Ch08 Electric Tambahan

8/9/2019 Ch08 Electric Tambahan

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Troy University, SCI 2233 2009

Electric charge

The Van de Graaff generator (left) that these children aretouching can produce very large charges of static electricity. Theballoon can then attract a stream of water (right), even thoughwater molecules themselves are electrically neutral.


Model of an atom

Every atom is composed of a positivelycharged nucleus surrounded bynegatively charged electrons

Each of the electrons in any atom has thesame quantity of negative charge andthe same mass. Electrons are identical toone another

Protons and neutrons compose thenucleus. Protons are about 1800 timesmore massive than electrons, but eachone carries an amount of positive chargeequal to the negative charge ofelectrons. Neutrons have slightly moremass than protons and have no netcharge

+

+

Charge:1 proton = 1 electron

Mass:

1 proton = 1800 electrons

1 proton ~ 1 neutron

-

-


See atoms

[001]

a)5 nm

0,50 nm

b)


Electrical polarization

When a charged rod is far from a neutral object, the atoms in the object

are undistorted. As the rod is brought closer, however, the atoms distort,

producing an excess of one type of charge on the surface of the object (in

this case a negative charge). This induced charge is referred to as a

polarization charge. Because the sign of the polarization charge is the

opposite of the sign of the charge on the rod, there is an attractive forcebetween the rod and the object.


Electric conductors

Materials like amber, in which charges are not free to move, are referred to as

insulators. Most insulators are nonmetallic substances, and most are also good

thermal insulators.

Most metals are good conductors of electricity, in the sense that they allow

charges to move about more or less freely.

Some materials have properties that are intermediate between those of agood conductor and a good insulator. These materials, referred to assemiconductors, can be fine-tuned to display almost any desired degree of

conductivity by controlling the concentration of the various components fromwhich they are made.

Exposure to light can sometimes determine whether a given material is an

insulator or a conductor. An example of such a photoconductive material is

selenium, which conducts electricity when light shines on it but is an insulator

when in the dark.


Photocopy


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Magnitude of an Electron's Charge

The charge of an electron isvery small

e = 1.60 10-19 C

Charles-Augustin

de Coulomb (June

14, 1736 August

23, 1806, Paris,

France)

The precise definition of

the coulomb is in terms

of electric current


Ions

Normally, an atom has as many electrons as protons. It has nonet charge

If an atom gains electrons, it has negative net charge. If anatom loses electrons, it has positive net charge. Atoms withmore or less electrons than protons are called ions

Material objects are made of atoms: protons, neutrons,electrons

Atoms have many electrons

The innermost electrons are attracted strongly to theoppositely charged atomic nucleus

The outermost electrons of many atoms are attracted moreloosely and can easily be dislodged


Conservation of charge

No electrons are created or destroyed.

Electrons are simply transferred from one

material to another. Charge is conserved

Conservation of charge, of energy, of

momentum are the fundamental principles in

physics


Coulombs law F = k q

1q2/d2

Unit of charge is C (Coulomb)

k = 9.000.000.000 Nm2/C2

Value of k (9109) is muchhigher than the value of G(6,6710-11)

Gravitational force isattractive, whereas the electricforce is either attractive orrepulsive +

-

The sun

The earth

Nucleus

Electron

http://www.youtube.com/watch?v=xx8fH-

r3Luk


Hydrogen atom

r = 5,2910-11 mmp = 1,6710

-27 kgme = 9,1110

-31 kgqp = -qe = 1,610

-19 CFg = 3,6310

-47 NFe = 8,2210

-8 N

Fe/Fg = 2,261039Attractive force of the earth < attractive force of the amber rod.


Electric field

Force field: electric force canact between things that arenot in contact with each other

due to the force field Force field: gravitational field,

electric field

Electric field configuration: line force

about a single positive charge and two

parallel plates

http://ocw.mit.edu/ans7870/8/8.02T/

f04/visualizations/electrostatics/06-

DipoleField3d/06-dipField320.html

http://ocw.mit.edu/ans7870/8/8.02T/

f04/visualizations/electrostatics/01-

VandeGraffAttract/01-

vdg_Attract_320.html


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Electric potential

A charged object in an electric field has anelectric potential

Electric potential =electric potential energy/amount of charge

1 volt = 1 joule/coulomb

A 1,5-volt-battery gives 1,5 joules of energy toevery 1 coulomb of charge flowing throughthe battery

http://www.youtube.com/watch?v=70gh5GrX4xI


Voltage source, electric current

To maintain a flow of charge in a conductor, agenerator arranges a difference in potential while

charge flows from one end to the other Generators separate charge by electromagnetic

induction

Electric current is a flow of charged particles. Thecharged particles in metals are conductionelectrons

Electric current is measured by ampere. 1 A = 1coulomb/1 second

Speed of electrons is 1 cm/s

Speed of electric field is the speed of light


Charge distribution on a conducting

sphere

(a) A charge placed on a conducting sphere

distributes itself uniformly on the surface of the

sphere; none of the charge is within the volume of

the sphere. (b) If the charge were distributed

uniformly throughout the volume of a sphere,

individual charges, like that at point A, would

experience a force due to other charges in thevolume. Since charges are free to move in a

conductor, they will respond to these forces bymoving as far from one another as possible; that

is, to the surface of the conductor.


Intense electric field near a sharp

point

Electric charges and field lines are more densely

packed near a sharp point. This means that the

electric field is more intense in such regions as well.


In a dramatic science-museum demonstration of electricalshielding (left), the metal bars of a cage provide excellentprotection from an artificially generated lightning bolt. A morepractical safeguard is the lightning rod (right).


Direct and alternative current

Direct current:

current does not

change with time

Alternativecurrent: 60 Hz,

220 V

time

time

Direct current

Alternativecurrent


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Direct Current Electric Motor

http://www.youtube.com/watch?v=Xi7o8c

MPI0E


Ohms law

Electric resistance: the movement electrons in

a wire is blocked. Electric resistance ismeasured in units of Ohm

Superconductor: no electric resistance

Current = voltage/resistance

Ampere = volt/ohm


Resistivity

When cooled below their critical

temperature, superconductors not only

lose their resistance to current flow but

also exhibit new magnetic properties, such

as repelling an external magnetic field.

Here, a superconductor (bottom) levitates

a small permanent magnet.http://www.youtube.com/watch?v=4VGAC

LNfZ8s


Electric shock

Current (A)

0,001 Can be felt

0,005 Is painful

0,010 Muscle contraction

0,015 Loss of muscle control

0,070 Go through the heart

Depending on the conditions, the electricresistance of the body can vary from 100

to 500.000 ohm


Electric circuits Any path along which

electrons can flow is acircuit

Series circuit

Parallel circuit

Safety fuse: if the current ishigher than the safetycurrent, the safety fuse willbreak the circuit


Electric power

Power = current voltage

1 watt = 1 ampere 1 volt

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Ch08 Electric Tambahan