Great Ideas in Science:
Lecture 4
Electricity and Magnetism
Professor Robert Hazen
UNIV 301
Great Idea: Electricity and magnetism
are two different aspects of one
Force — the electromagnetic force.
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
Tonight’s Outline
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Nature’s Other Forces
Magnetism
Static Electricity
Batteries and Electric Circuits
Magnetic Effects from Electricity
Electrical Effects from Magnetism
Electromagnetic Radiation
Magnetism
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Magnetic Force
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William Gilbert
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Natural magnetic minerals
Earth is a giant magnet
Used in navigation
All magnets are dipoles
Like repels, unlike attracts
Earth is a dipole magnet
Magnetic Field
Magnetism at Different Scales
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Northern Lights
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Magnetic Bacteria
Static Electricity
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Newton’s First Law
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Greeks’ Observations
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Electrical charge
Electricity
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Static electricity
Two kinds of charge
Opposites attract
Like repels
Benjamin Franklin, ca.1750
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Benjamin Franklin – single electrical fluid
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Transfer of electrons
Positive and negative charges
Atomic structure
Lightning
Lightning occurs when the negatively-charged
clouds interact with the positive charged ground
Lightning
Lightning occurs when the negatively-charged
clouds interact with the positive charged ground
The Lightning Rod
Electrons are conducted harmlessly
from clouds into the ground
Coulomb’s Law, ca.1785
kq1q2
F
2
d
Coulomb’s Law, ca.1785
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Charles Coulomb
 Experiments
kq1q2
F
2
d
Coulomb’s Law:
 Force is related to charge and distance
1 Coulomb = 6.3 x 1018 electrons
Force of Electricity vs. Gravity
Coulomb’s Law:
2
Fe = ke(q1 X q2)/d
Gravitational Force:
Fg = kg(m1 X m2)/d2
The Electrical Field
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The force felt by a
charge located at
a particular point
Differs from the
magnetic dipole
field
Frogs and Animal Electricity
ca. 1790
Frogs and Animal Electricity
ca. 1790
Alessandro Volta’s
Response (ca.1800)
Alessandro Volta’s
Response (ca.1800)
The Battery
The Battery
Three Parts of an Electric Circuits
1.
2.
3.
Source of electrical energy
Closed path
Device to use electrical energy
Electric Circuits
Electric Circuits
Two Kinds of
Electric Circuits
Series circuit vs. parallel circuit
Ohm’s Law
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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:
V  IR
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Load
 Equation:
P  IV
Hans Christian Oersted, 1820
The Electromagnet
Magnetic Effects
from Electricity
Magnetic Effects
from Electricity
The Electric Motor
North and South poles flip during rotary motion.
The Electric Motor
North and South poles flip during rotary motion.
Michael Faraday, 1831
Electrical Effects from Magnetism
Michael Faraday, 1831
Electrical Effects from Magnetism
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Electromagnetic induction
Electrical fields and currents can be
produced by changing magnetic fields
Electric Generators
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Move a coil of wires next to
magnets to create electricity
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Car alternator
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Turbines (power plants)
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Wind
Water
Steam - Coal/Nuclear power plants
James Clark Maxwell, 1855
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
The Electromagnetic
Spectrum
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Maxwell’s equations predict more waves
Hertz discovers radio waves