electricity form 5 chap 2 malaysia

8/20/2019 Electricity Form 5 Chap 2 malaysia

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Chapter 2

Form 5

Electricity



Understanding charges



Figure 1: A flow charges forms an

electric current

The Van de

raaff

generator isused to

produceelectric

charge.



! From acti"ity a#o"e$ we %now that an

electric current is caused #y a flow of

electric charge&

! 'n general$ we define

current as the rate of flow of electric

charge.

Current = Charge

Time or



! The (' unit of

)a* current : ampere )A*

)#* charge : coulom# )C*

! ! A current in a circuit is 1 ampere if the charge is

flowing at the rate of 1 coulomb per second.

! +y rearranging the formula ' , -.t$ we o#tain: - , 't

1C , 1A / 1s , 1As! Thus$ a coulomb is the charge which passes any pont

in a circuit when a steady current of 1 ampere isflowing in 1 second&



How many electron to make up 1 coulomb?

! The charge in an electron, e = - 1.6 x 10 ‾! C

! 'f 0 electron ma%e up 1 C of charge$ then:

"umber of electrons," x charge in an electron,e =Total charge,#

" x e = 1

! Therefore$ 6.$% x 10 1& electrons are reuired to ma%e up 1

coulomb of charge&

! 'f a current of 1 A is flowing on a point in a circuit$ it means &25

/ 1314

electrons pass through that point in 1 second



El t i t i d #



Electric current is measured #y

using an ammeter$ a milliammeter

or a micrometer



! To measure the current passing through a light

#ul#$ an ammeter is connected in series withthe light #ul#$ with its positi"e terminal )use awire in red colour* connected to the positi"eterminal of the #attery as shown in figure

a#o"e&



'irection of an electric current ! A circuit is a closed loop throughwhich charges can continuouslyflow&

(a) *xternal circuit: The path

ta%en #y the charges outsidethe cell or the #attery&

(b) +nternal circuit : The pathwithin the cell or the #attery&

! The electromoti"e force of thecell or #attery causes thenegatiel-charged electrons to flow from negatie terminal to the positie terminal

through the wires&6 The direction of a current in the

external circuit is from the positie

terminal of an electric cell or #attery

to the negatie terminal.



! The interaction of forces are reduced when thedistance #etween the charged #odies are

increased, and "ice "ersa if the charges are

close each other&



Electric Field 7ines! An electric field pattern is represented #y electric

field lines.! The electric field lines show the direction of

electrostatic forces e/perienced #y the two opposite

charges )or electrodes* in an electric field&

! Electric field lines are directed from the positie

electrode )positi"e charge* to the negatie electrode

)negati"e charge*&

! The electric field lines do not cross each other.! The closer the electric fields lines$ the stronger the

electrostatic forces& Furthermore$ the electric field

lines neither #ranch off nor intersect one another&



E/periment

Aim: To study electric field lines for differentarrangements of charges&

8ypothesis: The pattern of electric field lines

changes as the shape of electrodes changes&



Varia#les:

)a* 9anipulated : (hape of electrodes

)#* esponding : ;attern of electric fields

lines

)c* Fi/ed : potential difference of

electrodes

Apparatus . 9aterials

lass dish$ metal electrodes$ high "oltage

supply$ connecting wires$ oli"e oil and

semolina powder or grass seed&



;rocedure

1& The apparatus is set up as shown in figure

a#o"e&2& A glass dish is filled with oli"e oil until the pair

of electrodes is completely #elow the oilsurface&

<& (emolina powder is dispersed e"enly on thesurface of the oil&

=& The high "oltage supply is switched on&

5& The "oltage is increased until a pattern of the

electric field #ecome "isi#le&& >ther pattern of electric field are o#ser"edusing electrodes of different shapes such asspheres$ planes or com#ination of shapes&



esults

1& ?hen the high "oltage supply is switched on$ thesemolina powder on the oil surface #ecome

aligned in the direction of electric field and thusmaps out the electric field lines&

2& Figure #elow shows some patterns of the electricfield using different types of electrodes&



'iscussion

1. lie oil is used #ecause it is an insulator

which does not conduct electricity & Thisena#les a "ery high "oltage to #e maintained

across the two electrodes&

2& The semolina powder aligns itself along the

electric lines of force due to the action ofelectric field&

Conclusion

An electric field exists in the space surroundinga charged bod. The pattern of the electric

lines of force depends on the shape and the

num#er of charged o#@ects&



)a* (ingle positi"ely )#* (ingle negati"ely



)a* (ingle positi"ely )#* (ingle negati"ely

charged sphere charged sphere

A iti l h d



! A positi"ely charged

sphere and a negati"ely

charged sphere&

! The two opposite

charges attract each

other&

)pposite charge/ attract)

! Two positi"ely charged

spheres&

! The two similar charges

repel each other&

)ie charge / repel)



! A positi"ely charged plate

and a negati"ely charged

plate&! A uniform electric field

e/cept at the sides&

! A positi"ely charged plate

and a positi"ely charged

plate&! A neutral point at the

centre&



! A negati"ely charged


charged plate&! A neutral point #etween

charged sphere and

charged plate&! A positi"ely charged


charged plate&! A nonuniform electric

field&



Electric Current and ;otential Bifference



! The potential difference, 2 between points 3 and 4can be defined as the wor done in moing a unitcharge from 4 to 3 in an electric field.

! 5otential difference = wor done

charge! 2 =

#



! The potential difference #etween two pointsis 1 olt if 1 7oule of wor% is reuired to

mo"e a charge of 1 coulom# from one pointto the other&

! 1 "olt , 1 @oule

1 coulom# or

1V , 1 C D

6 For e/ample$ when 1 2 is applied across alight bulb$ 1 8 of electrical energ isconerted to heat and light when 1 C ofcharge flow passes through the light #ul#$

that is 1 V , 1 CD



! (ince wor% done is con"erted to other forms of

energy$ the definition can #e rephrased as

follow:

;otential difference can #e measured #y using a

oltmeter or millioltmeter which is

connected in parallel across the two points&



! The "oltmeter is

connected in

parallel with the

#ul#&



The differences #etween an ammeter and a

"oltmeter in a circuit



! The difference in the pressure of water $ represented #y theheight of the water column&

! The larger difference in the pressure of water causes ahigher rate of flow&

! What is the relationship between the electric current flow ina conductor and the electric potential difference across theconductor

! 'n 142$ 9eorge :imon hm$ a erman physics teacherfound relationship #etween V and '&

! The relationship named hm;s aw&



! The relationship can also #e represented #y a graph as shownin figure #elow&

! Conductor with 2-+

9raphs which are linear

and pass through the

origin o#ey >hmGs 7aw

and are said to #e

hmic conductors&

6 >ther materials which do not obe hm;s law are called non-

ohmic conductors.



i t



esistance

or 2 3‾



<ormulae from the factors affect



<ormulae from the factors affect

resistance<rom the factors affect resistance, the following can be

stated/

*xample/ :iler is a er good conductor with a er low

resistiit, .



(uperconductor



(uperconductor ! A Butch physicist$ 8ei%e

Hammerlingh >nnes

disco"ered in 1I11 that whenmercur is cooled to =&15 H)or 2I JC*$ its resistance isthe lowest )almost >ero* andbecomes a perfect

conductor )or asuperconductor*&

! For a metal, its resistance decreases at itstemperature decreases&! At er low temperature called the critical

temperature Tc$ its resistance approaches >ero andthe phenomenon of superconductiit is said to

occur&

! Ta#le shows some element

i h h i i i l



with their critical

temperatures.

! These materials offer no

resistance to flow of currentand act as perfect

conductors with

>ero resistiit.

! >nce the current is

set up in these

materials$ it needs

no applied oltage

to persist flowing

without an loss.

The application of superconductivity



The application of superconductivity

! The high current flow in the conductor generates aer strong magnetic field&

! This causes strong magnetic repulsion&

! Applications of superconductors

)a* inentions of ?agle train

)#* strong accelerator in nuclear research c* ?agnetic resonance +maging )9'*& 9' is used as

diagnostic tool in medicine&

)d* Computer chip which are faster and of smaller siKe&

6 (cientists are still finding alternati"e of superconductingmaterials which ha"e higher critical temperature#ecause the operating cost of superconducti"ity ater low temperature is not economicall "ia#le&



! ?hen a ball falls through acylindrical card#oard from aposition of highergraitational potentialenerg$ wor% is #eing done #y

the gra"itational field& 8ence$the man needs to do wor #y carrying the #all #ac% to aposition of higher gra"itationalpotential energy so as torepeat the process again.

! (imilarly$ an electric current flows froma higher electric potential )L terminal*to a lower electric potential ) terminal*as shown in figure a#o"e&

! 'n this case$ electric energy is conerted

to light energy and energy loss due toheat dissipated&

! The batter has an electromotie force (e.m.f.) which is a#le to drie the electric charges throughout the circuit&



or total energy supplied #y a cell to mo"e a unit of

charge through the complete circuit&

or



'nternal resistance )r*



raph V against '



;ower ating



g



! The la#el shows that the fan heater )a* wor%s at a "oltage of 223 V

)#* it consumes an electrical power of

2333? )or 2333 @oule per second*



(olution:



! 3n immersion heater is moresuita#le to boil a cup of coffee



#ecause

the immersion heater has a

lower rating.! The %ettle is more suita#le to #oil a

larger "olume of water&

! 3 small room normally has an airconditioner of 1 horse power (hp) rather than 2&5 hp which wasteselectrical energy and cools the roome/ceedingly&

! The airconditioners should hae thethermostat sstem to switch off andswitch on the airconditionersautomatically so as to control thetemperature of the room&

electricity form 5 chap 2 malaysia

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