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)