Transcript Electrification of Bodies

Electrification of Bodies Electrostatics
Description of Atom
Greeks used amber to pick up bits of lint or fluff
“elektron” = Greek word for amber
Elizabethan Era - amber,glass,etc bodies which could exert force
on a small test body if charged = electrified
Noted long range force acting over inches
19 th Century conclusion : only 2 kinds of “electricity”
Resinous - rubber rod + fur = rod charged negative
Vitreous - glass rod + silk = rod charged positive
Electroscope - description
Experimental Conclusions
1. Like charges repel each other ; unlike charges attract each other
2. Substances differ markedly in their electrical conductivity
e.g. insulators, semiconductors, conductors, superconductors
3. Electric charges are of two kinds
4. Electric forces act a distance
5. An object can be charged by induction
Charles Augustin Coulomb (1736 - 1806)
Scientific Priority : Daniel Bernoulli 1760,
Joseph Priestley 1767, John Robison 1769,
Henry Cavendish 1775
Coulomb’s Law : force law between two charged bodies
Coulomb used torsion balance (1788) similar in principle to
That used by Cavendish 10 years later for gravitation
Statement of Law - involves inverse square law for electric
force
Storage of Charge
Leyden jar forerunner of modern capacitor built in
1746 at the University of Leyden by Dutch scientist
Pieter van Musschenbroek (1692-1761) as device to
store large amounts of charge
Van de Graaff generator
Lightning rods - Ben Franklin
Shock from Electric Eel & Frogs’ Legs
Luigi Galvani (1737 - 1798) Italian Physiologist
Alessandro Volta (1745 - 1827) Italian Physicist
No device to give continuous source of charge
Natives of Africa & S.America familiar with fish
That delivered shock - effect reminiscent of Leyden
Jar if both ends are touched when charged
Frog legs (Bologna delicacy) - Galvani noticed legs
Suspended from copper hooks on his balcony jerked
As if alive when legs touched iron railing.
Lab Experiment - fork with Fe and Cu prong
Therefore - “animal electricity” (1791) - fork
Released electricity from frog legs ?
Nephew - Giovanni Aldini - awarded Copley Medal
Of Royal Society for using electrical discharge to
Briefly reanimate decapitated felon.
Volta confirmed Galvani’s results but realized
Galvani was wrong ; instead noted electricity
Produced by junction of dissimilar metals in
Water solution of a salt = Voltaic Pile (“30,40,
50 or more pieces of Cu applied to each a piece
Of Zn and many strata of cardboard soaked in
Lye or salt water” ) = first battery
Galvani’s Group - pioneered electrophysiology
(nerves produce & transmit electrochemical
Signals)
Andre Ampere (1775 - 1836) his name used for unit
Of electrical current
I = electrical current = rate of flow of charge
= charge/time (amperes)
For current to flow we need potential difference
(i.e. voltage difference) like water running downhill
Electric Potential (volts) = potential energy/charge
(joules/coulomb= volt)
For like charges to be brought together requires work
For unlike charges work is need to pull apart
Ex. Battery hooked to light bulb; analog is water
Pump with reservoir and paddle wheel
Power = work/time = (work/charge)(charge/time) = IV
Power & current of some household appliances
Consider wire of length L, cross-sectional area A
Hook up to battery, creates E field
One would suspect the larger the E field,the larger the
Current, i.e. they are proportional or I = (const) E
This is Ohm’s Law (theoretical form)
But, I = (const) V/L can incorporate R = L/(const)
To get I = V/R Ohm’s Law (practical form)
With R = (rho) L/A = resistance in units of Ohms
And implies longer wire more resistance, greater
Diameter less resistance
Revisit battery , switch & resistor (light bulb) circuit
Networks of Batteries
1. Series - Voltages add (see water analog)
Note completed circuit needed for sustained flow
ex: Auto Battery - six 2 volt cells in series
2. Parallel - Same voltage, but each supplies a fraction
of the current; keep bulb burning longer
Networks of resistors
1. Series - same current, total voltage is sum of
individual voltage drops
2. Parallel - same voltage across each, total current
is sum of individual currents
Electrical Safety
Current limiting devices : fuses, circuit breakers
Proper grounding - do not want current to take
Path through device user (i.e. person)
Three prong plugs, grounded receptacles,fuse boxes
And circuit breakers
Note: Current kills, not voltage itself.
About 100 mA causes ventricular fibrilation
By 1800’s international community of scientists -large
Journals,private correspondence,Royal Society,etc
Made possible for many physicists to contribute to
Understanding electricity & magnetism
Stand Out Contributor:
Michael Faraday (1791-1867), son of blacksmith
No opportunity much schooling (just basics);
Age 14 apprenticed as bookbinder (7 years)
Natural inquisitiveness,self taught
Sir Humphrey Davy - top chemist at Royal Institution
Credentials - lecture notes (Royal Society talk) used
To get job in lab
Ultimately recognized as talented researcher-chemistry
Age 34 director - Royal Institution 1st Government
Research Lab
Turning point (1831) age 40 electrical experiments
Reputation: ‘Greatest Experimental Physicist’
Lack of formal education - couldn’t visualize
Electrical and magnetic forces via ‘action at a
distance’,thus visual concept of field,using lines
To represent:
1. Direction of force
2. Strength - where lines are closest (greatest)
3. Number of lines arbitrary - only relative spacing
important
Electric Field Strength = E = force/positive charge
Magnetism
Effects recognized for centuries
Lodestones (natural magnets-iron oxides) were found
Near Magnesia -ancient city in Asia Minor
Greeks: lodestone attracts bits of iron
Chinese: 121 AD iron can be magnetized by being
Near a lodestone
Vikings: in the 11 th century navigated via crude
Magnetic compass
1820 Hans Christian Oersted - made connection
Between electricity and magnetism: compass needle
Deflects near current carrying conductor
Decade later: M.Faraday and Joseph Henry
Independently note a current exists in a circuit while
The current in a nearby circuit was being started or
Stopped = Electromagnetic Induction
Basic Effects
1. Like magnetic poles repel; unlike magnetic poles
attract
2. Force acts at a distance (Inverse square law)
Magnetic Fields
1. Bar Magnet
2. Long Straight wire
3. Loops of wire
4. Earth’s Magnetic Field
5. Domains
6. Electromagnets
Faraday’s Law Of Electromagnetic Induction
The time rate of change of the magnetic flux is
Proportional to the negative of the induced E.M.F.
(magnetic flux =magnetic field strength x area)
Heinrich Lenz - professor of physics St.Petersburg,
Russia published 1834 : Lenz’s Law - An induced
E.M.F. tends to set up a current whose action opposes
the change that created it
Examples: Transformers (step-up,step-down),
Galvanometer (heart of applied electricity),
Electromagnetic generator, electrical power
transmission