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Electrostatics and magnetism

Physics

Grade 10 Advanced level

2

Electrostatics and magnetism Unit 6

UNIT 6: Electrostatics and magnetism

Core Standards …………………………………..… 3

Objectives ………………………………..………… 4

Electrostatics ………………………………..……………………… 5

Friction and charge ………………………………..……………. 8

Electric Field ………………………………..…………………….. 19

Magnetism ………………………………..……………………….. 25

Magnetic Field ………………………………..…………………… 32

Questions ………………………………..….……… 35

References ………………………………..…..…… 39

30.1-

30.2

Contents

30.3

30.4

30.5

3


10A.30.1 Distinguish between conductors, semiconductors and insulators

with reference to moving electrons or ions; know how the properties of semiconductors can be influenced by the presence of small quantities of impurities.

10A.30.2 Know that friction can generate two kinds of electric charge on an

insulator and that opposite charges attract but like charges repel each other.

10A.30.3 Describe an electric field as an example of a field of force and

know that electric field strength can be defined as force per unit positive charge and that an electric field can be represented by means of field lines.

10A.30.4 Make magnets from magnetic materials by a variety of methods.

Know that they have north and south poles and that unlike poles attract and like poles repel each other.

10A.30.5 Describe a magnetic field as an example of a field of force and

know that it can be represented by means of field lines.

4


By the end of the unit, students generate electrostatic charge in insulators, know the rules of electrostatic attraction, know how to use an electroscope to investigate charge and understand distribution of charge on a conductor. They detect electric fields and know that they can exert a force on a charge They know that magnets have north and south poles and generate fields, the shape of which they plot, that exert forces on other magnets.

Students who progress further define electric field strength and draw and interpret field lines representing the strength and direction of electric and magnetic fields.

5


Have you ever placed your hand next to a television screen when the television has just been switched on? The hair on your hand will stand on end.

شؼشخ ارا ٠حذز شؼش ا١ذ ػذ الغح ٠ذن شاشح ارفاص ؟ذشغ١حظح

Try walking across a rug and reaching for a doorknob in a dry environment. You will experience a shock!

ي ظ مثغ اثاب ف ٠ ظاف تؼذ اش ػ ععادج اح .ظافح

.عرعشب اشؼس تظذح

33.1 Electrostatics

الكهرباء الساكنة

Lighting is a common occurrence during a heavy rain. Have you ever wondered how lighting is produced and why it is usually strikes during a heavy rain?

اـشج ف فىشخ و١ف ٠حذز اثشق؟ األ٠اظاشج ذحذز ف اثشق

Some Common Phenomena of Static electricity

6


Some objects (such as a glass rod or an ebonite rod) acquire a new property of being able to attract small pieces of paper after they have been rubbed with another material (such as silk or fur). This phenomenon belongs to the branch of Physics called electrostatics or static electricity.

It involves the study of static electric charges. Before rubbing, these objects do not attract small pieces of paper. This implies that friction due to rubbing has changed the nature of the surfaces of the rods. We say that friction has caused the rods to be 'electrified' or 'charged'.

. افش أتمـؼح احش٠ش دىظزب لظاطاخ اسق تؼذ ٠ىتؼغ ااد ومؼ١ة صظاض زا ٠ؼ .ذعزب لظاطاخ اسق أال ٠ى ا األظغالث ده . ح١س ذغ اظاشج تاىشتاء اغاوح

. اذه شح عـح امؼ١ة اضظاض ارؼادي أشاءاالحرىان أ

Table gives a summary of some electrostatic experiments and the corresponding observations and inferences.

. االعرراظاخ تؼغ ذعاسب اىشتاء اغاوح اشاذاخ

Experiment Observations Inferences

Experiment 1

The pith ball remains in its rest position when the uncharged glass rod is brought near to it.

ذثم وشج اث١غا عاوح ػذ ذمش٠ة لؼ١ة اضظاض غ١ش

.اشح ا

The gravitational force of attraction between the pith ball and the uncharged glass rod is too weak to cause any movement in the pith ball.

ل ارعارب اىر ػؼ١فح ظذا .ذحذز حشوح ىشج تح١س ال

Friction and charge

االحركان وانشحنح

Some electrostatic experiments

33.2

7


1. For Experiment 2, the pith ball is seen to move towards the glass rod after the rod has been rubbed with silk

اضظاض تؼذ تاذعاذعزب اىشج دى

2. For Experiment 3, the pith ball is seen to move towards the ebonite rod after the rod has been rubbed with fur.

3. The angles of displacement and of the pith ball are

generally large

ىشج ػادج and اضا٠ا .وث١شج ذى

1. Both the glass and ebonite rods are able to attract light objects

after they are rubbed with silk and fur respectively, i.e. they are electrified or charged.

دى٠شح لؼ١ة اضظاض تؼذ .اث١غاتاحش٠ش ف١عزب وشج

2. The attraction between the pith ball and each rod is fairly large, i.e. the electrostatic force is much stronger than the gravitational force between the pith ball and the rods.

ل ارعارب اىشت اوثش .ل ارعارب اىر

The angle of displacement of the pith ball is smaller

than or ,

اطغش تىص١ش اضا٠ح اضا٠ر١

The electrified states of the glass and ebonite rods tend to weaken each other. In other words, there exist two

electrified or charged states.

٠ؼ لؼ١ث اضظاض االت١د إػؼافاشح١ ػ .تؼؼا اثؼغ

8


The two charged glass rods are seen to repel each other. (Note: Repulsion also occurs between two charged

ebonite rods.)

٠حذز ذافش مؼ١ث صظاض .شح١

The electrified glass rods have similar or like charges.

.شحاخ لؼ١ث اضظاض رشات

The two charged rods are seen to attract each other.

. ٠حذز ذعارب ت١ امؼ١ث١

The glass rod and ebonite rod have different or unlike charges.

.شحاخ امؼ١ث١ خرفح

Based on the results of the experiments in Table we can conclude that: تؼغ ارائط ارعاسب اغاتمح.

1- Friction produces two different kinds of charges on different materials (Such as glass and ebonite) .

. ذ شحاخ خرفح ػذ ده اد خرفحاالحرىان ٠ -1

2- Like charges always repel each other. And unlike charges always attract each other.

اشحاخ ارشات ذرافش اخرفح ذرعارب -2 3- Only two kinds of charges exist.

.ان ػا اشحاخ -3 -4

9


1- Matter is made up of indivisible particles called atoms. Each atom has

negatively-charged electrons orbiting round a small massive nucleus which consists of positively-charged particles called protons and neutral particles called neutrons. Figure shows an atomic model of a beryllium atom.

اج ذحر تشذاخ ظثح يحااد ذرى رساخ ذحر شحاخ عاثح االىرشاخ ذذس -1 ١ذشاخ رؼادح

2- In charging by friction (such as rubbing a glass rod with silk), some

electrons from the surface atoms of one object (the glass rod, in this case) are transferred to another object (silk, in this case). This makes the glass rod positively charged as it is now short of electrons. The silk becomes negatively charged as it gains excess electrons. Charge is never made or destroyed in the friction process; it is transferred from one material to another.

االىرشاخ إ١اا ٠عؼ اادج ار ارمد ألخشػذ ده ادذ١ تؼغ االىرشاخ ذرم ادج -2

اىرشاخ لـؼح فمذازا ظؼ لؼ١ة اضظاض ظثا . عاثح ار ارمد ا االىرشاخ ظثح .ألخشذرم ادج إاذف أاشحاخ ال ذخك .احش٠ش عاثح الورغاتا اىرشاخ

The Table summarizes the charges produced on some common materials due to the friction process.

.تاألخشاعذي ارا ٠ث١ ع اشحح ػ و ادج ػذ دىا

Materials Positive charge

Negative charge

Glass rod rubbed with silk Glass Silk صظاض ش٠حش

Ebonite rod rubbed with fur Fur Ebonite فش اتا٠د

Perspex ruler rubbed with woolen duster Perspex Duster لاػ

Plastic comb rubbed with hair Hair Plastic comb شؼش

Polythene strip rubbed with woolen duster

Duster Polythene تص١

An explanation of charging by friction ذفسير انشحن عن طريك االحركان

10


The SI unit of charge is the coulomb (C). It is equal to the charge on about 6 million million million electrons, although it is not defined in this way. One coulomb is a relatively large quantity of charge, and it is often more convenient to measure charge in microcoulombs: 1 microcoulombs (µC) = 10-6 C the charge on a rubbed polythene rod is, typically, only about 0.005 µC.

ذؼشف ىا ال. إىرش١ ١ ١ 6 ذغا شحح . احذج اذ١ح شحح اىاشحح ػذ ده اثص١ ذمش٠ثا . مذاس وث١ش ظذا زا غرخذ حذج ا١ىشو . اـش٠مح تز

.١ىشو 0.005ذغا

When some materials gain charge, they lose it almost immediately. This is because electrons flow through them or the surrounding material until the balance of negative and positive charge is restored.

االىرشاخ ذرذفك خالي اادج حر أزا تغثة . ذفمذا تغشػح فئاػذا ذىرغة تؼغ ااد شحح ، . حح اغاثح اشحح اظثحشذراص ا

are materials that let electrons pass through them. Metals are the best electrical conductors. Some of their electrons are so loosely held to their Atoms that they can pass freely between them. These free electrons also make metals good thermal conductors. Most non-metals conduct charge poorly or not at all, although carbon is an exception.

اطالخ الحرائا ػ أفؼاؼاد ذؼرثش . اطالخ اد ذغح الىرشاخ تاشس خالاػؼ١فح إاؼذ١ح اؼاطش غ١ش ا. اىرشاخ ػؼ١فح االسذثاؽ تازسج زا ذرحشن تحش٠ح ت١ ازساخ

. غ١ش طح تاعرصاء اىشت أارط١

Unit of charge وحدج انشحنح

حذج اشحح

Conductors

Conductors and Insulators

11


are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move - although they can be transferred by rubbing. Insulators are easy to charge by rubbing because any electrons that get transferred tend to stay where they are.

ىا . ذغح الىرشاخ تاشس خالا ال اىرشاذا شذثـح تمج تزساذا اؼاصالخ اد ال. ذشح تغح ػذ دىا ال االىرشاخ ارمح ذ١ ثماء ح١س

These are 'in -between' materials. They are poor Conductors when cold, but much better conductors when warm.

.ذط١ال ػذ سفغ حشاسذا أوصشاطالخ ت١ اطالخ اؼاصالخ ذى اشثا

Induction is the process of charging a conductor without any contact with the charging body.

األخشطالخ ٠ر د الغح اعغ اشح عغ تارأش١شاشح

Insulators

Semiconductors

Charging conductors by induction

اطالخ تارط١شح

12


Step 1: The two conductors (metallic spheres) on insulator stands are brought into contact with one another (Figure 1)

. رالغا طال ؼذ١ا ػ حا ػاصح

Step 2: A negatively-charged rod is brought near to sphere A (Figure 2) . This causes the electrons from A to be repelled to the farthest side of sphere B. Under this condition, sphere A alone will have excess positive charge (due to electron loss) while B alone has excess negative charge (due to gain of electrons).

Bاتؼذ ظاة ط إا ٠ؤد رافش االىرشاخ Aمشب لؼ١ة شح تاغاة اط .اىرشاخ الورغاتعاثا Bاىرشاخ خغاسذظثا Aف١ظثح

Step 3: With the negatively-charged rod in place, the two spheres A and B are separated a distance apart using the insulating stands (Figure 3)

٠ث١ اطال رثاػذا تشحاخ خرفح تظد امؼ١ة اشح تاغاة 3اشى

(a) To charge two conductors with equal and opposite charges:

.شح ط١ تشحر١ خرفر١ رغا٠ر١ -ا

13


Step 4: The sphere A will now have induced positive charges while B will carry an equal number of induced negative charges. The charge on the charging rod remains unchanged (Figure 4)

. امؼ١ة اشح حظ ػ ط١ شح١ تشحاخ خرفح إصاحػذ

Step 1: Bring a charged rod (say positively charged) to the vicinity of the conductor held by an insulating stand (Figure 1) مشب لؼ١ة ظة اشحح ط رظ تحا ػاصي.

Step 2: The free electrons in the conductor will be drawn towards the end of the conductor nearer to the positively-charged rod. Leaving the other remote end to have excess positive charge. Note that the conductor is still electrically neutral despite the redistribution of the free electrons on it

(Figure 2).

، ط ظة اشحح خشا٢امؼ١ة ظة اشحح ف١ظثح اـشف تاذعاذرحشن االىرشاخ احشج . ذص٠غ االىرشاخ احشج ػ١ إػادجالحظح اط ٠ثم رؼادي وشت١ا ػ اشغ

(b) To charge a single conductor by induction.

تارأش١ششح ط -ب.

14


Step 3: With the positively-charged rod still in place, the conductor to be charged is earthed. This can be done by touching the conductor with our body momentarily. Being a relatively good conductor, our body will allow electrons to flow to the conductor to neutralize the excess positive charge on the far side of the conductor. Note that now the conductor will carry an excess negative charge (Figure 3).

تا١ذ حظح ف١غح اعغ تشس تالغرره تاألسػتظد امؼ١ة اشح ط اط

.اط رؼادي اشحاخ اظثح ف١ظثح اط حاال شحاخ عاثح فمؾ إاىرشاخ

Step 4: On removing the charging rod, the excess negative charge (electrons) will redistribute on the surface of the conductor to attain electrostatic equilibrium

(Figure 4) امؼ١ة اشح ٠ظثح اط عاة اشحح، ح١س ذرصع تئصاح . االىرشاخ ػ عـح رحمك اراص االىرشعراذ١ى

Note also that charging a single conductor by induction will always result in a charge that has the opposite sign to that of the charging rod.

.ذى شحح اط ػىظ شحح اشاح ارأش١شتاشح تاعـح : الحظ

15


(Figure 1&2) shows the structure of a typical electroscope used for the detection of charges and to test for the sign of the charge.

ال وزه ع اشحح أوا اعغ شح إاشى ٠ث١ ذشو١ة اىشاف اىشت از ٠غرؼ ؼشفح .وا شحا إ

Electroscope انكشاف انكهرتي

16


In Figure 3, the gold leaf is close to the brass plate. It is in a collapsed state. The brass cap, brass rod, brass plate and the gold leaf are electrically neutral (there is no excess charge).

اسلرا ازثرا رالغرا، اظف١حح احاع١ح اغاق احاع اسلرا )ىاخ اىشاف 3ف اشى ازثرا ظ١ؼا رؼادح اشحح

When a charged insulator (such as a positively-charged glass rod) is brought near to the brass cap, the free electrons from the brass and gold parts of the electroscope are attracted to the brass cap, leaving the brass plate and gold leaf positively charged. This causes the gold leaf to diverge due to the repulsion between like positive charges (Figure 4). In this way, we can conclude that the glass rod is charged.

أاامشص إترمش٠ة لؼ١ة شح تاظة امشص احاع ىشاف ذرحشن االىرشاخ احشج .امؼ١ة وا شحا أثح ذفشظا ا ؼشف ظ اىشاف ازثر١ فرظثحا راخ شحح سلر

(a) Detection of charge انكشف عن انشحناخ -ا

17


To test for the sign of a charge on a charged body, we need to charge the electroscope first. The electroscope can be charged easily by induction. The following diagrams show the sequence of charging the electroscope positively by induction (Figure 5).

ارا١ح ذث١ شح اىشاف ألشىايا. تارأش١شىشف ػ شحح ظغ ا حراض شح اىشاف ره تاشح .تارأش١شتشحح ظثح

.٠ص شاح شح اىشاف اىشت تشحح ظثح 5اشى

To charge the electroscope negatively, we can slide a negatively-charged rod on the brass cap of the electroscope (Figure 6) so that some electrons from the charged rod can be transferred to the cap causing the gold leaf to deflect or diverge.

ره تالغح . ٠ص شح اىشاف تشحح عاثح 6اشى تؼغ االىرشاخ أعاق عاثح تمشص اىشاف ح١س

.ذرم مشص ظاػح اسلرا ازثرا ذفشظا

(b) Testing for the sign of charge

انكشف عن نىع انشحنح -ب

18


If a charged rod with an unknown charge is brought near to a negatively charged electroscope and there is an increase in the divergence of the gold leaf, we can conclude that the charge on the charged rod is negative (Figure 7).

شحح وشاف شح ػذ ذمش٠ة ظغ عي اتاغاة صاد افشاض اسلرا ازثرا فا اعغ ٠ى

.شح تاغاة Similarly if a charged rod is brought near to a positively-charged electroscope and the divergence of the gold leaf increases, we can conclude that the charge on the charged rod is positive (Figure 8). Note that for both cases of testing the sign of the charge, only the phenomenon of repulsion between like charges is used. Do you know why attraction between unlike charges cannot be used to test the kind of charge? ػذ ذمش٠ة ظغ عي اشحح وشاف شح تاظة صاد افشاض اسلرا ازثرا فا اعغ .٠ى شح تاظة

الحظ ف احار١ اعرخذا ص٠ادج ارافش ت١ ) ارعارب ؼشفح ع اشحح آ١حاسلر١ غرخذ

(حاي ذفغ١ش ره

Negative charge detection

انكشف عن شحنح سانثح

Positive charge detection

يىجثحانكشف عن شحنح

19


• Michael Faraday developed an approach that helps us both

understand and calculate the influence of a charged particle on other

charged particles

فاسادا ترـ٠ش ف ٠غاػذا ف حغاب ذأش١ش اعغ١اخ اشحح ػ غ١شا مذ لا •

اعغ١اخ

• This approach is called the field concept زا اف ا ٠ؼشف تثذأ اعاي

• We say that a charged particle creates a field throughout all of space

ح ترى٠ عاي ذأش١ش خالي افشاؽ ٠رأشش ت اعغ١اخ اشحح اح١ـحذم اعغ١اخ اشح •

• Other charged particles then interact with this field

Here a charge Q sets up a field that points away from the charge in three-dimensional space.

عاال ح١ؾ تا ٠ؤشش ف ف زا اشى ٠ظذ شحح ف اعؾ ذى .اصالز اذعااخ

If we place a test charge at point P, we can measure the field at that point by looking at the force exerted on the test charge by the field.

٠ىا ل١اط ذأش١ش اعاي ػذ ذه امـح ػ ؿش٠ك امج ( P) ػذ امـح اخرثاس٠ػذ ػغ شحح .االخرثاس٠اؤششج ػ اشحح

30.3 Electric Field

انجال انكهرتي

20


We bring in a positive charge q0 as a test charge, which is carefully selected with a very small magnitude, so that it does not alter the locations of the other charges

The electric field E that exists at a point is the electrostatic force F experienced by a small test charge q0 placed at that point divided by the charge itself:

شحح ظثح فئ اعاي اىشت ػذ ذه امـح ٠غا امج qoطغ١شج ظذا اخرثاس٠إرا لشتا شحح .مغح ػ مذاس ذه اشحح االخرثاس٠اىشت١ح اغاوح ارأششج تاشحح

• The electric field is a vector, and its direction is the same as the

direction of the force F on a positive test charge

اظثح االخرثاس٠اعاي اىشت و١ح رع ٠ى اذعا فظ اذعا امج ػ اشحح •

• SI Unit of Electric Field: Newton per coulomb (N/C)

21


• It is the surrounding charges that create an electric field at a given

point.

.٠ح١ؾ اعاي اىشت تاشحح ارغثثح ف زا اعاي •

• Any charge q placed at the point with the electric field E will

experiences a force, F=qE. For a positive charge, the force points in

the same direction as the electric field; for a negative charge, the force

points in the opposite direction as the electric field.

• تاغثح . شحح ػػح تامشب عاي وشت عررأشش تمج ذحذد تاما ازوس عفا أ

شحح اظثح فئ امج ذؤشش ف فظ اذعا اعاي اىشت ػ ػىظ ا ٠حذز غ اشحح

.اغاثح

• At a particular point in space, each of the surrounding charges

contributes to the net electric field that exists there

ف مـح ا ف افشاؽ ، و شحح ح١ـح ذشاسن ف اعاي اىشت اى اظد ف زا •

.افشاؽ

Important about electric field

حمائك عن انجال انكهرتي

22


In Figure a the charges on the two metal spheres and the ebonite rod create an electric field E at the spot indicated. This field has a magnitude of 2.0 N/C and is directed as in the drawing. Determine the force on a charge placed at that spot, if the charge has a value of (a) qo=+18×10–8 C and (b) qo =–24×10–8 C.

ف اشى اػح اشحاخ اراظذج ػ اىشاخ اؼذ١ح ػ عاق األت١د ذذ عاال وشت١ا ػ ػ١ امج اؤششج ػ . اذعا وا ف اشع و/ ١ذ 2مذاس زا اعاي . امـح اػحح

ف qo=+18×10–8 C ( أ)اشحح اػػح ػذ ذه امـح إرا ػد أ مذاس اشحح ف احاح qo=–24×10–8 C ( ب)احاح

Example : Electric Field Leads to a Force

انجال انكهرتي يسثة لىج : يثال

23


An electric field can be visualized on paper by drawing lines of force, which give an indication of both the size and the strength of the field. Lines of force are also called field lines.

Field lines start on positive charges and end on negative charges, and the direction of the field line at a point tells you what direction the force experienced by a charge will be if the charge is placed at that point. If the charge is positive, it will experience a force in the same direction as the field; if it is negative the force will be opposite to the field.

خـؽ . ٠ى ذص١ اعاي اىشت تشع خـؽ ام ار ذث١ وال مذاس شذج اعاي اىشت .ذثذأ خـؽ اعاي اشحح اظثح ذر ػذ اشحح اغاثح. ام ذغ أ٠ؼا تخـؽ اعاي

The fields from isolated, individual charges look like this:

:اعاي ااشئ ػ اشحاخ اؼضح افشدج ذثذ وا ٠

Electric Field Lines

خطىط انجال انكهرتي

What does an electric field look like?

24


When there is more than one charge in a region, the electric field lines will not be straight lines; they will curve in response to the different charges. In every case, though, the field is highest where the field lines are close together, and decreases as the lines get further apart.

إرا وا ان أوصش شحح ف ـمح احذج ف ذثذ خـؽ اعاي غرم١ح ى ذح ذحد ذأش١ش ف و األحاي فئ لج اعاي اىشت ذى أوثش ا ٠ى ػذا ذرماسب خـؽ . خشاشحاخ األ

.اعاي ٠م ػذا ذرثاػذ خـؽ اعاي

Rank the magnitudes E of the electric field at points A, B, and C shown in the figure.

اػحح حغة لج اعايسذة ذأش١ش اعاي اىشت ػ اماؽ

A) EC>EB>EA

B) EB>EC>EA

C) EA>EC>EB

D) EB>EA>EC

E) EA>EB>EC

25


In our childhood, most of us might have come across a few objects that attracted pins,

needles, blades, etc. We realized that they did not attract things like paper, wooden

blocks, plastic scales, etc. Later, we learnt that such objects belonged to a class

called magnets. In this chapter we will study the types and properties of magnets.

تؼؼا ( ص اذتات١ظ اشفشاخ احذ٠ذ٠ح ) ف شاحا اذساع١ح اثىشج ػشفا أ ان تؼغ ااد ذعزب غاؿ١ظ

.اؿ١ظ از عذسط ا خاطػشفا تؼذ ره ا ٠غ تاغ. ال ذرأشش ت ص اسق اخشة

To understand the properties of a magnet, try and do the following:

:نفهى خىاص انغناطيس يكنه انمياو تاننشاط انراني

1. Suspend a bar magnet horizontally with the help of a string. When you

disturb the magnet, you will observe that the magnet starts oscillating. After

some time, it comes to rest in the same direction in which it was

undisturbed.

إرا حشود اغاؿ١ظ عرالحظ أ اغاؿ١ظ تذأ . ؿ١ظ أفم١ا تاعـح خ١ؾل ترؼ١ك لؼ١ة اغا

تاالرضاص تؼذ فرشج ذالحظ اعرمشاس ف فظ االذعا از تذأ ػذ االرضاص

30.4 Magnetism

انغناطيسيح

Properties of Magnets

طيساخ اخىاص انغن

26


2.Repeat the experiment. If you watch the magnet’s direction carefully you

will notice that the magnet always comes to rest in the earth’s north–south

direction. It is one of the inherent properties of magnets. On all bar

magnets, the end that points to the earth’s north is called the north pole – it

always marked with a white dot. The other end, pointing towards the Earth’s

south is called the south pole.

ف و اغاؿ١غاخ امؼ١ث١ح ٠ى اـشف از ٠ش١ش إ امـة اشا ألسع ػذ االعرمشاس

.امـة اشا غاؿ١ظ ػادج ا ٠شض تمـح ت١ؼاء ، ٠غ اـشف اصا تامـة اعت

A magnet’s directional property is used in a mariner’s compass. Keep the

compass in any direction; it comes to rest in the north–south direction. The

compass has a thin needle pivoted freely and it is made out of a magnet.

In ancient times, the compass helped travelers to navigate and know their

directions.

اثطح ذرشوة إتشج غاؿ١غ١ح ؼمح . ز اظفح االذعا١ح غاؿ١ظ ذغرخذ وثطح الح١

.ررحشن تحش٠ح رش١ش ػذ اعرمشاسا إ ألـاب األسع

We can therefore say that the magnets preferably attract iron and does not

attract copper or aluminum. This is known as the attractive property of a

magnet.

خاط١ح ارعارب غاؿ١ظ ذض ػ أ ان اد ذعزب غاؿ١ظ ص احذ٠ذ أخش ال ذعزب

ص احاط األ١

27


3. The iron filings are seen to be attracted only to the ends of the bar

magnet. Therefore, it can be concluded that the end regions of a magnet

have greater strength to attract iron filings. We call these regions of a

magnet – the north pole and the south pole.

اؿ١ظ ذرشوض ػ األلـاب ذعزب تشادج احذ٠ذ إ لـث اغاؿ١ظ فمؾ ا ٠ؼ أ امج اؼظ غ

4. Take two bar magnets. Try and bring the two like poles together. Then

try and bring two unlike poles together. You will observe that north–north

poles and south – south poles repel each other. Whereas north – south

poles of the two bar magnets attract each other.

:ألـاب اغاؿ١ظ ارشاتح ذرافش اخرفح ذرعارب ٠رؼح ره اشى ارا

This rule is similar to the rule for the forces between electric charges,

where like charges repel one another and unlike charges attract. But

28


there is a very important difference between magnetic poles and electric

charges. Whereas electric charges can be isolated, magnetic poles

cannot.

ى ان . ز اماػذج ذشث لاػذج ام ارذج ت١ اشحاخ اىشت١ح ح١س ارعارب ارافش

اخرالف ظش اشحاخ اىشت١ح اخرفح ٠ى ػضا ػ تؼؼا ت١ا ال ٠ى ػضي األلـاب

اغاؿ١غ١ح ػ تؼؼا

We see that the attraction or repulsion of the poles is observed even if

the poles are at a distance from each other. We say that there is a force,

a 'actions at a distance' force, that is acting on these magnets. The

magnetic poles are experiencing a force due to what we call 'magnetic

fields'.

ذؤشش ألـاب اغاؿ١ظ تمج ػ األظغا اغاؿ١غ١ح امش٠ثح ره ػ ؿش٠ك ا ٠غ تاعاي

.اغاؿ١غ

29


• A piece of magnetic material can be turned into a magnet if it is stroked by a magnet. As the magnet moves along the magnetic material, it causes the magnetic dipoles in the magnetic material to become aligned in one direction and give rise to a magnetic field.

٠ر اذه تـي امـؼح احذ٠ذ٠ح . ٠ى ذح٠ لـؼح حذ٠ذ٠ح إ غاؿ١ظ ػ ؿش٠ك دىا تغاؿ١ظغف اذعا احذ ١رى لـث١ ٠رى ر١عح زه عاي غاؿ١ .

30.4 Methods of making magnets

طرق صناعح انغناطيس

1- ‘Stroke’ method

طريمح اندنه

30


To make an electromagnet you may implement the following steps.

:غاؿ١ظ وشت ٠ىه ذف١ز اخـاخ ارا١حظاػح

Step 1 - Gather the Materials جع انىاد

One iron nail fifteen centimeters (15cm) long

ع 15 إتشج حذ٠ذ٠ح تـي

Three meters (3m) of insulated, copper wire

رش عه حاع ؼضي 3

One or more D-cell batteries تـاس٠ح أ أوصش احع اىث١ش

wire strippers لشاسج أعالن

Step 2 - Remove some Insulation ذمشير انسهه

Some of the copper wire needs to be exposed so that the battery can make a good electrical connection. Use a pair of wire strippers to remove a few centimeters of insulation from each end of the wire.

اؼضي ٠ر إصاح ػاصي اغه ترمش١ش ظضء ؿشف اغه احاع

2- Electromagnets انغناطيس انكهرتي

Activity نشاط

31


Step 3 - Wrap the Wire Around the Nail نف انسهه حىل انسار

Neatly wrap the wire around the nail. The more wire you wrap around the nail, the stronger your electromagnet will be. Make

certain that you leave enough of the wire unwound so that you can attach the battery.

تئذما ذا ف اغه حي اغاس ، وا صادخ ػذد افاخ وا صادخ لج اغاؿ١ظ

When you wrap the wire around the nail, make certain that you wrap the wire all in one direction.

.اذعا احذػذ ف اغه ذأوذ أ ٠ى اف و ف

Step 4 - Connect the Battery ذىصيم انثطاريح

Attach one end of the wire to the positive terminal of the battery and the other end of the wire to the negative terminal of the battery. If all has gone well, your electromagnet is now working.

اغه إ اـشف اظة ثـاس٠ح اـشف ا٢خش غه غ ؿشفط إحذ .ؿشف اثـاس٠ح اصا

Don't worry about which end of the wire you attach to the positive terminal of the battery and which one you attach to the negative terminal. Your magnet will work just as well either way. What will change is your magnet's polarity. One end of your magnet will be its north pole and the other end will be its south pole. Reversing the way the battery is connected will reverse the poles of your electromagnet.

إال إرا ود ذش٠ذ ػؼا ؼ١ا أللـاب ( لـث١ح ارط١ ) األؿشاف ذط ثـاس٠ح تأ ال ذشغ فغه .ااشئ ، فؼىظ ألـاب ارط١ ٠ؼىظ ألـاب اغاؿ١ظ اىشت ااشئ اغاؿ١ظ

32


A magnetic field is an invisible field which exerts magnetic force on substances which are sensitive to magnetism. A classic example of a magnetic field is the field created by an iron magnet; to see how the energy in such a field works, you can place a small magnet under a piece of paper and sprinkle iron filings on it. As the filings respond to the magnetic field, they will slowly orient themselves along an axis.

مه األمثيح اىمعتادج . اىمجاه اىمغىاعيسي مجاه غيش مشئي يؤثش تقج مغىاعيسيح عي اىماد اىمغىاعيسيح

اىمجاه اىمغىاعيسي ىقضية مغىاعيسي حيث يضع فق سقح ثم يىثش تشادج اىحذيذ حيث تتخز زي

. اىثشادج ماضعا في اىمجاه وتيجح تأثشا ت

Magnets are surrounded by magnetic fields. A magnetic field can be thought of as consisting of lines of force. The forces of magnetic attraction and repulsion move along the lines of force. The iron filings line up along the magnetic field lines of the magnet. Note the circular pattern of the field lines. By convention, we say that the field lines emanate from the north pole of the magnet and re-enter the magnet through the South Pole. Note also that the field lines are closer together at the poles than at the center of the magnet. More iron filings are attracted to the poles because the

30.5 Magnetic Field

المجال المغناطيسي

33


strength of the magnetic field is greater at the poles. Discover the affect on magnetic field lines when two magnets are placed close to each other.

اىمجاه اىمغىاعيسي يحيظ تاىمغىاعيس عي شنو خغط تسم أيضا خغط اىقج ص ق اىتجارب اىتىافش

تنن زي اىخغط مغيقح تحيث تخشج مه اىقغة اىشماىي اى . تتحشك في مساسا عثش زي اىخغط

ظ تجارب عذد امثش مه ىزا والح, تنن متقاستح جذا عىذ األقغاب تاىىسثح ىسظ اىمغىاعيس. اىقغة اىجىتي

اىشنو اىتاىي يضح . رىل الن شذج اىمجاه اىمغىاعيسي امثش عىذ األقغاب . تشادج اىحذيذ عىذ األقغاب

. متقاستيه هاىمجاه اىمغىاعيسي ىمغىاعيسيي

Larger examples of magnetic fields include the Earth's magnetic field,

. مثاه آخش ىيمجاه اىمغىاعيسي مجاه األسض اىمغىاعيسي

34


Earth is a huge magnet. For points near earth's surface, its magnetic field can be approximated as the field of huge bar magnet – a magnetic dipole – that straddles the center of the planet. Hence a suspended magnet or compass points northward. The compass aligns with the magnetic field of the earth.

ىزا فإن . تعتثش األسض مغىاعيس عمالق ، فاألسض تثذ في تأثيشا مأن مغىاعيس عمالق في جفا

اإلتشج اىمغىاعيسيح تتأثش في حشمتا تقغثي األسض

Earth's magnetic field

انجال انغناطيسي نألرض

35


1- The diagrams below show charged and neutral bodies being brought together.

. يرعادنح الررتد ين تعضها انثعض وأخرييشحىنح أجساياذثم األشكال

Questions

36


Indicate whether the objects attract or repel each other.

ذرافش أذرعارب األظغا زواد إت١

Diagram Observation

(a)

(b)

(c)

(d)

2. When someone pulls a plastic comb through their hair, the comb becomes negatively charged

. عاة اشحح ٠ظثح اشؾ اشأطػذ اعرخذا شؾ اثالعر١ه شؼش

a) Which ends up with more electrons than normal, the comb or the hair? اشؼش؟ أاشؾ أوصش٠حر اىرشاخ األؿشاف أ

b) Why does the hair become positively charged?

اشؼش ر شحح ظثح؟ أطثحارا

3. On the right, a charged rod is held close to a metal can. The can is on an Insulated stand

ف اشى عاق شحح الرشتد ػثح ؼذ١ح ػػح ػ لائ ػاصي

37


a) Copy the diagram. Draw in any induced charges on the can. . شحح ذغرحس ػ اؼثح اؼذ١ح أاسع اشى اسع -ا

b) Why is the can attracted to the rod even though the net (overall) charge on the can is zero?

حظح اشحاخ ػ١ا طفشا؟ أفغش ذعارب اؼثح غاق غ -ب

c) If you touch the can with your finger, electrons flow through it. In which direction is the flow?

٠حذز زا ارذفك؟ ااذع أا١ذ ذرذفك االىرشاخ ، ف تئطثغغد اؼثح أ -ض

d) What type of charge is left on the can after it has been touched? ا ع اشحح ارثم١ح ػ اؼثح تؼذ غا تا١ذ؟ -د

4- If you are given a negatively-charged rod. Describe how you can use it to charge a neutral electroscope positively.

.شح وشاف وشت رؼادي تشحح ظثح اعرخذاطف و١ف ٠ى . عاق شحح تاغاة أػـ١د إرا 5- Tow small light balls coated in metallic paint are Suspended by long insulating threads from points A and B as shown below.

.وا ف اشى A B ػاصح ؿ٠ح ػذ امؾ وشذا خف١فرا ظثغر١ تـالء ؼذ ؼمرا تخ١ؽ

a- Both balls are given a positive charge. Complete the figure below to show the new positions of the balls and their suspending threads.

.اشع رػح االغ اعذ٠ذج ىشذ١ وزه خ١ـ ارؼ١ك أو. شحد اىشذا تشحح ظثح راإ

38


b- The ball suspended from B is carefully moved and suspended from C without altering the charges on either of the two balls. Complete the figure below to show the new positions of the balls and their suspending threads.

اشع رػح أو. د ذغ١١ش ف شحح اىشذ١ Cتحزس ػمد ف Bحشود اىشج اؼمح ب إرا .خ١ـ ارؼ١ك االغ اعذ٠ذج ىشذ١ وزه

39


Resources:

Books:

1-Physics A Course for “ O” level.(Charles Chew. Leong See Cheng)

2- Complete physics (Stephen Pople)

3- Essentials of physics (Julie Quah. Chuen Wee Hong)

4-Longman GCSE physics (Brin Arnold . Steve Woolley)

5- Advanced physics for you

Websites:

1. http://library.thinkquest.org/10796/ch12/ch12.htm

2. http://www.colorado.edu/physics/2000/waves_particles/wavpart3.html

3. http://physics.bu.edu/~duffy/py106/Electricfield.html

4. http://www.physicsclassroom.com/class/estatics/u8l4b.cfm

5. http://www.titaneducation.com.au/samples/mag.pdf

http://library.thinkquest.org/10796/ch12/ch12.htm

http://www.colorado.edu/physics/2000/waves_particles/wavpart3.html

http://physics.bu.edu/~duffy/py106/Electricfield.html

http://www.physicsclassroom.com/class/estatics/u8l4b.cfm

http://www.titaneducation.com.au/samples/mag.pdf

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