1. Great Ideas in Science: Lecture 4 Electricity and Magnetism
Professor Robert Hazen
PROV 301
Great Idea: Electricity and magnetism are two different aspects of one
Force — the electromagnetic force.

2. Electricity and magnetism are two aspects of the same force.
All magnets have both north and south poles
Static electricity is manifest as a force between charged objects
Electricity produces magnetic effects
Moving magnets produce electricity

3. Tonight’s Outline Nature’s Other Forces Magnetism Static Electricity
Batteries and Electric Circuits
Magnetic Effects from Electricity
Electrical Effects from Magnetism
Electromagnetic Radiation

4. Magnetism Magnetic Force William Gilbert Magnetic Field
Natural magnetic minerals
Earth is a giant magnet
Used in navigation
William Gilbert
All magnets are dipoles
Like repels, unlike attracts
Earth is a dipole magnet
Magnetic Field

5. Magnetism at Different Scales
Northern Lights
Magnetic Bacteria

6. Static Electricity Newton’s First Law Greeks’ Observations Electricity
Electrical charge
Electricity
Static electricity
Two kinds of charge
Opposites attract
Like repels

7. The “Electric Kiss”, ca.1750

8. Benjamin Franklin, ca.1750 Benjamin Franklin – single electrical fluid
Transfer of electrons
Positive and negative charges
Atomic structure

9. Lightning
Lightning occurs when the negatively-charged clouds interact with the positive charged ground

10. Electrons are conducted harmlessly from clouds into the ground
The Lightning Rod
Electrons are conducted harmlessly
from clouds into the ground

11. Coulomb’s Law, ca.1785

12. Coulomb’s Law, ca.1785 Charles Coulomb Experiments Coulomb’s Law:
Force is related to charge and distance
1 Coulomb = 6.3 x 1018 electrons

13. Force of Electricity vs. Gravity
Coulomb’s Law:
Fe = ke(q1 X q2)/d2
Gravitational Force:
Fg = kg(m1 X m2)/d2

14. The Electrical Field
The force felt by a charge located at a particular point
Differs from the magnetic dipole field

15. Frogs and Animal Electricity ca. 1790

16. Frogs and Animal Electricity ca. 1790

17. Alessandro Volta’s Response (ca.1800)

18. Alessandro Volta’s Response (ca.1800)

19. The Battery (“Vital” effects)

20. Three Parts of an Electric Circuits
Source of electrical energy
Closed path
Device to use electrical energy

21. Electric Circuits

22. Electric Circuits

23. Two Kinds of Electric Circuits
Series circuit vs. parallel circuit

24. Ohm’s Law Flow of Electricity Current (amps) 1 amp=1 coulomb/sec
Voltage (volts)
Amount of power in circuit
Electrical Resistance (ohms)
Ohm’s Law
Current is directly proportional to voltage and inversely proportional to resistance
Equation:
Load

25. Hans Christian Oersted, 1820

26. The Electromagnet

27. Magnetic Effects from Electricity

28. Magnetic Effects from Electricity

29. North and South poles flip during rotary motion.
The Electric Motor
North and South poles flip during rotary motion.

30. North and South poles flip during rotary motion.
The Electric Motor
North and South poles flip during rotary motion.

31. Michael Faraday, 1831 Electrical Effects from Magnetism

32. Michael Faraday, 1831 Electrical Effects from Magnetism
Electromagnetic induction
Electrical fields and currents can be produced by changing magnetic fields

33. Electric Generators
Move a coil of wires next to magnets to create electricity
Car alternator

34. Electric Generators
Move a coil of wires next to magnets to create electricity
Turbines (power plants)
Wind
Water
Steam - Coal/Nuclear power plants

35. James Clark Maxwell, 1855

36. Maxwell’s Laws

37. Maxwell’s Laws Coulomb’s Law of static electricity
There are no magnetic monopoles
Magnetic fields are produced by electricity
Electric fields are produced by moving magnets

38. Maxwell’s Laws, 1855
Four equations describe the behaviors of electricity and magnetism
1. Coulomb’s Law of static electricity
2. All magnets have both north and south poles
3. Electricity produces magnetic effects
4. Moving magnets produce electricity
These equations lead to prediction of waves:
1. Waves travel 186,000 miles per second
2. Light is a consequence of electricity and
magnetism switching back and forth

39. A Hierarchy of Scientific Ideas
Fact (a confirmed observation)
Hypothesis (an educated guess)
Law (a predictive mathematical description of nature)
Theory (a well established explanation of nature)

40. The Electromagnetic Spectrum
Maxwell’s equations predict more waves
Hertz discovers radio waves