Lesson 26 Electrostatics

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Physics 30 Lesson 26 ElectrostaticsIn lessons 24 and 25 we learned about work, energy, and the principle of conservation.Now we turn our attention to electricity and magnetism. A carry over idea which will beused e!tensively, in fact throughout the course, is the idea of conservation ."onservation of different #uantities is one of the most important principles in science.

In this lesson we shall discuss the properties of static $not moving% electric charges. Infuture lessons we will learn about dynamic $moving% electric charges which is calledcurrent electricity.

I. Historical background for electricity and magnetism(o tional!

1) Amber is a semi transparent solid $yellow or brown% which is fossili&ed sap thatoo&ed from softwood trees in the distant past. In '(( )", *hales recogni&ed that ithad a property of attraction if rubbed vigorously against a cloth. *his is the firstrecorded evidence of electrostatic attraction.

2) +odestone is a mineral $we call magnetite% that has a chemical formula of e - 4. Ithas the unusual property of attracting iron. /hen suspended or floated in a li#uid itwill always turn to one particular $North 0outh% orientation. It was used as acompass by the 1ikings and the "hinese.

3) ffluvium *heory. +ucretius was the first scientist to attempt an e!planation of theattractive properties of both amber and lodestone. 3e suggested that the amberand lodestone had an efflu! $or flowing out% of minute particles that would captureother small ob ects on their way back to the lodestone or amber. +ucretius did notattempt to distinguish between the type of attraction or take into account that amber

and lodestone did not attract the same materials.4) de Magnete . 0ir /illiam ilbert $6544 6'(-% was the chief physician to 7ueen

li&abeth I of ngland. 3e was also a scientist and, since 7ueen li&abeth wasvery healthy, he had lots of time to work on his scientific ideas. 3e published hiswork in a document called 8de 9agnete: in the year 6'((.

In de Magnete , ilbert advanced the idea that the earth had a magnetic field. 3ereasoned that if small pieces of lodestone always line up the same way on a largerpiece of lodestone and that they also align themselves in one direction with respectto the earth, then the earth must be a IAN* lodestone.

In de 9agnete, ilbert also attempted to draw a distinction between the propertiesof lodestone and amber $in other words, a distinction between electric and magneticattraction%. ilbert introduced the term electric to describe bodies that attract theway amber does. *he word ;electric< comes from ;electron< which is the reek wordfor amber. ilbert categori&ed the properties as=

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electric $i.e., amber% lodestone $magnetic%i% attracts only when rubbed i% always attractsii% attracts small particles of most

ob ectsii% attracts only iron or other ferromagnetic

materialsiii% attracts from one center of attraction iii% attracts from two centers $poles% of

attraction

iv% only e!amples of attraction e!isted $in6'4', 0ir *homas )rowne founde!amples of electric repulsion%

iv% attraction and repulsion had beenobserved

0ir /illiam ilbert was the first scientist to distinguish between electric andmagnetic fields.

II. Electrostatics " Early ideas (o tional!

*hales $'(( )"% discovered that if substances like amber are rubbed with a piece of

cloth, they can pick up little shreds of cloth or other small pieces of matter.ilbert $6'(( A>% showed that many substances can be electrified by rubbing and

as a result they can attract small bits of matter.

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>istribution of "harges

All substances will allow electric charges to flow over or through them with differentdegrees of ease. Conductors are materials which allow electricity to flow easily throughthem $metals, ionic solutions, etc.% Insulators do not allow electricity to flow very easily$wood, plastics, glass, etc.% *he terms conductor and insulator are relative , since somemetals are better conductors than others, while some materials insulate better thanothers.

0ince like charges repel each other, charged particles on an ob ect will try to move asfar away from one another as possible. If the charges are on a conductor they arerelatively free to move away from one another. If the charges are on an insulator $fur,plastic, ebonite rod, glass rod% the charges cannot easily move away from one anotherand are therefore forced to remain in close pro!imity to one another.

I*. %harging ob+ects

%harging ,y -riction (%onduction!

0ome substances ac#uire an electric charge when rubbed with another substance. or e!ample, an ebonite rod becomes negatively charged when rubbed with fur. /e cane!plain this phenomenon with the help of the model of the electrical structure of matter.

An atom holds on to its electrons by the force of electrical attraction to its positivelycharged nucleus. 0ome atoms e!ert stronger forces of attraction on their electronsthan others. /hen ebonite and fur are rubbed together, some of the electrons from thefur are GcapturedG by the atoms of the ebonite, which e!ert stronger forces of attractionon them than do the atoms making up the fur. *hus, after rubbing, the ebonite has ane!cess of electrons and the fur has a deficit. /e call this charging by conduction,which implies physical contact is made between the ob ects.

*he same e!planation holds for many other pairs of substances, such as glass and silk.*he electrostatic series table below lists many of the substances that can be chargedby friction. If two substances in the table are rubbed together, the substance that is

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Negatively chargedmetallic sphere

Negatively charged plasticrod

n a conductor, the electronsare free to move as far awayfrom one another as possible.

n an insulator, the electronsare forced to stay at one end ofthe rod since they are not freeto flow to the uncharged end.


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higher in the table becomes negatively charged, while the other substance becomespositively charged.

Electrostatic Series

hold on to electrons tightlysulphur brass

copper eboniteparaffin wa!silkleadfur woolglass

B hold on to electrons loosely

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Electric %harge $he $ransfer of Electric %harge/cti ity

Part / Pre'in estigation hand'in assignment

Answer the following #uestions=

6. 0tate the +aw of lectric "harges.

2. /hat is the +aw of "onservation of "hargeE

-. If an ob ect is electrically charged, what is this a result ofE /hich charges arerelatively free to move and which are not free to move in a solid substanceE

4. /hat is the difference between a conductor and an insulatorE

5. ne can transfer electrons from one ob ect to another through friction by rubbingthe two ob ects together. or each of the following pairs of materials that arerubbed together, state which material will become positively charged and whichwill become negatively charged=

brass HHHH and paraffin wa! HHHH wool HHHH and glass HHHH wool HHHH and lead HHHH paraffin wa! HHHH and ebonite HHHH ebonite HHHH and fur HHHH glass HHHH and silk HHHH

'. If electrons are removed from an ob ect it will have a net HHHHHHHHHHH charge. Ifelectrons are added to an ob ect it will have a net HHHHHHHHHHH charge.

?. *wo identical metal spheres have different charges. ne has a charge of B6( "and the other has a charge of F2( ". If the spheres are allowed to touch eachother for a time and then are moved away from each other, what is the finalcharge on each sphereE !plain.

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Part , In estigations

lease read the following instructions carefully= In groups of two conduct the following investigations with the materials provided. >o the lab work together as a group and then discuss the 7uestions together. ach member of the group should write the answers to the 7uestions for each of

the investigations. >raw good diagrams, e!plaining what they mean. Jemember, every member of the group is responsible for the knowledge gained in

these labs. n oy yourselves and do not hesitate to ask for assistance $i.e. electrostatics is a

black art%KK

In estigation Induced %harge 1e aration

roblem=3ow can a charged ob ect cause charges on a nearby neutral ob ect to moveE

9aterials=9etre stick ebonite rod L fur $negative charge%

lass lens acetate L paper towel $positive charge%

Note= !periments with charged ob ects work best on cool, dry days. /arm humid air,contains many positive and negative ions. If such air comes into contact with acharged ob ect, the ions in the air will neutrali&e the charged ob ect by contact. Also,moisture gets on charged surfaces and creates discharge paths. If your ob ect seemsto be GleakingG its charge, this may be due to the humidity of the surrounding air.

rocedure=6. lace the edge of the metre stick on the glass lens so that it is balanced. *ouchthe metre stick with your finger to neutrali&e it. Jub the ebonite rod with the furand bring it close to the one end of the metre stick. "arefully observe the motionof the metre stick. >o not let the rod touch the metre stick.

2. Jepeat the procedure using the acetate strip rubbed with a paper towel.

7uestions=6. As the charged ebonite rod was brought near the metre stick, which way did the

metre stick begin to moveE /hyE >raw a sketch to show the metre stick, with thecharged rod near it, and the effect of the rod on the positive and negative chargeson the neutral metre stick.

2. /hich way does the metre stick move when the charged acetate strip is broughtclose to itE /hyE >raw another sketch showing the new distribution of charge onthe neutral metre stick.

-. /hat happens to the metre when the charged rods are removedE /hyE /hat isthe net charge on the metre stickE

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4. /hy does touching an ob ect with your hand ensure that it is neutralE )e sureyour e!planation covers both cases= when the ob ect has an e!cess of electrons,as well as when it has a deficit. /hen a nearby charged ob ect causes a changein the distribution of charge on a neutral ob ect, it is called an induced chargeseparation. Is it possible to create an induced charge separation on an insulatorE

n a conductorE !plain your answers.

In estigation 2 Induced %harge 1e aration Part 2roblem="an a charged ob ect cause a stream of water moveE

9aterials=ebonite rod and fur faucet and sink

rocedure=6. Jub the ebonite rod with the fur to give a charge $negative charge%. *urn on the

faucet so that a small stream of water is flowing.2. 9ove the rod close to the stream of water and observe.

7uestions=6. /hich way did the stream of water moveE /hich way did the electrons in the

water moveE

In estigation 3 ancing a er

roblem=/hy are small pieces of paper attracted to a charged rodE

9aterials=- or 4 small slips of paper $(.5 cm ! 6.( cm% ebonite rod and fur

rocedure=6. lace the slips of paper on a table. "harge the rod and then place it over the bits

of paper. Note what happens.

7uestions=6. Msing diagrams, e!plain why the slips of paper were attracted to the charged rod.

2. /hy do the slips of paper stick to the ebonite rod rather than being repulsed bythe ebonite rod after contactE$ or this #uestion, it is wise to check your answer with your teacher.%

In estigation *an er 4raaf 4enerator

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roblem=3ow does a 1an >er raaf enerator workE

9aterials=ebonite rod and fur faucet and sink

rocedure=6. )efore turning on the generator, carefully lift the metallic bulb and observe theapparatus. ou should see an acrylic belt, that will rub against metal teeth. *hefriction will cause electrons to be transferred from the belt to the metal bulb.Jeplace the metal bulb.

2. *urn on the generator and wait appro!imately 65 seconds. *ouch the metal bulbwith a finger and observe the transfer of electrons from the bulb to you.Oust before contact is made between you and the generator, youmay have observed a small spark composed of Gover an!iousG electrons.

-. lace a hand on the metal bulb and leave it there for appro!imately 65 secondswhile the generator is running. /ithout removing your hand, touch a metalob ect with your other hand. 0ee #uestion 6.

4. lace a small amount of paper circles on the generator while it is running. 0ee#uestion 2.

7uestions=6. >escribe the flow of electrons when your hand was placed on the gernator and

you touched a metal ob ect with your other hand. /hat was your net chargeprior to touching the metal ob ectE

2. !plain why the paper was originally attracted to the generator, and thenrepulsed a few seconds later.

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Lesson 26 Electrostatics

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