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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

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electrification of bodies electrostatics
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

slide2

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

slide3

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

slide4

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

slide5

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

slide6

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.

slide7

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)

slide8

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

slide9

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)

slide10

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

slide11

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

slide12

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

slide13

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

slide14

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’

slide15

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

slide16

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

slide17

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

slide18

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

slide19

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