 amps?
P=IV
Power = (1.36 amps) (110 V) = 150 W
Electric Energy
Electric companies provide your house with
electrical energy that you can then transform into
other forms of energy….thermal (toaster),
mechanical (washing machine), etc.
Electric energy = electrical power (kilowatts) x
time (h)
E=Pxt
Check for Understanding
A microwave has a power rating of 1000 Watts
and the family uses it for 30 minutes a day. How
much electrical energy has this microwave
used?
Check for Understanding
A microwave has a power rating of 1000 Watts
and the family uses it for 30 minutes a day. How
much electrical energy has this microwave
used?
Electric energy = electrical power (kilowatts) x
time (h)
E=Pxt
1000 W = 1 kW and 30 minutes = 0.5 h
E = 1 kW X 0.5 h = 0.5 kWh
Check for Understanding
Super challenge! Put it together!
The current flowing through an appliance
connected to a 120 V source is 2 A. How many
kilowatt-hours of electrical energy does this
appliance use in 4 hours?
Check for Understanding
Super challenge! Put it together!
The current flowing through an appliance
connected to a 120 V source is 2 A. How many
kilowatt-hours of electrical energy does this
appliance use in 4 hours?
P = I V = 2 A x 120 V = 240 Watts = 0.240 kW
E = P x t = 0.240 kW x 4 h = 0.96 kWh
Electric Energy
Kilowatt-hours are read on an electric meter and you
are charged for each kilowatt-hour by the electric
company
It’s the total number of kilowatts used by all
appliances times the total number of hours those
appliances are used
Magnets
More than 2,000 years ago
Greeks discovered
deposits of a mineral
called magnetite
In the twelfth century
Chinese sailors used
magnetite to make
compasses that improved
navigation
Today, the word
magnetism refers to the
properties and interactions
of magnets
Characteristics of Magnets
Every magnet has two poles
(N and S) and is surrounded
by a magnetic field
This field exerts a force on
other magnetic material
Magnetic poles are where
the magnetic force exerted
by the magnet is strongest
Depending on which ends of
the magnets are close
together, the magnets either
repel or attract each other
What else do you know of that
has a North and South Pole?
If you answered Earth you
were right! It is one giant
magnet with two magnetic
poles surrounded by a
magnetic field
This field is what causes a
compass needle to point in
different directions the
poles of a magnet to point
either north or south
N and S poles do move in
time
Magnetic Field Direction
A magnetic field also has
direction shown by the arrows
The north pole of a compass
points in the direction of the
magnetic field
This direction is always away
from a north magnetic pole
and toward a south magnetic
pole
Magnetic Materials
All metal objects are not
attracted by magnets
Only a few metal (iron, cobalt,
and nickel) are attracted to
magnets or can be made into
permanent magnets
Every atom contains electrons
and these electrons have
magnetic properties
These magnetic properties
don’t cancel out in magnetic
materials
Magnetic Domains
Atoms that have magnetic fields
can exert a force on other nearby
atoms
This means all the N magnetic
poles in the group point in the
same direction
These groups of aligned atoms are
called magnetic domains


Behave like magnets with a N and S
pole
Contain an enormous number of
atoms, yet the domains are too small
to be seen with the unaided eye
Permanent Magnets
A permanent magnet can be made by placing a
magnetic material, such as iron, in a strong
magnetic field
The strong magnetic field causes the magnetic
domains in the material to line up
The magnetic fields of these aligned domains
add together and create a strong magnetic field
inside the material
This field prevents the constant motion of the
atoms from bumping the domains out of
alignment. The material is then a permanent
magnet
•
How are Electricity and Magnets
Related?
By the 1700s, scientists
everywhere had made many
advances in both electricity
and the study of magnetism
They then wondered if
there was a possible link
between the two fields.
The first to discover such a
connection was a Danish
physicist, Hans Christian
Oersted (1777-1851).
Magnets and Electricity
It is now known that moving
charges, like those in an
electric current, produce
magnetic fields
The direction of the
magnetic field around the
wire reverses when the
direction of the current in
the wire reverses
As the current in the wire
increases the strength of
the magnetic field increases
Magnets and Electricity
Magnets are used to
generate, or produce,
electricity and vice versa
When a conductive wire is
wrapped around a piece of
metal (like a nail) and
current is run through it,
magnetism can be induced
in the metal
This is called
electromagnetic induction
Electromagnets
Electromagnet- a
temporary magnet made
by passing electric current
through a wire coiled
around an iron bar
Can see them in junkyards


A crane holding a huge
electromagnet can be used
to pick up scrap metal when
current flows through it
When the crane operator
wants to drop the scrap, they
shut off the current
Generators
Generators also use electromagnetic induction as
they transform mechanical energy into electrical
energy


An outside source (hands, wind, fossil fuels) provide
mechanical energy that causes the turbine to rotate
The rotating turbine rotates the coil of wire past the
magnet, inducing an electrical current
Motors
Motors also use electromagnets, but they convert
electrical energy into mechanical energy
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

An outside source like a battery or plug provides electrical
energy and the current creates an electromagnet
The magnet rotates due to the attractive and repulsive
forces between the coil and the magnet causing the shaft
to rotate
See Fig 21
Sound
Sound from a CD is
produced by a loudspeaker
that contains an
electromagnet connected
to a flexible speaker cone
that is usually made from
paper, plastic, or metal
The electromagnet
changes electrical energy
to mechanical energy that
vibrates the speaker cone
to produce sound
Electricity for Your Home
Electrical energy comes from a power plant with
huge generators
The coils in the generators have many coils of
wire wrapped around huge iron cores
The rotating magnets are connected to a
turbine- a large wheel that rotates when pushed
by water, wind, or steam
These rotating magnets create electric current
Hoover Electric Dam
Electricity for Your Home
Some power plants first
produce thermal energy by
burning fossil fuels or using the
heat produced by nuclear
reactions
This thermal energy is used to
heat water and produce steam
Thermal energy is then
converted to mechanical
energy as the steam pushes
the turbine blades to create
electricity