Transcript Electricity - SFSU Physics & Astronomy

ELECTRICITY
Chapters 22 - 23
Electric charge
• Electron theory of
charge
– Ancient mystery:
“Amber effect”
– J. J. Thompson:
identified negatively
charged electrons
• Today:
– Basic unit of matter =
atom
Discovery of the electron
J. J. Thomson (late
1800’s)
• Performed cathode ray
experiments
• Discovered negatively
charged electron
• Measured electron’s
charge-to-mass ratio
• Identified electron as a
fundamental particle
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
(charge by induction)
Charging by friction
and then by contact
Charging by induction
Stages of charge induction by grounding
Measuring electric charge
• Unit of charge = coulomb (C)
– Fundamental metric unit (along with m, kg and s)
– Negative charge of 1 C requires > 6 billion billion electrons
• Electron charge = 1.60 x 10-19 C
– Fundamental charge of electron (and proton)
– Smallest seen in nature
– All charged objects have multiples of this charge
Measuring electric forces
Coulomb’s law
• Relationship giving force
between two charges
• Force between two charged
objects:
– repulsive if q1 and q2 are same
– attractive if q1 q2 different
• Both objects feel same force
• Distance between objects
increases: strength of force
decreases
– Double distance, force reduced by 1/4
Electric Field
Force fields
Model of a field considers condition
of space around a charge
Charge produces electric field
Visualized by making map of field
(Michael Faraday 1791-1867)
Electric field lines indicate strength
and direction of force the field
exerts on field of another charge
E = F/q
Field lines
Point outward around positively
charged particles
Point inward around negatively
charged particle
Spacing shows strength
Lines closer; field stronger
Lines further apart: field weaker
Figure 22.20: Electric Shielding
Potential Difference (Voltage)
Electric energy
Storage
(Capacitor)
Electric Current
Electric Current
Flow of charge
• 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)
– Electric field moves back and
forth through wire
– Current flows one way then the
other with changing field
I = 1.00 amp
Resistance
Electrical conductors and
insulators
• Electrical conductors
– Charge flows easily
– Many loosely attached electrons are free to move from atom
to atom
– Examples: metals, graphite (carbon)
• Electrical insulators
– Charge does not easily flow
– Electrons are held tightly, electron motions restricted
– Examples: Glass, wood, diamond (carbon), rubber
• Semiconductors
– Conduct/insulate depending on circumstances
– Applications: Computer chips, solar cells, ...
Resistance
Resistance factors
– Type of material
• Conductors have less electrical resistance, insulators have more
– Length
• Longer the wire, more resistance
– Cross sectional area
• Thinner the wire, the more resistance
– Temperature
• Resistance increases with increasing temperature
Electric circuits
• Energy source (battery,
generator)
– Necessary for
continuing flow
• Charge moves out one
terminal, through wire
and back in the other
terminal
• Circuit elements
– Charges do work
• Light bulbs, run motors,
provide heat …
Electrons move very
slowly in DC circuit.
The electric field moves
near the speed of light.
Electrical resistance
• Loss of electron
current energy
• Two sources
– Collisions with other
electrons in current
– Collisions with other
charges in material
• Ohm’s law
Electrical power and work
Three circuit elements
contribute to work
Power in circuits
• Voltage source
• Electrical device
• Conducting wires
Power
Includes time factor
Measured in watts (joule/sec)
Electric utility charge
Cents per kilowatt-hour
Electric bills
Dry Cell
• Produces electrical energy
from chemical reaction
between ammonium
chloride and zinc can
• Reaction leaves negative
charge on zinc and positive
charge on carbon rod
• Always produces 1.5 volts
regardless of size
– Larger voltages produced by
combination of smaller cells
(battery)
Household Circuits and Safety
• Parallel Circuit
– Current can flow through any
branch without first going through
any other
• Circuit breaker (or fuse)
– Disconnects circuit when a preset
value (15 or 20 amps) reached
• Three-pronged plug
– Provides grounding wire
• In case of a short circuit, current
will travel through grounding wire
to ground
• Ground-fault interrupter (GFI)
– Detects difference in loadcarrying and system wire
– If difference detected, opens
circuit within a fraction of second
(much quicker than circuit
breaker)
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
Earth’s magnetic field
• Shaped and oriented as if
huge bar magnet were inside
– South pole of magnet near
geographic north pole
• Geographic North Pole and
north magnetic pole different
– Magnetic declination = offset
Electric currents and
magnetism
• Moving charges
(currents) produce
magnetic fields
• Shape of field
determined by
geometry of current
– Straight wire
– Current loops
– Solenoid
Electromagnetism
Electromagnet
•
•
•
•
•
Loops of wire formed into cylindrical coil (solenoid)
Current run through coil produces a magnetic field
Can be turned on/off by turning current on or off
Strength depends on size of current and number of loops
Widely used electromagnetic device
Solenoid switches
• Moveable spring-loaded iron core responds to solenoid field
• Water valves, auto starters, VCR switches, activation of bells and
buzzers
Galvanometer
• Measures size of
current from size of its
magnetic field
• Coil of wire wrapped
around an iron core
becomes an
electromagnet that
rotates in field of a
permanent magnet
• This rotation moves a
pointer on a scale
Electromagnetic induction
Causes:
•
•
Relative motion between magnetic
fields and conductors
Changing magnetic fields near
conductors
– Does not matter which one moves or
changes
Effect:
•
Induced voltages and currents
Size of induced voltage depends on:
•
•
•
Number of loops
Strength of magnetic field
Rate of magnetic field change
Direction of current depends on
direction of motion
Generators
• Device that converts mechanical energy
into electrical energy
Structure
• Axle with many loops in a wire coil
• Coil rotates in a magnetic field
– Turned mechanically to produce electrical
energy
Transformers
• Steps AC voltage up or down
• Two parts
– Primary (input) coil
– Secondary (output) coil
• AC current flows through primary coil,
magnetic field grows to maximum
size, collapses to zero then grows to
maximum size with opposite polarity
• Growing and collapsing magnetic field
moves across wires in secondary coil,
inducing voltage
• Size of induced voltage proportional to
number of wire loops in each coil
– More loops in secondary coil – higher
voltage output (step-up transformer)
– Fewer loops in secondary coil – lower
voltage output (step-down transformer)