Download Electricity

PowerPoint Lectures
to accompany
Physical Science, 6e
Chapter 6
Electricity
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and magnetism
Electric charge
• Electron theory of charge
– Ancient mystery: “Amber
effect”
– J. J. Thompson:
identified negatively
charged electrons
• Today:
– Basic unit of matter =
atom
– Atoms made up of
electrons and nuclei
containing positively
charged protons and
neutral neutrons (See
Ch. 8)
Electric charge and electrical
forces
• Charges in matter
– Inseparable property of
certain particles
– Electrons: negative
electric charge
– Protons: positive electric
charge
• Charge interaction
– Electric force
– “Like charges repel;
unlike charges attract”
• Ions: non-zero net charge
from loss/gain of electrons
Electrostatic charge
• Stationary charge
confined to an object
• Charging mechanisms
– Friction
– Contact with a charged
object
– Polarization
(reorientation induced
without changing net
charge)
Electrical conductors and
insulators
• Electrical conductors
– Electrons are free to move throughout material
– Added charge dissipates
– Examples: metals, graphite (carbon)
• Electrical insulators
– Electron motions restricted
– Added charge tends to remain on object
– Examples: Glass, wood, diamond (carbon)
• Semiconductors
– Conduct/insulate depending on circumstances
– Applications: Computer chips, solar cells, ...
Measuring electric charge
• Unit of charge = coulomb
(C)
– Equivalent to charge of
6.24x1018 electrons!
– Fundamental metric unit
(along with m, kg and s)
• Electron charge
– Fundamental charge
– Smallest seen in nature
– Quantity of charge and the
number of electrons
Measuring electric forces
Coulomb’s law
• Relationship giving
force between two
charges
• Similar to Newton’s law
of gravitation
• k versus G implies
gravity weaker
• 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.
Force fields
How do forces act through
space?
• Charges surrounded by
electric fields (Vector
fields/directional)
• Fields and charges
inseparable
• Fields act on other charges
– Direction of fields = motion
of positive test charge in the
field
– Visualized with lines of force
• Same ideas apply to gravity
and magnetism
• 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
• Scalar field associated
with potential energy
• Units = volts (V)
• Related to work involved
in positioning charges
• Potential difference
important in producing
forces and moving
charges
• Analogous to moving
masses in gravitational
fields
– 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
Earlier - electrostatics
• Charges more or less fixed in place
Now - charge allowed to move
• Electric current
– Flow of charge, not flow of current
– Reason for charge flow - potential (voltage)
differences
• Electric circuits
– Structures designed to localize and utilize currents
Electric circuits
Structure
• Voltage source
– Energy input
– Necessary for continuing
flow
• Circuit elements
– Energy used up as heat,
light, work, …
• Current flow convention:
from high potential to low
potential through the
external circuit
• Water/pump analogy
– 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
• Historically - nature of
“electrical fluid”
unknown
• Current thought to be a
flow of positive charge
• Reality - more
complicated, depending
on material
• Opposite correct in
metals, current =
electron flow
Current mechanisms
Liquids and gases
• Both positive and negative
charges move, in opposite
directions
Metals
• Delocalized electrons free to
move throughout metal
• “Electron gas”
• Drift velocity of electrons
slow
• Electric field moves through
at nearly light speed
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.
More current details
• Current = charge per
unit time
• Units = ampere, amps
(A)
• Direct current (DC)
– Charges move in one
direction
– Electronic devices,
batteries, solar cells
• Alternating current (AC)
– Charge motion oscillatory
– No net current flow
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
• Loss of electron
current energy
• Two sources
– Collisions with other
electrons in current
– Collisions with other
charges in material
• Ohm’s law
– 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.
More on resistance
• Resistance factors
–
–
–
–
Type of material
Length
Cross sectional area
Temperature
• Superconductors
– Negligible resistance
at very low
temperatures
Electrical power and work
Three circuit elements
contribute to work
Power in circuits
• Voltage source
• Electrical device
• Conducting wires
– Maintain potential difference
across device
– Input wire at higher potential
than output wire
– Output wire = "ground" for
AC circuits
– No potential difference, no
current (bird on a wire)
Electric bills
– The work done by a voltage source is equal to the
work done by the electrical field in an electrical
device.
• W=Vq
• 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
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
Earliest ideas
• Associated with naturally occurring magnetic
materials (lodestone, magnetite)
• Characterized by “poles” - “north seeking” and “south
seeking”
• Other magnetic materials - iron, cobalt, nickel
(ferromagnetic)
Modern view
• Associated with magnetic fields
• Field lines go from north to south poles
Magnetic poles and fields
• Magnetic fields and
poles inseparable
• Poles always come in
north/south pairs
• Field lines go from north
pole to south pole
• Like magnetic poles
repel; unlike 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.
Sources of magnetic fields
• Microscopic fields
– Intrinsic spins of subatomic particles (electrons,
protons, …)
– Orbital motions of electrons in atoms
• Macroscopic fields
–
–
–
–
Permanent magnets
Earth’s magnetic field
Electric currents
Electromagnets
Permanent magnets
• Ferromagnetic materials
• Atomic magnetic moment
– Electron/proton intrinsic
moments
– Electron orbital motion
• Clusters of atomic
moments align in domains
• Not magnetized - domains
randomly oriented
• Magnetized - domains
aligned
Earth’s magnetic field
• Originates deep beneath the
surface from currents in
molten core
• Magnetic “north” pole =
south pole of Earth’s
magnetic field
• Magnetic declination = offset
• Direction of field periodically
reverses
– Deposits of magnetized
material
– Last reversal - 780,000 yrs.
ago
• 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.
– 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
• Moving charges
(currents) produce
magnetic fields
• Shape of field
determined by
geometry of current
– Straight wire
– Current loops
– Solenoid
Electromagnets
• Structure
– Ferromagnetic core
– Current carrying wire wrapped around core
• Field enhanced by the combination
• Can be turned on/off
• Widely used electromagnetic device
Electric meters
• Instrument for
measuring current
(ammeter)
• Uses magnetic field
produced by the current
• Magnetic field and,
hence, deflection
proportional to current
• Modified, can measure
– Potential (voltmeter)
– Resistance (ohmmeter)
Electromagnetic switches
Relays
• Use low voltage control
currents to switch larger,
high voltage currents on/off
• Mercury switch/thermostat
Solenoid switches
• Moveable spring-loaded iron
core responds to solenoid
field
• Water valves, auto starters,
VCR switches, activation of
bells and buzzers
Telephones and loudspeakers
Coupling acoustic waves to electric currents
Telephone
• Sound vibrates carbon
granules changing
resistance
• Changing resistance varies
current
Speaker
• Varying current changes field
of electromagnet, moving
permanent magnet
• Moving magnet vibrates
spring attached to paper
cone producing sound
Electric motors
• Convert electrical energy to
mechanical energy
• Two working parts
– Field magnet - stationary
– Armature - moveable
electromagnet
• Armature rotates by
interactions with field magnet
– Commutator and brushes
reverse current to
maintain rotation
Electromagnetic induction
Causes:
•
•
Relative motion between
magnetic fields and conductors
Changing magnetic fields near
conductors
Effect:
•
Induced voltages and currents
Induced voltage depends on
•
•
•
Number of loops
Strength of magnetic field
Rate of magnetic field change
Generators
Structure
• Axle with main wire loops in
a magnetic field
• Axle turned mechanically to
produce electrical energy
AC generator
• “Alternating current”
• Sign of current and voltage
alternate
DC generator
• “Direct current”
• Sign of current and voltage
constant
• 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.
Transformers
• Problems in power
transmission
– High currents - large
resistive losses
– High voltages - dangerous
potential differences
• Solution: transformers
boost/lower AC currents and
voltages
• Basic relationships
– Power in = power out
– Number of coils to voltage
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