1. PowerPoint Lectures to accompany Physical Science, 7e
Chapter 6
Electricity
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Electricity
and magnetism

3. Concepts of electricity
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)

4. Electric charge 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

5. Static electricity Electrostatic charge Charging mechanisms
Stationary charge confined to an object
Charging mechanisms
Friction
Contact with a charged object
Polarization (reorientation induced without changing net charge)

6. Electrical conductors and insulators
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, ...

7. Electric charge Unit of charge = coulomb (C) Electron charge
Equivalent to charge of 6.24x1018 electrons!
Metric unit of charge
Electron charge
Fundamental charge
Smallest seen in nature
Quantity of charge and the number of electrons

8. Electrostatic forces Coulomb’s law
Relationship giving force between two charges
Similar to Newton’s law of gravitation
k versus G implies gravity weaker

9. 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

10. 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

11. Electric current Earlier - electrostatics
Charges more or less fixed in place
Now - charge allowed to move
Electric current
Flow of charge
Reason for charge flow - potential (voltage) differences
Electric circuits
Structures designed to localize and utilize currents

12. The electric circuit 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

13. 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 in metals, current = electron flow

14. Current mechanisms Liquids and gases Metals
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

15. 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

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

17. More on resistance Resistance factors Superconductors Type of material
Length
Cross sectional area
Temperature
Superconductors
Negligible resistance at very low temperatures

18. Electrical power and work
Three circuit elements contribute to work
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)

19. 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

20. 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

21. 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

22. 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

23. 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

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

25. Electromagnets Structure Field enhanced by the combination
Ferromagnetic core
Current carrying wire wrapped around core
Field enhanced by the combination
Can be turned on/off
Used in many applications: meters, switches, speakers, motors…

26. Telephones and loudspeakers Coupling acoustic waves to electric currents
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

27. 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

28. 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

29. 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

30. 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

31. Transformers Problems in power transmission
High currents - large resistive losses
High voltages - dangerous potential differences
Solution: transformers boost/lower AC currents and voltages

32. Transformers
Basic relationships
Power in = power out

33. Circuit connections Alternating current Direct current
Practically all generated electricity
Transmitted over high voltage lines and stepped down for use in homes and industry
Direct current
Used in automobiles, cell phones, mp3 players, laptops, …
Movable and portable applications
Main current source is chemical batteries, also fuel cells & solar cells

34. Voltage sources in circuits
Series circuit
Negative terminal of one cell connected to positive terminal of another cell
Total voltage is the sum of single cell voltages (volts)
Single current pathway

35. Voltage sources in circuits
Parallel circuit
All negative terminals connected; all positive terminals connected
Resultant voltage determined by the largest cell voltage
Makes greater electrical energy available (amps)

36. Resistances in circuits
Series connection
Total resistance is the sum of all components
Overall resistance increases as more elements are added
Current drops as more elements are added

37. Resistances in circuits
Parallel connection
More than one current path available
Overall resistance drops as more elements are added
Total circuit current increases as more elements are added

38. Household circuits Combination series and parallel circuit
Light fixtures in a room in parallel
Appliances in parallel
Same voltage available to each
Circuit breakers and fuses prevent overloads

39. Household circuit safety
Potential difference from two wires per device
Energized load carrier
Grounded or neutral wire
Three-pronged plug
Provides another grounding wire
Other devices: polarized plugs, ground-fault interrupter (GFI)

40. Important Equations Eq. 6.1 q = ne Eq. 6.2 F = kq1q2/d2
Eq V = IR, R = V/I, I = V/R
Eq P = IV, P = I2R, P = V2/R
Eq Cost = (watts)(time)(rate)/1,000W/kW
Eq Vp/Np = Vs/Ns
Eq VpIp = VsIs
Eq Rtotal = R1 + R2 + …
Eq /Rtotal = 1/R1 + 1/R2 + …

42. Summary of Circuits Type of Overall Circuit Resistance Current
Series Increases with Decreases as
additional more elements
components are added
Parallel Decreases with Increases as
additional more elements

43. Circuit Example
A 60Ω resistor, a 100Ω resistor, and a 200Ω resistor are connected in series with a 120 V source.
What is the overall resistance of the circuit?
What is the current flowing in the circuit?

44. Circuit Example
A 60Ω resistor, a 100Ω resistor, and a 200Ω resistor are connected in parallel with a 120 V source in a parallel circuit.
What is the overall resistance of the circuit?
What is the current flowing in the circuit?