electricity chapters 22 - 23. electric charge electron theory of charge –ancient mystery: “amber...
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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 frictionand 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 fieldsModel of a field considers
condition of space around a charge
Charge produces electric fieldVisualized 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/qField lines
Point outward around positively charged particlesPoint inward around negatively charged particleSpacing shows strength
Lines closer; field strongerLines further apart: field
weaker
Figure 22.20: Electric Shielding
Potential Difference (Voltage)
Electric energy
Storage
(Capacitor)
Electric Current
Electric CurrentFlow 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, ...
ResistanceResistance 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
• Voltage source• Electrical device• Conducting wires
Power
Includes time factor
Measured in watts (joule/sec)
Electric utility charge
Cents per kilowatt-hour
Power in circuits
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 load-
carrying 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 inductionCauses:• 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)