# Electromagnetism

March 6, 2014### A Century of Electromagnetic Discovery

1751 Franklin—iron needles magnetized by an electric discharge

1781 Coulomb—1/r^{2} law of electric force

Force between charges is proportional to product of the charges divided by square of the distance between them

F = q_{1}q_{2}/r^{2}

1789 Aepinus—capacitor

1791 Galvani—electric current from frog

1800 Volta—battery

1820 Oersted-–magnet causes current-carrying wire to move in a circle

1820 Biot Savart and

1821—Ampere—Currents attract—pinch!

Force between two current carrying wires proportional to length, product of currents, divided by distance between them

F/L=2I_{1}I_{2}/Cr

1824—Sturgeon invention of electromagnet—circular current—magnet

1825—Ampere current creates a circular magnetic force

B=I/5r

1831—Faraday changing electric current creates magnetic field, changing magnetic field produces current—still perpendicular force—magnetic lines of force—Joseph Henry

1833—Gauss, Morse—telegraph

Faraday’s law—the rate of change of magnetic field proportional to perpendicular electric field

1846—Faraday speculates vibrations of E field could be radiation

1830’2-1860’s development of electric dynamos and motors

1861—Maxwell “On physical lines of force”—hydrodynamic analogy—magnetic lines are vortices—(also Helmholtz—1859) centrifugal forces—changing magnetic field generates E, changing E field generates B