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.

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

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

Magnetism at Different Scales Northern Lights Magnetic Bacteria

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

The “Electric Kiss”, ca.1750

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

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

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

Coulomb’s Law, ca.1785

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

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

The Electrical Field 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 (“Vital” effects)

Three Parts of an Electric Circuits 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 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

Hans Christian Oersted, 1820

The Electromagnet

Magnetic Effects from Electricity

Magnetic Effects from Electricity

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

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

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

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

James Clark Maxwell, 1855

Maxwell’s Laws

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

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

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)

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