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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.

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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

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

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

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Magnetism at Different Scales Northern Lights Northern Lights Magnetic Bacteria Magnetic Bacteria

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

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

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

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Lightning

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

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Coulomb’s Law, ca.1785

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Charles Coulomb Charles Coulomb Experiments Experiments Coulomb’s Law: Coulomb’s Law: Force is related to charge and distance Force is related to charge and distance 1 Coulomb = 6.3 x 10 18 electrons 1 Coulomb = 6.3 x 10 18 electrons

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Force of Electricity vs. Gravity Coulomb’s Law: F e = k e (q 1 X q 2 )/d 2 Gravitational Force: F g = k g (m 1 X m 2 )/d 2

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

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Frogs and Animal Electricity ca. 1790

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Alessandro Volta’s Response (ca.1800)

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The Battery

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Three Parts of an Electric Circuits 1. Source of electrical energy 2. Closed path 3. Device to use electrical energy

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Electric Circuits

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Two Kinds of Electric Circuits Series circuit vs. parallel circuit

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

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Hans Christian Oersted, 1820

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The Electromagnet

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Magnetic Effects from Electricity

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

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

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Michael Faraday, 1831 Electrical Effects from Magnetism

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Electromagnetic induction Electromagnetic induction Electrical fields and currents can be produced by changing magnetic fields Electrical fields and currents can be produced by changing magnetic fields

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

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James Clark Maxwell, 1855

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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: These equations lead to prediction of waves: 1. Waves travel 186,000 miles per second 1. Waves travel 186,000 miles per second 2. Light is a consequence of electricity and 2. Light is a consequence of electricity and magnetism switching back and forth magnetism switching back and forth

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

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