Lecture 5 - Field & Energy

4 Pages

History and Philosophy of Science and Technology
Course Code
Chen- Pang Yeang

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HPS211 Lecture 5 MAY29/2012 Field th - Energy and field important in 19 century physics th  Both have a relationship to the technological context of the 19 century - 19 century British focused on fields and the idea of a mechanical model of aether I. Early Studies of electricity and magnetism - Electricity and Magnetism in Antiquity  “Amber effect” – rubbing of crystal, causes friction – leads to “electricity”  Lodestone from Magnesia, Asia Minor  attract/ repel each other or iron  Magnetism = magnesia - Study of electricity and magnetism remained qualitatively explained - William Gilbert, De Magnete (1600) st  1 to observe, propose that the earth is a giant magnet  Observe/perform classification of electric/magnetic fields  Believed that electricity as outcome of “effluvia”  a purely mechanical picture  Magnetism as outcome of “sympathth and “athipathy” - Electricity saw great progress in the 17 and 18 century because of new instruments for electrical experiments  i.e. electric friction machine (similar to Amber effect) and Leyden jar (mid-18 c., stores electricity) II. Electric current, electrodynamics, electromagnetic induction - Charles Coulomb, a French engineer performed experiments (measure quantitatively the force between 2 things) in the 1780s - Coulomb’s Force Law of Electricity  Electrified bodies carry positive or negative charge  “like” charges repel, opposite charges attract  Electric force is inverse-square central:  Force can be attractive and repulsive!  Magnetic force follows same law, but with two poles (Carl Gauss) th  “Electrostatics” in the early 19 century  Hallmark of French physics  The study of magnetic and electric force led to the development of “electrostatics”  Coulomb’s force put an end to the effluvia theory - Alessandro Volta – tried to investigate the source of electricity  He came up with the Voltaic pile  a series of pile and wet paper – connects to Leyden jar and he saw how it produced energy  Voltaic pile liken to a battery  Voltaic pile as a steady source of electricity  electric current - Hans Christian Orsted, Danish  1819, experimented with Voltaic pile  Accidentally discovered that electricity couples with magnetism  Electric currents can repel magnet or produce a magnetic effect (?)  Electromagnetism (1819)  Not a central force st  One of the 1 instances that scientists showed the relationship between characters - André-Marie Ampère: French mathematics and chemist  Electromagnetism implies force between currents  Different in kind from Coulomb force  “Electrodynamics”  Applications: electromagnet, motor, ammeter III. Faraday’s concept of field - Michael Faraday detected the Ampere Effect (1831) Electromagnetic induction  Change of magnetic state induced electromotive force HPS211 Lecture 5 MAY29/2012  Application: electric generator - Three Electric and Magnetic Effects by the Mid-19 century  Electrostatic/magnetostatic force between charges (Coulomb/Gauss)  Electrodynamic force between currents (Orsted-Ampère)  Electromotive force from electromagnetic induction (Faraday)  How to account for these effects? - Wilhelm Weber’s Corpuscular Theory (1840s)  Electric and magnetic forces as sums of actions between constituent charged corpuscles  Electrostatic: distance R  Electrodynamic: velocity  Electromotive: acceleration  Action at a distance (like Laplacian interpretation of gravity) - Faraday’s Concept of Field (1840s)  Electric (magnetic) field at a location is the electric (magnetic) force on a point electric (magnetic) charge at that location.  Connecting the field in space forms “lines of force” - Electric and Magnetic Lines of Forces  To Faraday, the lines of forces are real entities, not math constructs  Field is not action at a distance. Lines of force are contiguous actions  Coulomb/Gauss, Orsted-Ampère, and Faraday effects are effects of electric and magnetic fields  How do electric and magnetic fields transmit in space? - Newton’s ray optics  prism experiment - Augustin-Jean Fresnel’s interference experiment IV. Luminiferous aether (ether) - Luminiferous Aether (Ether) and Wave Optics  By the 1820s, Fresnel established wave optics  Light is a transverse wave  Polarization defined by its plane of oscillation  Light wave propagates in luminiferous aether, a subtle medium -
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