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Electrostatics



THE NEUTRAL ATOM

The atom

consists of anucleuscontainingprotons and

neutrons surrounded by acloud of electrons.

Number ofprotons =

number ofnumber of

electronselectrons in a

neutral atom.

-

--

-

-

-

-

-

+ +++

++



- -

-

-

-

-

Rubbing materials

Electron clouds from the

atoms interact with eachother.

Atoms from dierentsubstances usually attract

electrons with dierentstrengths.

Electrons from the atoms inone substance can get

rubbed o onto the atoms ofthe other substance.

The substance receivingreceiving

electronselectrons becomes NEGAT!ENEGAT!E

the substance losingelectrons becomes "#$T!E.

- -

-

-

-

-

- -

-

-

-

-

- -

-

-

-

-

-

ELE!TRON TRAN"#ERRE$

!HAR%E$ ATOM" &'ON"(#ORME$



"e)arating !*arge

'f a )olyt*ene rod is rubbed+it* clot* electrons aretransferred to t*e rod.

T*e total number of negati,esand )ositi,es are still eual.

%harges *a,e sim)ly beenseparated c*arge *as notbeen created.

T*e La+ of !onser,ation ofc*arge states t*at/

%&A'GE %ANN#T (E%'EATE) #' )E$T'#*E)#N* T'AN$,E''E) ,'##NE "A%E T# AN#T&E'.

+/0 + -/0 1 2



A charged object experiences a force in thevicinity of a second charged object.

We say there is anWe say there is an

electric fieldelectric fieldpresent in thepresent in thespace around thespace around thecharged object.charged object.

An electric fieldelectric field is defined as a spacespace in which a chargedchargedobjectobject will experience an electrostatic force.

,orce of repulsion

Electric0eld



,actors aecting force

"i1e of t*e c*arge/ 23 4 etc. As t*e charge 'N!REA"E" t*e force 'N!REA"E".

$istance bet+een c*arges. As t*e distance 'N!REA"E" t*e force $E!REA"E".

d

,orce of repulsion

,

ore charge 1bigger force3

4 4

,

(igger distance 1smaller force3

,



!*arges and #orce

Li5e c*arges re)el

Unli5e c*arges attract.

-

RE6EL

ATTRA!T77



6olari1ation An unc*arged ob8ect is ATTRA!TE$ by a c*arged ob8ect.

T*is is due to 6OLAR'"AT'ON. T*e c*arged ob8ect attracts opposite charges in t*e ob8ect.

T*is causes a !HAR%E "E6ARAT'ON or 6OLAR'"AT'ON in t*eob8ect.

An electrostatic force is t*en establis*ed bet+een t*e t+o

ob8ects.

+

--

--

+

++

+



6olari1ation An unc*arged ob8ect is ATTRA!TE$ by a c*arged ob8ect.

T*is is due to 6OLAR'"AT'ON. T*e c*arged ob8ect re)els li4e charges in t*e ob8ect.

T*is causes a !HAR%E "E6ARAT'ON or 6OLAR'"AT'ON in t*eob8ect.

An electrostatic force is t*en establis*ed bet+een t*e t+o

ob8ects.

- - -

-

-

-

--

--

++

+

+



The distribution of charge on aconductor depends on its shape.

---

- -- ----

---

-

- - -----------

Electric c*arge is concentrated around a )oint.



1 Coulomb of charge is an enormouscharge.

1 electron has a charge of -1! x 1"-1# C - this iscalled the elementary charge.

The number of electrons in 1 C is$

% !&' x 1"1( electrons



1 Coulomb of charge is an enormouscharge.

1 electron has a charge of 1! x 1"-1# C - this iscalled the elementary charge.

The number of electrons re)uired toa charge C is$

% +o. of electrons in the charge



When the collecting sphere of a large ,an der raaffgenerator is charged sufficiently and the spar* is 1"cm long there is only ""1 C on the sphere.

Typical everyday charges are of theorder of

picocoulombs pC/x1"-1&

C/ ornanocoulombs nC/ x1"-#C/.microcoulombs µC/ x1"-!C/



---

------

-

---

------

-

---

-----

--

!*arged

Neutral&)olarised(

Eac*7#inal c*arge on eac* s)*ere = &->?2;-9 (

= -2.@ ? 2;-9 !Ho+ many electrons +ere transferred onto t*e neutral s)*ere

No. Of e- = c*argeB&-2.9?2;-2< ( = -2.@?2;-9 B&-2.9?2;-2< ( = <.C?2;24

electrons7



Electrosco)es

%old leaf electrosco)es areused to detect t*e)resence ofelectrostatic charge.

D*en t*e electroscope ischarged t*e gold leafis repelled by t*e s*aft

&L'E !HAR%E"( and itstands out to t*e side.

Metal dis5

Metals*aftGold

foilleaf0

!ontainer +it*

glass+indo+s"im)li0ed

diagram

UN!HAR%E$

%&A'GE)7

'N"ULAT'ON



!*arging Electrosco)es

'f a positively charged

ob5ect is broug*t closeto t*e electrosco)e )late- electrons would beattracted to the plate lea,ing t*e bottom of

the shaft and goldleaf6 positive.

T*e gold leaf would nowrepel t*e s*aft.

THE 6O"'T'FE !HAR%E" $O

NOT MOFE7777

- - - - - -

- - - - --

"ome of t*e electrons are nowtransferred to the rod.

T*e electrosco)e *as no+ ac7uired a

positive charge.

- - -



!*argingElectrosco)es'f a negatively charged

ob5ect is broug*tclose to t*eelectrosco)e )late -electrons would berepelled by the

plate ma5ing t*ebottom of the shaftand gold leaf6negative.

T*e gold leaf would

now repel t*e s*aft. THE 6O"'T'FE !HAR%E"

$O NOT MOFE7777

- -

- -

- -

- -

-

- -

- - -

- -

- -

++++++

"ome of t*e electrons are nowtransferred to the EE%T'#$%#"E.

T*e electrosco)e *as no+ ac7uired aNEGAT!E charge.

- - - -

+ ++

-

- -- -- -

- -- -

- -

- -- -

- -



Mili5anGs E?)eriment

• Milikan used a microscope to observe the oil droplets between the plates.• These droplets are charged as they are forced out of the nozzle.• As the plate voltage increases some of the drops fall more and more slowly until the drops stop

moving.• At this point the electric fore is equal to the weight of the oil droplet.• The electric force on the object is equal to the weight.

qV = mg d

q=Droplet Charge V= Holding Voltage d= Distance between plates m=droplet mass g=

Acceleration due to gravity.• From his experiments Milikan determined that the charge on an electron was 1.6×10-19 C.• By timing how long it takes for a droplet to fall with the plates switched off he could calculate the

mass of the droplet.



F d %raaf %enerator

T*e brus*es transferelectrons to and fromt*e belt.

T*ese electrons t*en

accumulate on t*emetal dome.

T*e insulating stand5ee)s t*e c*arge from

being transferred tot*e ground.

brus*es

'nsulating stand

motore-

e-

e-

e-

e-

e-

e-

e-

e-

e-

hgkujhguyg yrty rty rty rty rt ybuy guyhgu yghuiyh ug ujh

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