1. ELECTRICITY
Chapters

2. Electric charge Electron theory of charge Today:
Ancient mystery: “Amber effect”
J. J. Thompson: identified negatively charged electrons
Today:
Basic unit of matter = atom

3. 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

4. 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

6. Electrostatic charge Stationary charge confined to an object
Charging mechanisms
Friction
Contact with a charged object (charge by induction)

7. Charging by friction and then by contact

8. Charging by induction

9. Stages of charge induction by grounding

10. 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 C
Fundamental charge of electron (and proton)
Smallest seen in nature
All charged objects have multiples of this charge

11. 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

12. Electric Field

13. 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 )
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

14. Figure 22.20: Electric Shielding

15. Potential Difference (Voltage)

16. Electric energy
Storage
(Capacitor)

17. Electric Current

18. 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

20. Resistance

21. Electrical conductors and insulators
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, ...

22. Resistance Resistance factors Type of material Length
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

23. Electric circuits Energy source (battery, generator) Circuit elements
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 …

24. Electrons move very slowly in DC circuit.
The electric field moves near the speed of light.

25. Electrical resistance
Loss of electron current energy
Two sources
Collisions with other electrons in current
Collisions with other charges in material
Ohm’s law

26. 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

27. 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)

28. 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)

29. Magnetism Earliest ideas Modern view
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

30. 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

31. 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

32. Electric currents and magnetism
Moving charges (currents) produce magnetic fields
Shape of field determined by geometry of current
Straight wire
Current loops
Solenoid

33. Electromagnetism Solenoid switches 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

34. 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

35. 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

36. 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

37. 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)