Lec 7: Forces and Fields to EM Theory Flashcards
Faraday 1930 - Kinds of electricity?
1832, Faraday tries to relate different kind of electricities
- > searched for unity between electrostatic generators, voltaic cells, thermocouples, dynamos, electric fixshes
- > those like John Davy’s (Humphry’s bro) thought E effects were complex combinations of many powers
Studies conduction in liquids
- > I flows through all water solutions
- > up to this point, can’t measure quantities of electricity
- > not sure if different electricity -> same effects
Designs an apparatus to measure electricity
- > products of electrolysis proportional to their chemical combining weights
- > second law of electrolysis
With William Whewell, introduces terms
- > electrolysis
- > electrolyte
- > electrode
- > anode
- > cation
- > cathode
Ion = electrified substances transported through liquids in electrolysis
Realised there was a proportion of elements set free to a standard current
- > Helmholtz shows this is charge of electron
- > but Faraday don’t like atomic theory, or electrical atoms
Faraday’s Electricity Measuring Device
- > glass tube, with two electrodes in it
- > both electrodes go to a test tube filled with water
- > connecting the terminals to electricity passes current through water
- > water –> H + O, as gas in top of each tube
- > can see amount of gas produced
- > which in proportion to electricity passed through it
- > later version, each electrode gets own tube
- > H and O in individual tubes
Electricity as a Force?
Faraday observed static electricity discharging through air via blotting paper (soaked with an electrolyte)
- > challenges standard electrical theories of action at a distance
- > no poles?
- > electricity is A FORCE, not a MATERIAL
- > “current” is rupture/re-establishment of bonds as ions migrate through solution
This explains insulators and currents through wires
- > insulator = can sustain high Electric force without the molecular chain breaking
- > good conductor = weak, can sustain very little intermolecular strain - the strain builds and breaks rapidly
Thought this explained all phenomena of electricity
- > and E as a force and interactions with materials can explain Magnetism
- > if E is a force, B is a force (and not a liquid!)
- > but why does B only act on certain materials (iron, nickel, cobalt), unlike E?
Magnetic Fields and Light?
Faraday tried using a powerful electromagnet rather than static
- > B can affect a ray of light!
- > pass a polarized ray of light through a highly refractive substance along a magnetic line of force
- > will rotate plane of polarization, proportional to |B|
Led him to Universality of Magnetism
- > tried to show all bodies affected by a B
- > leads to DIA/PARAMAGNETISM
- > also tries to link gravity/B, fails
Diamagnetics and Paramagnetics?
Dia: Conduct B poorly
->in a uniform B, lines of force diverge
Para: Conducts B well
- > in uniform B, lines of force converge on the body
- > the lines of force cluster on entry and exit from the body
- > these are the POLES of induced magnets
JCM (1831-1879)
Sees Faraday’s description of E & M good for mathematical description
- > formulates it all into mathematical equations
- > known as ELECTROMAGNETIC THEORY
Such a based man, Faraday’s influence is primarily through him. However, JCM modified F’s theory:
- > JCM said matter and the field are different entities
- > Newton says each body exerts spooky force, the field is just book keeping to express it
- > JCM says the bodies interact indirectly via their fields
- > but he still separated matter & force
- > electric force consists of forces per unit charge, NOT fields of force
- > but he still separated matter & force
Electromagnetic Theory
Magnetism and Electricity cannot exist separately!
- > shows E field created by oscillation of I
- > radiates outward from source at 186k miles/s
- > problem with C - so light is an electric charge too!
Heinrich Rudolf Hertz (1857-1894)
Created a circuit that would produce long-wave radiation, if light was a kind of EM radiation
- > metal rod with gap at midpoint
- > sparks cross gap, causing violent high f oscillations
- > proved they are transmitted through air, by detecting them some distance away
Showed this radiation was like light
- > they could reflect, refract, polarize
- > SAME SPEED but longer wavelength
- > 1M times the size
- > “Hertzian waves”, confirm Maxwell’s prediction of EM waves
- > leads to radio communication, TV, radar
Guglielmo Marconi (1874-1937)
- > saw application of Hertz’s waves to Morse code
- > used to form Marconi’s Wireless Telegraph Co
- > first wireless internation transmission, En-Fr
- > 1901, cross Atlantic
