ElectrostaticsElectrostaticsFrom our previous From our previous

discussions in class you discussions in class you do remember the four do remember the four

fundamental interactions fundamental interactions in the universe,in the universe,

Can you name them?Can you name them?

electron

neutron

proton

Four Kinds of ForcesStrong Nuclear Force:

A strong force that holds the particles of the nucleus of an atom together. Short range attractive force that is much larger in magnitude to the gravitational or the electromagnetic forces.

Weak Force:Force involved in transmutation of particles within the nucleus. Only observed/viewed in radioactive decay. Stronger only than the gravitational force.

Four Kinds of ForcesGravitational Force:

Attractive force that exists between all objects. The gravitational force between the Earth and the moon keeps the moon in orbit. It may be the most evident but it is the weakest of all the forces.

Electromagnetic Force:Charged particles at rest or in motion exert electric forces on each other. They give materials their strength, their ability to bend, squeeze, stretch or shatter. When charged particles are in motion they produce magnetic forces on each other. Electric and magnetic forces are both considered to be aspects of this single force.

Electromagnetic Force

We are going to study electromagnetic force and its effects and importance on our world for the next couple

of monthsTo start we look at electric charges at rest called……….

ElectrostatElectrostaticsicsStudy of properties and results of

electric charges at rest

AtomAtom is Neutrally Charged – Positive charge on the nucleus is exactly balanced by the negative charge of electrons

An Electron can be removed from

an atom to create a positive

ion

Freed electron can be:Unattached and Free – Creating a negative charged particleAttached to an atom – Creating a negative Ion

ElectroElectronsns

Electric ForcesElectric Forces• There are two kinds of electric charges:

– Positive – protons – p+– Negative – electrons – e-

• Charges exert a force on other charges over a distance.

• Like charges repel.• Unlike charges attract.• Electroscope = Instrument to

determine charge.

Materials can be of three kinds:

Insulators – Materials that inhibit the flow of free charged particles

Examples: wood, air & rubberConductors – Materials that allow

the flow of free charged particlesExamples: metal & waterSemiconductors – Intermediate

class, conduction between an insulator and conductor

Charging of ObjectsAn object can be charged either through conduction or induction

Charging by

Conduction

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-

-

-+

+-

-

Charging a neutral body by touching it with a charged body

Charging of Objects

--

-

-

-

+

+

InductiInductionon

An object can be charged either through conduction or induction

Charging a neutral object by bring a charged body close to but not touching the object.

+

Charging of Objects

--

-

+ +

+ +

-

-

-

-

An object can be charged either through conduction or induction

ChargiCharging by ng by

InductiInductionon

+

+

Charging of ObjectsAn object can be charged either through conduction or induction

ChargiCharging by ng by

InductiInductionon

+ +

+ +

- -

- ---

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Charles CoulombCharles Coulomb (1738-1806)

was a French physicist and military engineer. Because of his expertise with simple machines, he was able to build an apparatus to measure the electrical force between two charged objects. He derived a law, Coulomb’s Law, which gives the relationship between charges, their separation and the electrical force of attraction or repulsion.

Coulombs Coulombs LawLaw

Coulombs Law states that the size of the electric force between two charged particles depends on the size of the charges and the distance between them.

FKqq

r

'2

q is a unit of charge measured in coulombs “C”

r is the distance between the charged objects

K is a constant = 9.0 x 109 Nm2/C2

Coulombs LawCoulombs LawThe charge on one electron or one proton is called an elementary charge

e- = -1.60 x 10-19 C

One coulomb of charge has6.25 x 1018 electrons or protons

One lightninglightning bolt may have 10 C of charge

p+ = +1.60 x 10-

19 C

Millikan Oil Drop Experiment

Robert Millikan

American physicist who determined the charge on an electron using charged oil drop

experiments in 1909.

1868-1953

What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this unit charge.

Millikan Oil Drop Experiment

The Millikan oil-drop experiment was far superior to previous determinations of the charge of an electron. Where other workers had attempted to measure the quantity by observing the effect of an electric field on a cloud of water droplets, Millikan used single drops, first of water and then, when he found these evaporating, of oil. The experiment had broader significance than a simple refinement of a number. Millikan emphasized that the very nature of his data refuted conclusively the minority of scientists who still held that electrons (and perhaps atoms too) were not necessarily fundamental, discrete particles. And he provided a value for the electronic charge which, when inserted in Niels Bohr's theoretical formula for the hydrogen spectrum, accurately gave the Rydberg constant—the first and most convincing proof of Bohr's quantum theory of the atom.