Transcript Document

•Electricity
The importance of electrical power seems obvious in a
modern industrial society. What is not so obvious is the
role of electricity in magnetism, light, chemical change,
and as the very basis for the structure of matter. All matter,
in fact, is electrical in nature, as you will see.
• Electric Charge
• Electron Theory of Charge.
– Electric Charge and Electrical Forces.
• Electrons have a negative electrical charge.
Joseph J. Thomson
• Protons have a positive electrical charge
• These charges interact to create an electrical force.
– Like charges produce repulsive forces.
– Unlike charges produce attractive forces.
A very highly simplified model of an atom has most of the
mass in a small, dense center called the nucleus. The
nucleus has positively charged protons and neutral
neurons. Negatively charged electrons move around the
nucleus a much greater distance than is suggested by this
simplified model. Ordinary atoms are neutral because
there is a balance between the number of positively
charged protons and negatively charged electrons
– Electrostatic Charge.
• Electrons move from atom to atom to create ions.
– positively charges ions result from the loss of
electrons and are called cations
– Negatively charge ions result from the gain of
electrons and are called anions
(A) A neutral atom has
no net charge because
the numbers of electrons
and protons are
balanced. (B) Removing
an electron produces a
net positive charge; the
charged atom is called a
positive ion. (C) The
addition of an electron
produces a net negative
charge and a negative
ion.
Arbitrary numbers
of protons (+) and
electrons (-) on a
comb and in hair
(A) before and (B)
after combing.
Combing transfers
electrons from the
hair to the comb by
friction, resulting
in a negative
charge on the comb
and a positive
charge on the hair
• The charge on an ion is called an electrostatic charge.
• An object becomes electrostatically charged by
– Friction ,which transfers electrons between two
objects in contact
– Contact with a charged body which results in the
transfer of electrons
– Induction which produces a charge redistribution
of electrons in a material
– Charge is transferred in all three cases, it is not
created or destroyed.
Charging by induction. The comb has become charged by
friction, acquiring an excess of electrons. The paper (A)
normally has a random distribution of (+) and (-) charges.
(B) When the charged comb is held close to the paper,
there is a reorientation of charges because of the repulsion
of the charges. This leaves a net positive charge on the
side close to the comb, and since unlike charges attract,
the paper is attracted to the comb
– Electrical Conductors and Insulators.
• Electrical conductors are materials that can move
electrons easily
– Good conductors include metals.
• Electrical nonconductors are materials that do not
move electrons easily
– These are also known as insulators
• Semiconductors are materials that vary in their
conduction and nonconduction, sometimes conducting
sometimes not conducting.
• Measuring Electrical Charges.
– The magnitude of an electrical charge is dependent upon
how many electrons have been moved to it or away from
it.
– Electrical charge is measured in coulombs.
• A coulomb is the charge resulting from the transfer of
6.24 X 1018 of the charge carried by an electron
• A very large amount of charge
• A lightning discharge may transfer 200 coulombs of charge
• A charged comb is less than 1 microcoulomb
– The fundamental charge is the electrical charge on an
electron and has a magnitude of 1.6021892 X 10-19 C
– To determine the quantity of an electrical charge you
simply multiple the number of electrons by the
fundamental charge on an electron or:
• q=ne
• Where q is the magnitude of the charge, n is the number of
electrons, and e is the fundamental charge.
Coulomb constructed a
torsion balance to test the
relationships between a
quantity of charge, the
distance between the
charges, and the
electrical force produced.
He found the inverse
square law held
accurately for various
charges and distances
• Measuring Electrical Forces.
– Force is proportional to the product of the electrical
charge and inversely proportional to the square of the
distance.
– Coulomb’s Law
1 2
F k
qq
d
2
• F is the force
• k is a constant and has the value of 9.00 X 109
newtonmeters2/coulomb2 (9.00 X 10 9 Nm2/C2)
• q1 represents the electrical charge of object 1 and q2 represents
the electrical charge of object 2
• d is the distance between the two objects.
• Force Fields.
– The condition of space around an object is changed by
the presence of an electrical charge.
– The electrical charge produces a force field, that is called
an electrical field since it is produced by electrical charge
– All electrical charges are surrounded by an electrical field
just like all masses are surrounded by gravitational fields.
– A map of the electrical field can be made by bringing a
positive test charge into an electrical field.
• When brought near a negative charge the test charge is
attracted to the unlike charge and when brought near a
positive charge the test charge is repelled.
• You can draw vector arrows to indicate the direction
of the electrical field
• This is represented by drawing lines of force or
electrical field lines
– These lines are closer together when the field is
stronger and farther apart when it is weaker.
A positive test
charge is used by
convention to
identify the
properties of an
electric field. The
vector arrow points
in the direction of
the force that the
test charge would
experience
Lines of force diagrams
for (A) a negative charge
and (B) a positive charge
when the charges have
the same magnitude as
the test charge.
• Electrical Potential.
– An electrical charge has an electrical field that surrounds
it.
– In order to move a second charge through this field work
must be done
– Bringing a like charge particle into this field will require
work since like charges repel each other and bringing an
opposite charged particle into the field will require work
to keep the charges separated.
• In both of these cases the electrical potential is changed.
Electric potential
results from moving a
positive coulomb of
charge into the electric
field of a second
positive coulomb of
change. When 1.00
joule of work is done
in moving 1.00
coulomb of charge,
1.00 volt of potential
results. A volt is a
joule/coulomb.
– The potential difference (PD) that is created by doing
1.00 joule of work in moving 1.00 coulomb of charge is
defined as 1.00 volt
• A volt is a measure of the potential difference between
two points
• electric potential =work to create
.
potential charge moved
• PD=W
•
Q
• The voltage of an electrical charge is the energy
transfer per coulomb.
– The energy transfer can be measured by the work that is
done to move the charge or by the work that the charge
can do because of the position of the field.
• Electric Current.
• Introduction
– Electric current means a flow of charge in the same way
that a water current flows.
– It is the charge that flows, and the current is defined as
the flow of the charge, it would be redundant to speak
of a “flow of current”.
• The Electric Circuit.
– In order to have an electric current there must be a
separation of the charge maintaining the electrical field (a
potential difference).
• This potential difference can push a charge through a
conductor.
– An electrical current is maintained by pumping charges
to a higher electrical potential and the then do work as
they flow back to a lower potential
The falling water
can do work in
turning the water
wheel only as
long as the pump
maintains the
potential
difference
between the upper
and lower
reservoirs.
– An electrical circuit contains some device that acts as a
source of energy as it gives charges a higher potential
against an electrical field.
• The charges do work as they flow through the circuit to a lower
potential
• The charges flow through connecting wires to make a
continuous path.
• A switch is a means of interrupting or completing the circuit.
– The source of the electrical potential is the voltage
source.
– The device where the charges do work is the voltage
drop.
A simple electric circuit has a voltage source (such as a
generator or battery) that maintains the electrical
potential, some device (such as a lamp or motor ) where
work is done by the potential, and continuous pathways
for the current to follow.
– Voltage is a measure of the potential difference between two
places in a circuit.
• Voltage is measured in joules/coloumb.
– The rate at which an electrical current (I) flows is the quantity (q)
that moves through a cross section of a conductor in a give unit of
time (t)
I=q/t
• the units of current are coulombs/second.
• A coulomb/second is an ampere (amp)
– In an electrical circuit the rate of current is directly proportional to
the difference in electrical potential between two parts of the
circuit IPD.
A simple electric circuit carrying a current of 1.00
coulomb per second through a cross section of a
conductor has a current of 1.00 amp.
• The Nature of Current.
– Conventional current describes current as positive
charges that flow from the positive to the negative
terminal of a battery.
– The electron current description is the opposite of the
conventional current.
• The electron current describes current as a drift of negative
charges that flow from the negative to the positive terminal of a
battery.
• It is actually the electron current that moves charges.
• Actually it does not matter which description is used, since
positive charges and negative charges are mathematically
equal
A conventional current describes positive charges moving
from the positive terminal (+) to the negative terminal (-).
An electron current describes negative charges (-) moving
from the negative terminal (-) to the positive terminal (+)
(A) A metal conductor
without a current has
immovable positive
ions surrounded by a
swarm of chaotically
moving electrons. (B)
An electric field causes
the electrons to shift
positions, creating a
separation charge as the
electrons move with a
zigzag motion from
collisions with
stationary positive ions
and other electrons.
Electrons move very
slowly in a direct
current circuit. With a
drift velocity of 0.01
cm/s, more than 5 hr
would be required for
an electron to travel
200 cm from a car
battery to the brake
light. It is the electric
field, not the
electrons, that moves
at near the speed of
light in an electric
circuit.
– The current that occurs when there is a voltage depends
on:
• The number of free electrons per unit volume of the
conducting material.
• The fundamental charge on each electron.
• The drift velocity which depends on the electronic
structure of the conducting material and the
temperature.
• The cross-sectional area of the conducting wire.
– It is the electron field, and not the electrons, which does
the work.
• It is the electric field that accelerates electrons that are
already in the conducting material.
– It is important to understand that:
• An electric potential difference establishes, at nearly
the speed of light, an electric field throughout a
circuit.
• The field causes a net motion that constitutes a flow of
charge.
• The average velocity of the electrons moving as a
current is very slow, even thought he electric field that
moves them travels with a speed close to the speed of
light.
What is the nature of the electric current carried by
these conducting lines? It is an electric field that
moves at near the speed of light. The field causes a
net motion of electrons that constitutes a flow of
charge, an alternating current. As opposed to DC.
• Electrical Resistance.
– Electrical resistance is the resistance to movement of
electrons being accelerated with an energy loss.
• Materials having the property of reducing a current and this is
electrical resistance (R).
– Resistance is a ratio between the potential difference
(PD)between two points and the resulting current (I).
• R=PD/I
• The ratio of volts/amp is called an ohm ()
– The relationship between voltage, current, an resistance
is
• V=IR
• Ohms Law
– The magnitude of the electrical resistance of a conductor
depends on four variables.
• The length of the conductor.
• The cross-sectional area of the conductor.
• The material the conductor is made of.
• The temperature of the conductor.
The four factors that influence the resistance of an
electrical conductor are the length of the conductor, the
cross-sectional area of the conductor, the material the
conductor is made of, and the temperature of the
conductor
• Electrical Power and Electrical Work.
– All electrical circuits have three parts in common.
• A voltage source.
• An electrical device
• Conducting wires.
– The work done by a voltage source is equal to the work
done by the electrical field in an electrical device.
• W=PDq
• The electrical potential is measured in joules/coulomb
and a quantity of charge is measured in coulombs, so
the electrical work is measure in joules.
• A joule/second is a unit of power called the watt.
• power = current (in amps) X potential (in volts)
– P=IV
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.
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
• Magnetism.
• Magnetic Poles.
– A North seeking pole is called the North Pole
– A South seeking pole is called the South Pole
– Like magnetic poles repel and unlike magnetic poles
attract.
Every magnet has ends, or poles, about which the
magnetic properties seem to be concentrated. As this
photo shows, more iron filings are attracted to the poles,
revealing their location.
• Magnetic Fields.
– A magnet that is moved in space near a second magnet
experiences a magnetic field.
• A magnetic field can be represented by field lines.
– The strength of the magnetic field is greater where the
lines are closer together and weaker where they are
farther apart.
These lines are a map of the magnetic field around a bar
magnet. The needle of a magnetic compass will follow the
lines, with the north end showing the direction of the
field.
The earth's magnetic field.
Note that the magnetic
north pole and the
geographic North Pole are
not in the same place. Note
also that the magnetic
north pole acts as if the
south pole of a huge bar
magnet were inside the
earth. You know that it
must be a magnetic south
pole since the north end of
a magnetic compass is
attracted to it and opposite
poles attract
This magnetic declination map shows the approximate
number of degrees east or west of the true geographic
north that a magnetic compass will point in various
locations
• The Source of Magnetic Fields.
Since electrons are charges in motion, they produce
magnetic fields as well as an electric field.
• magnetism is a secondary property of electricity
• the strength of the magnetic field increases with
the velocity of the moving charge. The magnetic
field does not exist if the charge is not moving
• A magnetic field is a property of the space around a
moving charge.
A bar magnet cut into halves always makes new, complete
magnets with both a north and a south pole. The poles
always come in pairs, and the separation of a pair into
single poles, called monopoles, has never been
accomplished.
Oersted discovered that a
compass needle below a wire
(A) pointed north when there
was not a current, (B) moved
at right angles when a current
flowed one way, and (C)
moved at right angles in the
opposite direction when the
current was reversed
He had discovered an electric
current produces a magnetic
field!
• The Source of Magnetic Fields.
– Permanent Magnets.
• Since electrons are charges in motion, they produce
magnetic fields.
• In most materials these magnetic fields cancel one
another and neutralize the overall magnetic effect.
• In other materials such as iron, cobalt, and nickel, the
electrons are oriented in such a ways as to impart
magnetic properties to the atomic structure.
– These atoms are grouped in a tiny region called the
magnetic domain.
(A) In an unmagnetized piece of iron, the magnetic
domains have random arrangement that cancels any
overall magnetic effect. (B) When an external magnetic
field is applied to the iron, the magnetic domains are
realigned, and those parallel to the field grow in size at the
expense of the other domains, and the iron is magnetized
– Earth's Magnetic Field.
• The Earth’s magnetic field is thought to originate with
moving charges.
• The core is probably composed of iron and nickel,
which flows as the Earth rotates, creating electrical
currents that result in the Earth’s magnetic field.
• How the electric currents are generated is not yet
understood
• There seems to be a relationship between rate of
rotation and strength of planet’s magnetic field.
• Electric Currents and Magnetism.
A magnetic
compass
shows the
presence and
direction of
the magnetic
field around a
straight
length of
currentcarrying wire
Use (A) a right-hand rule of thumb to determine the
direction of a magnetic field around a conventional
current and (B) a left-hand rule of thumb to determine the
direction of a magnetic field around an electron current
• Current Loops.
– A current-carrying wire that is formed into a loop has
perpendicular, circular field lines that pass through the
inside of the loop in the same direction.
• This has the effect of concentrating the field lines, which
increases the magnetic field intensity.
• Since the field lines pass through the loop in the same direction,
the loop has a north and south pole.
– Many loops of wire formed into a cylindrical coil are
called a solenoid.
• When a current is in the solenoid a magnetic field around the
solenoid is created that acts like a magnetic field and is called
an electromagnet
(A) Forming a wire into a loop causes the magnetic field
to pass through the loop in the same direction. (B) This
gives one side of the loop a north pole and the other side a
south pole.
When a current is run
through a cylindrical
coil of wire, a solenoid,
it produces a magnetic
field like the magnetic
field of a bar magnet
that can be turned on
and off, and is called
an electromagnet. The
strength depends on
current, loops and
presence of a soft iron
core.
• Applications of Electromagnets.
– Electric Meters.
• The strength of the magnetic field produced by an
electromagnet is proportional to the electric current in the
electromagnet.
• A galvanometer measures electrical current by measuring the
magnetic field.
• A galvanometer can measure current (ammeter), potential
difference (voltmeter), and resistance (ohmmeter).
– Electromagnetic Switches.
• A relay is an electromagnetic switch device that makes possible
the use of low voltage control current to switch a larger, high
voltage circuit on and off
A galvanometer measures the direction and relative
strength of an electric current from the magnetic field it
produces. A coil of wire wrapped around an iron core
becomes an electromagnet that rotates in the field of a
permanent magnet. The rotation moves pointer on a scale
You can use the materials shown here to create and detect
an electric current.
A schematic of a relay
circuit. The mercury vial
turns as changes in the
temperature expand or
contract the coil, moving
the mercury and making
or breaking contact with
the relay circuit. When
the mercury moves to
close to the relay circuit,
a small current activates
the electromagnet, which
closes the contacts on the
large-current circuit
(A) Sound waves are
converted into a changing
electrical current in a
telephone. (B) Changing
electrical current can be
changed to sound waves in
a speaker by the action of
an electromagnet pushing
and pulling on a permanent
magnet. The permanent
magnet is attached to a stiff
paper cone or some other
material that makes sound
waves as it moves in and
out
– Electric Motors.
• An electrical motor is an electromagnetic device that
converts electrical energy into mechanical energy.
• A motor has two working parts, a stationary magnet
called a field magnet and a cylindrical, movable
electromagnet called an armature.
• The armature is on an axle and rotates in the magnetic
field of the field magnet.
• The axle is used to do work.
A schematic of a simple electric motor
• Electromagnetic Induction.
• Introduction
– If a loop of wire is moved in a magnetic field a voltage is
induced in the wire.
• The voltage is called an induced voltage and the resulting
current is called an induced current.
• The interaction is called electromagnetic induction.
– Electromagnetic induction occurs when the loop of wire
cuts across magnetic field lines or when magnetic field
lines cut across the loop.
– The magnitude of the induced voltage is proportional to:
• The number of wire loops cutting across the magnetic
field lines.
• The strength of the magnetic field.
• The rate at which magnetic field lines are cut by the
wire.
A current is induced in a coil of wire moved through a
magnetic field. The direction of the current depends on the
direction of motion
• Generators.
– A generator is basically an axle with many wire loops
that rotates in a magnetic field.
• The axle is turned by some form of mechanical
energy, such as a water turbine or a steam engine.
(A) Schematic of a
simple alternator
(ac generator) with
one output loop.
(B) Output of the
single loop turning
in a constant
magnetic field,
which alternates the
induced current
each half cycle
(A) Schematic of a simple
dc generator with one
output loop.
(B) Output of the single
loop turning in a constant
magnetic field. The split
ring (commutator)
reverses the sign of the
output when the voltage
starts to reverse, so the
induced current has halfcycle voltages of a
constant sign, which is the
definition of direct
current.
• Transformers.
– A transformer has two basic parts.
• A primary coil, which is connected to a source of
alternating current
• A secondary coil, which is close by.
– A growing and collapsing magnetic field in the primary
coil induces a voltage in the secondary coil.
– A step up or step down transformer steps up or steps
down the voltage of an alternating current according to
the ratio of wire loops in the primary and secondary
coils.
• The power input on the primary coil equals the power
output on the secondary coil.
• Energy losses in transmission are reduced by stepping
up the voltage.
(A) This step-down
transformer has 10 turns
on the primary for each
turn on the secondary
and reduces the voltage
from 120 V to 12 V.
(B) This step-up
transformer increases the
voltage from 120 V to
1,200 V, since there are
10 turns on the
secondary to each turn
on the primary
Energy losses in transmission
are reduced by increasing the
voltage, so the voltage of
generated power is stepped up
at the power plant. (A) These
transformers, for example,
might step up the voltage
from tens to hundreds of
thousands of volts. After a
step-down transformer
reduces the voltage at a
substation, still another
transformer (B) reduces the
voltage to 120 for
transmission to three or four
houses