School Science Lessons
(UNPh31)
2024-07-26
Electrostatics
Contents
31.2.0 Electric charge
31.1.0 Electrostatics, static electricity
31.3.0 Induced charge
31.4.0 Insulators and conductors
31.1.0 Electrostatics, static electricity
Experiments
31.1.02 Electrostatic series, triboelectric series, static electricity from rubbing, ranking of insulators
31.1.03 Electrostatics and humidity
31.1.1 Electrostatics, static electricity
31.1.2 Aluminium balls on gramophone record
31.1.3 Aluminium foil precipitator
31.1.4 Attract water to a comb
31.1.5 Balloon charge
31.1.6 Balloon stays in place
31.1.7 Body sparks, rug scuffing
31.1.8 Coin stays on the cupboard door
31.1.9 Comb attracts and repels, bent water stream, air cleaner
31.1.10 Comb on a turntable
31.1.11 Clothes brush and flannel
31.1.12 Cellulose acetate sheet
31.1.13 Cotton thread rises
31.1.14 Cottrell smoke precipitator
31.1.15 Ebonite rods and glass rods
31.1.16 Electric charge, conventions from effects of friction between substances
31.1.17 Electric charge movement, charging by induction
31.1.18 Static electricity from rubbing
31.1.19 Faraday's ice pail experiment
31.1.20 Gold leaf electroscope
31.1.21 Many charges from one source
31.2.4 Measure electrostatic charge
31.1.23 Mercury shaker
31.1.24 Neon lamp on Styrofoam
31.1.25 Newspaper stays on the wall
31.1.26 Paper dancers
31.1.27 Paper horse race
31.1.28 Paper loop attracted by plastic ruler
31.1.29 Paper strips diverge
31.1.30 Paper snake strikes
31.1.31 Paper shapes dance
31.1.32 Pepper photocopy
31.1.33 Pith ball indicator, static electricity indicator
31.1.34 Pith ball indicator, electroscope
31.1.35 Polythene bag for a nylon stocking
31.1.36 Puffed rice jumps
31.1.37 Repulsing balloons
31.1.38 Rubbed balloon stays in place
31.1.39 Static electricity detector
31.1.40 Two kinds of static charge
31.1.41 Volta's experiment, voltaic pile, metal on Perspex
31.1.42 Wool flannel lights a fluorescent light tube
31.2.0 Electric charge, the coulomb, C, Coulomb's law
31.2.01 Electric charge, the coulomb, C, Coulomb's law
Experiments
31.2.1 Balloons rubbed with wool or fur repel each other
31.2.2 Charging by conduction and induction
31.2.3 Coulomb's law balance
31.2.4 Measure electrostatic charge
31.2.9 Metal balls play music
31.2.6 Pith balls and Styrofoam balls show electric charge
31.2.8 Separate pepper from salt and pepper, jumping pepper
31.3.0 Induced charge, shielding and charging by induction
Experiments
31.3.1 Charge electroscope by induction
31.3.2 Charge metal objects by induction
31.3.3 Deflection of s compass needle
31.3.4 Electrostatic shielding
31.3.5 Faraday cage, electrostatic shielding, electric field lines
31.3.6 Faraday's flashlight
31.3.7 Franklin's bell, lightning warning device
31.3.8 Pith balls repel and attract
31.3.9 Proof plane to test charge on an object
31.4.0 Insulators and conductors
Experiments
31.4.1 Conductor and non-conductor
31.4.2 Test whether an object is a conductor or insulator
31.1.02 Triboelectric series, electrostatic series, ranking of insulators
Produce static charge by rubbing or touching, contact charging.
The triboelectric series is a list of items sorted according to polarity of charge produced by rubbing.
Rub together any two of the substances in the triboelectric series.
The substance higher in the list will lose electrons to become positively charged.
The substance lower on the list will gain electrons and become negatively charged.
For example, if rub glass (6) with silk (14), glass becomes positive and silk becomes negative.
If rub ebonite (25) with wool (9), ebonite becomes negative and wool becomes positive.
If rub Perspex (23) with wool (9), Perspex becomes negative and wool becomes positive.
The further apart the materials the greater the charge produced.
The relative polarity depends on the specimen's molecular structure and the nature of the surface of that specimen.
The list below is a composite list from several authorities with no two lists in complete agreement.
If substances are touched instead of being vigorously rubbed, the sequence may change.
Positive polarity, +, collects positive charges
Most positive, <-- 1. Dry air.
2. Human skin,
3. Leather,
4. Asbestos, (Do NOT use asbestos in any experiment!),
5. Rabbit fur,
6. Glass rod, (borosilicate glass),
7. Mica,
8. Human hair,
9. Wool knitted, flannel, wool felt cloth,
10. Nylon stocking,
11. Cat fur,
12. Glass, polished glass, window glass,
13. Lead rod,
14. Silk cloth,
15. Aluminium foil, chocolate wrapping, ("silver paper"),
16. Paper, (newspaper, filter), paper, puffed rice, popcorn, pepper),
17. Cellulose acetate, combs, (acetyl cellulose, photographic film, overhead projector transparency),
18. Cotton handkerchief, flannelette,
19. Steel, (iron compounds).
Negative polarity, -, collects negative charges
20. Dry wood, pith, cork (sunflower stem, artichoke stem, elder tree pith),
21. Amber rod,
22. Perspex (Lucite), plastic ruler, (Plexiglas, PMMA, polymethyl methacrylate thermoplastic, optical lenses, tubing),
23. Paraffin wax, resin (sealing wax, esters, beeswax, shellac, turpentine),
24. Ebonite rod, hard rubber rod (polymerized isoprene resin + sulfur, vulcanite, motor car tyres),
25. Polycarbonate polymer, car battery casing (PC, Lexan, spectacle lenses),
26. Brass rod, copper, nickel, cobalt, silver (metals),
27. Gold, platinum,
28. Sulfur,
29. Rayon, (cellulose, artificial silk, tyre cord),
30. Celluloid, ping-pong ball (nitrocellulose polymer, spectacle frame),
31. Styrofoam, plastic Petri dish (Polystyrene, styrene resin),
32. Saran wrap (vinylidene chloride and vinyl chloride chlorofibre),
33. Orlon, polyesters (polyacrylonitrile polymer, acrylic resin, Acrilan, imitation fur, carpets),
34. Polyurethane polymer, paints, rubber, foam plastics, tough linings, car body parts,
35. Polythene strip, plastic bag, cling film (Polyethylene, "Scotch Tape", "Glad Wrap", bin liners, wash bottles),
36. Rubber, balloon (soft rubber, India rubber),
37. Polypropylene rod, plastic bucket (tubing connectors, heavy duty bottles),
38. PVC rod, gramophone record (polyvinyl chloride polymer, Vinyl, Vinylite, poly-chloroethane, tubing for burners, electrical cable),
39. Silicon, 40. Teflon, non-stick surface of frying pans (polytetrafluoroethene polymer,
PTFE, on-lubricated bearings), ---> Most Negative.
31.1.03 Electrostatics and humidity
Detecting electrostatic charge, sand bath drying oven
Electrostatics experiments may not work if the equipment is damp or stored in a damp place or if the humidity is high.
In some countries, electrostatics experiments may not work at all during the humid summer months.
So teach electrostatics in the winter months when humidity is low is best.
Some equipment, e.g. electrophorus, works better if first dried in front of a radiator.
31.1.1 Electrostatics, static electricity
1. Thales of Miletus, 636-546 BC, Greece, observed that amber rubbed with wood attracts pieces of fluff and fragments of straw.
The amber had become electrically charged.
electricity, (Greek ēlektron amber)
In 1600, William Gilbert, England, found that many substances rubbed together attract other substances.
In 1733, Charles du Fay, France discovered two different "types" of electricity, which he called "vitreous electricity" when glass is rubbed with silk, and "resinous electricity', when amber is rubbed with fur.
After observing and doing electricity experiments in 1746, Benjamin Franklin, America, renamed vitreous charge as positive charge and resinous electricity as negative charge.
Why he used this positive and negative convention is not known, but since then it has never been changed.
Nowadays, we say that when a glass rod is rubbed with a silk cloth, electrons are lost from the atoms in the glass and transferred to atoms in the silk cloth, leaving the glass rod with more positive than negative charge, and so a net positive charge.
The convention is that a glass rod rubbed with silk has a net positive electrical charge.
2. Electrostatics is the study of charges, mainly at rest, on non-conductors and the effects it creates.
Electric charge can be either positive or negative and a neutral atom has equal quantities of positive and negative charge.
Like charges repel each other and opposite charges attract each other.
Electrification is the process by which an object becomes electrically charged.
The two forms of electrical charge are positive and negative.
When two electrically charged objects are placed near each other, they exert a force on each other.
If the objects have the same charge then they will repel and if they carry different charges, they will attract each other.
So bodies with like charges repel each other and bodies with unlike charges attract each other.
The nature of a static charge is due to an excess or shortage of electrons so the "natural unit" of electric charge should be the charge on an electron.
A negatively charged body contains more electrons than protons.
A positively charged body contains fewer electrons than protons.
Use simple electrostatic generation and testing equipment to show the nature of attraction, repulsion and induction laws.
Electrostatic charge refers to electric charge at rest on the surface of an insulator or insulated conductor.
Electrostatic charge has an associated electric field, E.
3. An object becomes electrically charged when it either loses or gains electrons.
When most plastics are rubbed with wool, negative charge is taken off the wool.
So the wool loses electrons leaving it positively charged and the plastic gains electrons leaving it negatively charged.
However, when perspex is rubbed with wool, negative charge is taken of the perspex.
So the perspex loses electrons, leaving it positively charged and the wool gains electrons, leaving it negatively charged.
4. An object is electrically charged when there is a difference between the number of positive and negative charges, so there is a net charge on the object.
An object with ten positive charges and eight negative charges carries a net positive charge.
An object can never lose or gain positive charges, because only negatively charged electrons can be transferred between objects.
Conservation of charge refers to the fact that in a closed system the charge always remains the same and the charge in the universe always remains the same.
5. Conductor allow charges to move over them or through them, e.g. metals.
Insulators do not allow charges to move over them or through, e.g. plastics.
Use antistatic paper to stop static cling on television screens, sewing thread, panty hose, Venetian blinds and to collect cat hair.
31.1.2 Aluminium balls on gramophone record
1. Use aluminium foil or "silver paper" to cut out the shape of a soccer player and make a soccer ball.
Attach it to the edge of an old 78 rpm gramophone record.
Rub the record with wool cloth and put it on an inverted drinking glass.
Put a tin sheet from a jam tin in front of the player.
Attach a thread to the aluminium foil soccer ball.
Hold the thread so that the ball is suspended near the foot of the player.
The extra electrons on the record spread into the player and attract the ball that has no charge.
The ball touches the foot of the player, receives extra electrons, then is forced away from the player, because player and ball are now negatively charged.
The ball hits the tin sheet, loses its electrons and becomes attracted to the player again.
2. Rub an old gramophone record with wool and put it on glass.
Drop small aluminium foil balls onto the gramophone record.
They jump from the gramophone record in all directions.
Static electricity produced by rubbing is irregular.
The aluminium balls take up the negative charges and are repelled, but they are attracted to positive charges still on the gramophone record.
3. Use aluminium foil or "silver paper" to make little balls to represent grasshoppers.
Rub an old 78 rpm gramophone record with wool and sprinkle the little aluminium balls onto the record.
The grasshoppers will they will jump away from each other, because they have the same charge.
31.1.3 Aluminium foil precipitator
See diagram 31.9.29: Electrostatic precipitator.
In an electrostatic precipitator smoke and other particles become charged when they pass by sharp pointed electrodes inside the chimney.
The charged particles are removed by charged plates near the mouth of the chimney.
Build a simulation experiment to show the action of an electrostatic precipitator.
The outer electrode is a cylinder of aluminium foil inside the walls of the jar.
Turn on the aquarium pump.
Air passes hydrogen chloride gas from the hydrochloric acid to react with the concentrated ammonia solution from the next test-tube to form a smoke of ammonium chloride.
Notice the amount of smoke from the chimney.
Gradually increase the flow of air from the pump then turn on the induction coil to supply the high voltage.
Note any change in the smoke from the chimney.
Adjust the air flow by constricting the tube from the air pump.
31.1.4 Attract water to a comb
1. Attract water to a comb.
Adjust a tap so that a very thin stream of water flows from it.
Give a comb a charge by running it through the hair several times.
Hold the comb 2 cm from the stream of water.
The water is strongly attracted by the electrical charge on the comb.
2. Turn on a tap so that a thin continuous stream of water flows.
Charge a comb by combing your hair several times.
The friction between you hair and the comb cause excess electrons on the comb so it becomes negatively charged.
Hold the comb near the stream of water where it comes out of the tap.
The comb attracts the water, because of the negative electrical charges
on the comb attract positive charges in the water molecules.
The charges on the water molecules inside the water stream neutralize each other, but the charges on the surface of the stream opposite the comb cannot be neutralized, so the side of the stream opposite the comb is attracted to the comb pulling the rest of the stream along with it.
3. Repeat the experiment using "Golden Syrup" or treacle or thin honey instead of water.
4. Bend water with static electricity
Turn on the faucet and slowly turn down the water until a very thin stream of water flows.
Brush a plastic comb through your hair ten times.
Slowly bring the comb close the flowing water, without touching the water.
The stream of water bend towards the comb.
31.1.5 Balloon charge
See diagram 31.2.3.1: Balloon charge.
1. Suspend two equally charged balloons by triangular suspension.
The forces on one balloon are as follows: F electric force, T tension in string, mg weight of a balloon.
Measure the distance d to the vertical and then weigh a balloon mg.
F / mg = d / sqrt (s2 - d2).
2. Fill a toy balloon with air and tie the necks tightly.
Rub the balloon with wool.
Attach the rubbed part to a wooden door.
The balloon will still stay there.
Rub the balloon against a woollen sweater then attach the rubbed part to a student's face.
The air balloon will also stay there.
Some electrons have been rubbed from the wool onto the balloon to give it a negative charge.
These electrons repel the electrons nearby on the wooden door or student's face and these charges are then attracted to the positive charges left on the door.
The balloon will stick to any uncharged surface, but the charge will gradually leave the balloon especially if the humidity is high or if placed under running water.
The balloons filled with air will stick to the ceiling for hours, because they are attracted to the uncharged ceiling.
After some time the extra electrons on the balloons pass to the ceiling so the balloons no longer kiss the ceiling.
31.1.6 Balloon stays in place
Blow up a toy balloon and rub it with a piece of fur.
Place it against the wall and note that it stays where you place it.
Repeat the experiment by rubbing the balloon on your hair.
31.1.7 Body sparks, rug scuffing
1. Separating charge in common ways to separate charges, e.g. scuff a rug and then discharge through a neon bulb.
Identify charges with an electroscope charged with known sign to test other charged objects.
2. Walk on a carpet or rug made of synthetic fibres while wearing rubber shoes.
Electrons are attracted from the carpet and spread across the body.
When a metal rail or a door handle is touched, some electrons will jump away from the fingers as a small spark.
Touch the metal again.
Nothing happens, because the fingers are no longer charged.
3. Hold a fluorescent lamp tube while rubbing the shoes strongly on the carpet.
Keep hold of the tube and touch the other end on a metal railing.
The fluorescent tube produces a brief flash of light.
4. Comb the hair vigorously on a day when the humidity is low.
Your hair may stand on end!
5. Pull off a woolly jumper over a silk shirt.
On a very dry day listen for a crackling sound!
6. Motor car tyres can pick up extra electrons
form the ground that spread through the car and cover the body.
On a cold dry day, drivers may get out of the car and experience a slight electric shock when the extra electrons jump back to earth.
The car builds up an electrical charge and "earths", i.e. flows through the driver, when the driver steps onto the ground.
The driver act as an electrical conductor and so gets an electrical shock.
Some vehicles, e.g. petrol tankers, have a chain or a metalled strip always touching the ground to get rid of extra electrons picked up by the tyres from the road.
The vehicles are always "earthed".
31.1.8 Coin stays on the cupboard door
This experiment can work on unpainted wood, but it works best on waxed surfaces.
Rub a coin up and down on the cupboard door with a brisk action.
Remove you hand and the coin stays on the vertical door.
The rubbing action causes increased pressure and friction so that the air under the coin becomes hotter and expands.
Some of this hotter air leaks out from under the coin as it moves across the surface of the cupboard door.
When you stop rubbing, the remaining air under the coin cools and contracts causing a partial vacuum.
So the atmospheric pressure on the outside of the coin is greater than the pressure of air under the coin and the coin stays pressed against the cupboard door.
31.1.9 Comb attracts and repels, bent water stream, air cleaner
1. Pull a sheet of tissue paper, e.g. toilet tissue, Kleenex, into very small pieces and let them fall onto a sheet of plastic.
Rub a comb very vigorously with the sleeve of a woollen jumper.
Bring the charged comb close to the paper.
The comb attracts the light paper.
A charged comb causes bits of light paper to become polarized such that the sides nearest the comb get a charge opposite the charge on the comb.
So an attractive interaction between the comb and the bits of paper occurs.
The bits of tissue paper at first stick to the comb, but later they fall off or shoot off the comb as the charge on the comb is distributed around the surface of the bits of paper.
2. Rub a comb or plastic ruler vigorously with wool.
Bring it near a ping-pong ball on a smooth table.
The charged comb attracts the uncharged ping-pong ball.
Let the comb touch the ping-pong ball.
4. Bring the charged comb to a bowl of puffed wheat or puffed rice.
They are at first attracted to the comb then shoot off.
5. Adjust the water tap to give a thin stream of water.
Comb the hair or rub the comb with wool.
Bring the comb close to the thin stream of water.
The water stream bends towards the comb as the negatively charged comb attracts the neutral water.
Let the water touch the comb.
The comb loses its charge to the water and can no longer attract a water stream.
6. To show deflection of a water stream, rub a plastic spoon with wool, turn the tap on gently and hold the spoon near the fine stream.
The falling water is pulled towards the spoon.
The plastic spoon becomes negatively charged, because electrons are rubbed off the wool.
The plastic spoon attracts the uncharged water particles.
When the water touches the spoon it can no longer attract a water stream, because the extra electrons flow onto the water surface from the spoon.
7. Use a comb as an air cleaner.
Rub a comb with wool.
Wave the comb through the air then look at the teeth of the comb with a magnifier.
See the dust from the air now attracted to the comb.
This method is used to clean smoke in chimneys by "scrubbers", which attract dust by electrostatic attraction.
31.1.10 Comb on a turntable
Make a turntable for electrostatic experiments to study the two kinds of static charge.
Put the top of an inverted test-tube into the centre hole of a big cork or a rubber stopper.
Hammer four pins symmetrically into the cork's upper plane without destroying the balance of the cork.
A swivel can be used in many static electricity experiments.
Use it to show that like charges repel each other.
Put a plastic comb, after being rubbed against a piece of pelt, on the swivel table and make the comb's centre of gravity coincide with the centre of the cork.
Then observe the rotation of the comb on the swivel table when another comb, after being rubbed against a piece of pelt, close up the comb on the swivel table.
31.1.11 Clothes brush and flannel
Brush a piece of dry flannel or brown paper with a clothes brush and note how the flannel will then cling to the wall.
31.1.12 Cellulose acetate sheet
See diagram 31.9.30: Measure electrostatic force.
Stick 3 lengths of elastic yarn to the edges of a triangular piece of polystyrene foam.
Suspend the polystyrene triangle horizontally from the tripod stand so that one angle is outside the legs of the tripod.
Record the height of point 1.
Roll cellulose acetate sheet to form a cylinder and rub with a dry cloth.
Raise the cellulose acetate cylinder slowly under the exposed angle of the polystyrene triangle until the exposed angle dips down and sticks to it.
Record the height of point 1.
Lower the cellulose acetate cylinder.
Record the height of point A when the polystyrene triangle pulls away from the cellulose acetate cylinder.
Remove the cellulose acetate cylinder.
Put squares of graph paper on point A until the height is the same as before.
The weight of the squares of graph paper = the electrostatic force between the polystyrene triangle and the cellulose acetate cylinder.
31.1.13 Cotton thread rises
Hold a long cotton thread from each in front of a television.
The thread rises as electrons from inside the television set hit the screen and give it an electric charge that attracts the uncharged thread.
31.1.14 Cottrell smoke precipitator
See diagram 31.4.44: Smoke precipitator.
Clearly visible smoke passes through a glass tube "chimney" containing a central electrode and an outer earthing mesh.
Apply high voltage to the central electrode to reduce the smoke coming from the top of the chimney.
31.1.15 Ebonite rods and glass rods
1. Ebonite rod rubbed with wool becomes negative, because electrons leave the wool and stick on the surface of the ebonite.
Rub two ebonite rods with wool.
Suspend one rod.
Bring the other rod near it.
Note the repulsion.
2. Glass rod rubbed with silk becomes positive because electrons leave the glass and stick to the surface of the silk.
Note that glass rods dried in the flame of a Bunsen burner may become negative when rubbed with silk.
However, glass rods dried in a sand oven always become positive when rubbed with silk.
Lead glass may charge better than soda glass.
Use glass rods dried in a sand bath oven.
Rub the ends of glass rods with silk.
Suspend one rod so that it hangs horizontally.
Bring the rubbed end other rod near it.
Note the repulsion as the suspended rod moves away.
Note the force of attraction between the rubbed ends of both glass rods and the silk.
With one charged rod on a pivot use another of the same or opposite charge to show attraction or repulsion.
3. Suspend a charged ebonite rod.
Bring a charged glass rod near it.
Note the repulsion.
Suspend a charged ebonite rod.
Pass another charged rod gently through the hand then bring it near the suspended rod.
Note there is no repulsion.
Rub this uncharged rod on the back of the hand.
Bring it near the suspended rod.
Note the repulsion.
Rubbing on the skin has recharged the rod.
Pass the charged rod quickly through a Bunsen burner flame.
Bring it near the suspended rod and note its loss of charge.
4. Fit a brass or other metal tube over the end of an uncharged ebonite rod.
Rub the brass tube only with fur.
Bring it near a suspended charged rod of ebonite.
Note the repulsion.
The brass is also negatively charged.
31.1.16 Electric charge, conventions from effects of friction between substances
By convention, a rubber rod rubbed with fur has negative electrical charge.
A glass rod rubbed with silk has a positive electrical charge.
An uncharged body has equal amounts of positive charge and negative charge.
1. Rub a rubber rod with fur.
Suspend two pith balls so that they hang parallel and vertically.
Touch separately the two suspended pith balls with the rubbed rubber rod.
Bring the two pith balls together.
They swing apart, so a repulsive force now exists between them.
Rub the wool felt cloth on a PVC rod to produce negative charge on the rod.
Rub the cellophane on an acrylic rod to produce positive charge on the rod.
2. Rub a glass rod with a silk cloth.
Suspend two pith balls so that they hang parallel and vertically.
Touch separately the two suspended pith balls with the glass rod.
Bring the two pith balls together.
They swing apart, so a repulsive force now exists between them.
Bring together one of each pair of pith balls.
They swing together, so an attractive force now exists between them.
3. Rub a glass rod with a silk cloth then bring it near an insulated conductor.
The silk is now negatively charged and the glass is positively charged, so the side of the insulated conductor nearest the glass rod is negatively charged and the side of the insulated conductor away from the glass rod is positively charged.
The total charge on the insulated conductor is unaltered.
If the original charge on the insulated conductor was zero then it is still zero.
Verify these charges with a proof plane, an aluminum-covered conductive disc attached to an insulated handle.
It is used to sample the charge density on charged conductive surfaces.
By touching the insulated conductor at different points, it will be the same potential as that point and this can be verified with an electroscope.
4. Inflate a party balloon and tie it shut very tightly.
Rub the balloon vigorously on the head of a person with dry, non-oily hair.
Touch the balloon on a fluorescent light bulb.
The bulb gives off a "ghostly" glow when electrons from the balloon flow into it to excite the phosphorus coating and produce light.
31.1.17 Electric charge movement, charging by induction
See diagram 31.5.13: Charging by induction.
1. An electric current is a flow of electrons.
An electron carries one negative basic charge and a proton carries one positive basic charge.
The original positive charge is the charge on a glass rod when rubbed with silk.
Taking away electrons forms a positive charge on an object.
Adding electrons forms a negative charge on an object.
2. A flow of electric charge is associated with the following:
Heating effects,
Lighting effects,
Resistance, conductors and insulators,
The motor effect,
The electrolysis process,
Magnetic effects,
Currents in nerve tissue.
3. Electric charge can be either positive or negative, and a neutral atom has equal quantities of positive and negative charge.
Like charges repel each other.
Unlike charges attract each other.
Rubbing a plastic bag with wool takes electrons from the wool to leave the plastic bag negatively charged and wool positively charged.
4. Insulators do not allow charges to move over their surface or through them, so any charge remains on the surface.
Conductors allow charges to move over their surface or through them.
5. Bring a charged object near a metal conductor.
The like charge is repelled along the metal conductor to the far end, and the end near the charged object is left with opposite charge.
Charges separated in this way are called induced charges.
6. The testing for type of charge is repulsion by a like charge.
If an isolated conductor has charges induced on it, the charge that is repelled can be made to go to earth by touching the conductor briefly with a finger.
The opposite charge will be left on the conductor.
31.1.18 Static electricity from rubbing
See diagram 31.39: Electricity from rubbing.
1. Make circular pieces of paper with a hole puncher or make a pile of cork particles by filing or cutting a cork.
Rub a plastic comb or plastic rule or plastic ball pen case with a woollen jumper or your dry hair, or rabbit fur or flannel or silk.
Note which rubbing attracts the most circular pieces of paper or pieces of cork.
2. Static electricity is electricity not flowing as a current.
It is a form of energy caused by charged particles, e.g. protons, electrons, accumulating statically.
Make circular pieces of paper with a hole puncher or make a pile of cork particles by filing or cutting a cork.
Rub a plastic comb or plastic rule or plastic ball pen case with a woollen jumper or your dry hair, or rabbit fur or flannel or silk.
Note which rubbing attracts the most circular pieces of paper or pieces of cork.
3. Make a pile of finely divided cork particles by filing a cork.
Cut up some thin paper into small pieces.
Use very clean objects such as a plastic comb, a plastic pencil, a plastic fountain pen, a piece of wax, a rubber balloon, a glass or china dish and any other non-metallic objects you may find.
Rub each of these things briskly with your dry hair or a piece of fur and then bring near the pile of cork particles.
Rub again and bring near the pile of thin paper.
Observe what happens.
Repeat the experiment, rubbing each article in turn with a silk cloth.
Repeat using a piece of flannel.
31.1.19 Faraday's ice pail experiment
See diagram 31.1.4: Faraday's ice pail experiment, (Wikipedia).
The experiment shows how charge is distributed over a metal conductor and how electrostatic induction occurs.
A metal sphere, C, is given a net negative charge.
An ice pail, A (metal ice bucket), on an insulated stand, B, is electrically neutral, having charges evenly distributed over its inner and outer surfaces.
The metal sphere is lowered completely into the ice pail so as not to touch the sides.
The negative charges on the surface of the metal sphere repel the free electrons from the inside to the outside of the ice pail, where the excess electrons move down a wire connected to a gold leaf electroscope, E.
So now the inside of the ice pail has a net positive charge and the leaves of the gold leaf electroscope are displaced.
The induced charge on the electroscope is the same size as the inducing charge on the metal sphere.
If the metal sphere touches the inside of the ice pail, charges move between them and the metal sphere and inside of the ice pail become electrically neutral.
However, when the metal sphere is then removed from inside the ice pail, the leaves of the gold leaf electroscope remain displaced, because the outside of the ice pail keeps the same charge.
1. To show the equality of charges, rub a rubber rod against a similar rod covered with wool and insert them separately and together in the ice pail.
2. Charge the sphere several times by rubbing a charged rod on its surface.
Touch the proof plane to the outside of the sphere and then to the electroscope.
The electroscope charges, indicating there is a charge on the outside of the sphere.
Repeat the same test on the inside of the sphere.
The electroscope does not charge.
31.1.20 Gold leaf electroscope
See diagram 31.5.3: Gold leaf electroscope.
The gold leaf electroscope is used for detecting and measuring electric charges.
It consists of two gold leaves attached to a brass rod in a glass vessel.
A metal cap is attached to the end of the rod.
If leaves have same sign charges they repel each another like an inverted V, if charge is increased separate more widely, if the charge is decreased they fall together.
The gold leaves may be charged by induction or by contact.
Suppose that they are charged negatively.
The sign of the charge on any charged body may be found by bringing it up to the terminal of the electroscope.
If a positively charged body is held close to the terminal, electrons from the leaves will be attracted up to the terminal by the positive charge, and the charge on the leaves will be decreased and they will fall together.
If a negatively charged body is held close to the terminal, electrons will leave the terminal and go down to the gold leaves to increase their charges and so the leaves will separate more than before.
So if the gold leaves are charged together negatively, they fall together for a positively charged body, and separate for a negatively charged body.
To use a negatively charged body to charge the gold leaf electroscope positively, place the negatively charged body near the terminal, so that electrons will flow down to the gold leaves, leaving the terminal positively charged.
When the rod is earthed, grounded by touching with the finger, electrons flow from the leaves to the earth, leaving the leaves neutral.
When the earth connection is broken, and the charging body then removed, the positive charge from the terminal distributes over the terminal and leaves, leaving the electroscope with a net charge of opposite polarity to the charged body.
To use a negatively charged body to charge the gold leaf electroscope negatively, place the negatively charged body on the terminal to make direct contact between the terminal and charged body.
An electroscope used for measuring electric charges may have only one leaf.
The case is metal and is also charged.
The movement of the leaf indicating the magnitude of the charge or potential measured.
31.1.21 Many charges from one source
See diagram 31.50: Produce many charges of static electricity from one source, electrophorus.
Many charges from one source:. Use a piece of aluminium or a cake tin.
Heat the metal evenly over a flame.
Touch a wax candle to the centre of the aluminium until it melts and sticks solidly to it as a handle.
Use a plastic dish pan or bowl larger than the cake tin.
Put the bowl or pan on a table and stroke the inside bottom of the pan briskly with a piece of fur or flannel for half a minute.
Put the aluminium on the plastic and press it down hard with your fingers.
Remove the aluminium pan, put your finger near the metal and you should get a spark.
You can take many charges from the plastic without more rubbing.
Press the metal against the plastic, press with your fingers and lift by the handle.
31.1.23 Mercury shaker
(This experiment not allowed in some school systems!)
Shake mercury in a bottle.
Put some mercury in a plastic bottle with a conducting rod sticking through a stopper.
Shake the mercury and invert to charge the rod for a positive charge.
Invert a second time for negative.
A glass tube containing some mercury is covered with tin foil on one end.
Either positive or negative charge may be produced.
A mercury tube that emits light when shaken.
31.1.24 Neon lamp on Styrofoam
Use a voltage tester, shaped like a screwdriver, with a handle containing a small neon lamp.
In a dark rom, hold one metal end firmly and rub the other on a piece of hard Styrofoam plastic.
The lamp begins to glow.
Since the plastic is soft, its layers are rubbed against one another by the movement of the lamp and become strongly charged with electricity.
The electrons collect on the surface, flow through the core of the tiny lamp, which begins to glow, and into the body.
31.1.25 Newspaper stays on the wall
See diagram 31.44: Newspaper stays on the wall.
1. Spread out a sheet of newspaper and press it smoothly against a wall on a dry day.
Stroke the newspaper with a pencil or your hand all over its surface several times.
Pull up one corner of the paper and then let it go.
Notice how it is attracted back to the wall.
If the air is very dry, hear the crackle of the static charges.
If you hold the charged paper near your cheek, you may receive a tickling feeling.
Repeat the experiment by rubbing the paper with wool, fur, nylon, plastic or celluloid.
2. Warm a sheet of newspaper paper in an oven.
Put the newspaper on a table and rub vigorously with a polythene shopping bag or dry cleaner's cover.
Put a round metal top from a glass jar on it.
Hold each end of the newspaper and lift it.
Ask another student to bring a finger near the round top.
On a dry day a spark jumps.
Repeat by rubbing with a nylon stocking.
Repeat by rubbing with a woollen jumper.
Compare the sizes of the sparks.
3. Hold a piece of paper on a slate blackboard and rub it with fur.
Rub paper with cat fur while holding it on the board.
31.1.26 Paper dancers
Use very light paper to cut out the shapes of dancers.
Put the dancers on a metal plate.
Put a glass roof over the dancers supported by two books. Rub the glass
with wool.
Be careful! Do not rub too hard and break the glass!
The dancers start dancing when the uncharged dancers are attracted to the charged glass, pick up extra electrons from the charged glass, become negatively charged then repel other dancers with the same charge.
The dancers fall down onto the metal plate, give their extra electrons to the plate and are again attracted to the glass.
31.1.27 Paper horse race
See diagram 31.1.3.1: Paper horse race.
Cut some small horses from folded dried paper and stand them on a table.
Rub a plastic article, such as a ruler, vigorously with wool and use it to attract the "horses" in a race across the table.
31.1.28 Paper loop attracted by plastic ruler
Cut a piece of paper 5 cm x 1 cm and join the ends with adhesive tape to make a paper loop.
Rub a plastic ruler vigorously and hold it on the table in front of the loop.
The loop rolls towards the ruler.
React quickly enough to pull the ruler away from the loop so it will follow the ruler and roll across the table.
31.1.29 Paper strips diverge
See diagram 31.1.3.2: Paper strips diverge.
Cut newspaper strips about 5 cm wide and 40 cm long.
Dry them in an oven or warm place.
Hold two strips in one hand.
Stroke them downward several times with the finger and thumb of the other hand.
The strips fly apart.
Use another two strips.
Hold these two strips in one hand.
Stroke them downward with folded polythene plastic shopping bag or wool cloth.
The strips fly apart more strongly.
Use paper strips as a "strip test" to see which materials produce the greatest charge.
In this way, produce a triboelectric series.
31.1.30 Paper snake strikes
Cut a spiral-shaped coil from a piece of very thin paper, tissue paper, so that it looks like a snake.
Draw two eyes and a mouth on the head of the snake.
Put it on a tin lid and bend its head up.
Rub a plastic ball point pen case, or plastic comb, or plastic ruler with wool cloth and hold it over the snake.
The paper coil rises like a snake and tries to bite you!
The rubbed plastic takes electrons from the wool and attracts the uncharged paper.
On contact, the paper falls, because it takes part of the negative electric charge and gives it up immediately to the metal lid, which is a good conductor.
Since the paper is now uncharged again, it is again attracted upwards until the ball pen has lost its charge.
31.1.31 Paper shapes dance
Put a sheet of glass over a metal plate.
Cut out shapes from tissue paper.
Rub the glass with wool.
The tissue paper shapes jump up and do a funny dance.
Rubbing charges the glass and attracts the tissue paper shapes and charges them.
The shapes repel each other, fall back on the metal plate, give their charge
to the metal plate and again are attracted to the glass.
31.1.32 Pepper photocopy
Evenly scatter finely ground pepper over the bottom of a plastic Petri dish.
Cut a piece of paper to exactly match the plastic lid of the Petri dish.
Use sharp scissors to cut a shape out of the paper, e.g. a diamond shape.
Hold the paper over the Petri dish lid and rub vigorously with wool over the diamond shape.
The pepper jumps up and sticks to the underside of the cover in a diamond shape.
The diamond-shaped area of the plastic cover rubbed with wool developed a negative charge that induced positive charges on the upper surfaces of the pepper particles.
So the pepper particles were attracted to the plastic lid.
Pepper particles do not have a regular shape and pepper is an electrical insulator.
So the pepper particles remain charged and clinging to the plastic lid.
31.1.33 Pith ball indicator
Use the white pith from inside a plant stem.
Dry the pith thoroughly and then press it tightly into small balls 5 mm in diameter.
Coat the pith balls with aluminium powder in egg white, colloidal graphite or metal paint.
Attach each pith ball to a silk thread or fishing line 15 cm in length.
Bring objects rubbed with silk, fur or flannel near the pith ball and note how it behaves.
This equipment is an electroscope.
In place of pith balls, use grains of puffed wheat, puffed rice, expanded polystyrene, Styrofoam balls, ping-pong balls, or any light object.
31.1.34 Pith ball indicator, electroscope
Use the white pith from inside a plant stem.
Dry the pith thoroughly and then press it tightly into small balls 5 mm in diameter.
Coat the pith balls with aluminium powder in egg white, colloidal graphite or metal paint.
Attach each pith ball to a silk thread or fishing line 15 cm in length.
Bring objects rubbed with silk, fur or flannel near the pith ball and note how it behaves.
This equipment is an electroscope.
In place of pith balls, use grains of puffed wheat, puffed rice, expanded polystyrene, Styrofoam balls, ping-pong balls, or any light object.
31.1.35 Polythene bag for a nylon stocking
Rub the palms the hands with blackboard chalk to make them dry.
Rub a polythene shopping bag vigorously between the palms of the hands.
Pick up the bag.
The side move out, because of repulsion.
Cut a strip of polythene from the bag and hang it over a dry finger so that the ends hang down.
The ends repel each other.
Hold a nylon stocking against the wall and rub with a polythene bag.
Pull it from the wall.
The stocking fills out to its shape as the sides repel with the same charge.
Bring the polythene near the stocking and they will attract each other.
31.1.36 Puffed rice jumps
Rub a plastic spoon with wool.
Hold it over a dish of puffed rice cereal.
The grains jump up, hang on the spoon then jump off in all directions.
The uncharged puffed rice is attracted to the negatively charged spoon and hang on to it.
Then electrons spread from the spoon into the puffed rice both have the same charge.
The charged puffed rice grains then move in all directions, because like charges repel each other.
31.1.37 Repulsing balloons
Blow up two balloons and tie with strings one metre long.
Rub each balloon with fur.
Hold the strings together and note how they repel.
Put your hand between them and note what happens.
Bring one balloon near your face.
Repeat, using three balloons.
31.1.38 Rubbed balloon stays in place
1. Blow up a toy balloon and rub it briskly with a piece of fur.
Place it against the wall and observe that it stays there.
Repeat the experiment by rubbing the balloon on the hair.
2. Blow up a toy balloon and rub it with a piece
of fur or clothing.
Place it against the wall and note that it stays there.
The electrons collected on the rubber balloon from the fur or clothing repel electrons in the surface of the wall leaving a positive charge on the wall that attracts the surface of the balloon opposite the wall, which pulls the rest of the balloon with it.
Repeat the experiment by rubbing the balloon on the hair.
3. Rub inflated balloons against wool, cotton or hair.
Let them stick to the blackboard or the wall.
The material rubbed against loses some electrons and the balloons gain some electrons giving them a negative charge.
As the balloons approach the blackboard the negative charges on the balloon repel the negative charges on the blackboard leaving a positively charged surface on the blackboard near the approaching balloon.
So the balloon sticks to this positively charged area of the blackboard.
Later, as electrons flow from the balloon to the blackboard, the balloon becomes electrically neutral and falls, especially in humid weather.
31.1.39 Static electricity detector
See diagram 31.142: Static electricity detector.
Cut a strip 2 cm by 10 cm from thin cardboard.
Fold it in half lengthways and balance it on a pencil point.
The pencil point should indent, but not perforate the paper, so that the paper can turn easily.
Charge a comb by rubbing on hair or wool and hold it near one end of this detector.
31.1.40 Two kinds of static charge
See diagram 31.146A: Positive and negative charges attract each another.
See diagram 31.49.2: Use an uncharged pith ball electroscope.
See diagram 31.49.3: Use a charged pith ball electroscope.
The basic observations of electrostatics are as follows:
Observation 1. Rub a plastic comb with fur.
The plastic comb becomes -ve and the fur becomes +ve.
Observation 2. Rub a glass rod with silk.
The glass rod becomes +ve and silk becomes -ve.
2. Like static charges repel each other and unlike charges attract each other.
Make a turntable by driving a long nail through a wood base.
Push a test-tube into a hole made in a large flat cork.
File the end of the nail to a sharp point and invert the test-tube over it.
Set pins in the top surface of the cork, they brace the objects you put on the turntable.
Use two test-tubes or glass rods, a piece of silk, two plastic combs, an ebonite rod, some wool, and a piece of fur or flannel.
Rub a comb with fur and set it on the turntable.
Rub the other comb with fur and bring it near the comb on the turntable.
3. Rub a glass rod with silk and put it on the turntable.
Again rub a comb with fur and bring it near the glass rod.
Repeat until you are sure of your observations.
When you rub the comb with fur, the plastic takes a negative charge of electricity and the fur takes a positive charge.
When you rub glass with silk, the glass takes a positive charge and the silk a negative charge.
4. Rub an ebonite rod with a piece of wool and bring the rod near an uncharged pith ball electroscope.
Note that the pith ball is first attracted and then repelled.
5. Rub a glass rod with a piece of silk and bring the rod near an uncharged pith ball electroscope.
The pith ball is at first attracted to the glass rod and then repelled.
6. Charge a pith ball negatively by touching it with an ebonite rod rubbed with wool.
When you bring a negatively charged plastic comb near the negatively charged pith ball, they repel each other.
When you bring a positively charged glass near the negatively charged pith ball, they attract each other.
31.1.41 Volta's experiment, voltaic pile, metal on Perspex,
See diagram 31.1.19: Volta's experiment.
Volta's experiment
Fit a tin plate metal disc with a plastic handle and attached a metal screw to the metal disc.
Rub the top face of a sheet of Perspex plastic with wool to make a positive charge on the Perspex.
Take hold of the plastic handle and lay the metal disc on the charged sheet of Perspex.
Touch the metal disc briefly to earth it.
Hold the plastic handle and pick up the metal disc.
The metal disc now has a positive charge.
In the dark, bring a finger close to the screw in the metal disc and observe an electrical discharge.
Note how many times the discharge can be repeated without rubbing the Perspex sheet again.
31.1.42 Wool flannel lights a fluorescent light tube
Rub a fluorescent light tube in the dark with a piece of flannel.
31.2.01 Electric charge, the coulomb, C, Coulomb's law
(Charles Augustin de Coulomb, France, 1785)
The coulomb, the SI unit of electrical charge, is the quantity of electricity transferred in one second by a current of one ampere.
An electrically charged object has a quantity of net charge that can be measured.
The coulomb, C, is the practical unit of charge.
1 C = 6.25 x 1018 elementary charges, i.e. the number of electrons in 1C of charge = 6.25 x 1018 electrons.
1 elementary charge, e = 1.60 x 10-19 C, so the charge on a single electron is -1.6 x 10-19 C,
and the charge on a single proton is +1.6 x 10-19 C, the same value, but opposite in sign.
The charge on a neutron is zero.
SI unit of electric charge, C, is the quantity of electricity conveyed in one second by a current of one ampere.
(1 coulomb = 1 ampere second).
1. Electrostatic force, F, between two charged spheres varies directly on the charges on the spheres, Q1 and Q2, and varies inversely on the square of the distance, r, between the centres of the spheres.
The electrostatic force between two charged spherical objects obeys an inverse square law.
F = kQ1Q2 / r2, where Q1 and Q2 are the two charges, r = the separation of the centres of the two charged objects, and k = the constant depending on the medium between the two charges, called the coulomb constant.
ε0 is called "the permittivity of free space" or "vacuum permittivity", so ε is the "relative permittivity" or just "permittivity" of a medium.
Permittivity is measured by the SI unit, farads per meter, F / m, so for charges in a vacuum, k = 1/4πε0, where ε0 = 8.85 10-12 Fm-1, so k = 8.9 X 109 mF-1.
2. A conductor carrying one amp for one second is carrying one coulomb of charge.
Coulomb's law relates electrical charge to mechanical force.
The French physicist Charles Augustin de Coulomb invented a torsion balance that he used in 1785 to show the forces of attraction or repulsion between two charged spheres.
3. Coulomb's law
Like charges repel and opposite charges attract with force proportional to product of the charges and inversely proportional to square of the distance between them.
Coulomb's Law states the electrostatic force between two charged spheres depends on the charges on the spheres and inversely as the square of the distance between their centres.
If two charges, q1 coulombs, C, and q2 coulombs, C, have the same sign (+ or -) and are distance, r, apart, the force on one charge from the other charge, F = constant, k x (q1q2) / r2.
The Coulomb's law constant, k = 8.99 x 109 Nm2 (newton m2 / C2), about = 1 / (4π ε), where ε, permittivity of the medium = 8.85 x 10-12 C2 / N.m2.
So F = q1q2 / 4πEr2.
F = force measured in newton (N)
r = the distance between the centres of the two objects measured in metres (m)
q = the charge on the two objects measured in Coulombs.
4. When air is between the charges, the Coulomb constant, k = constant of proportionality, permittivity of free space, has value 8.9874 x 109 newton m2 / C2, Nm2C-2.
5. This force can cause the objects to repel or attract depending on the charge of the objects.
The formula refers to the force exerted by point charges and varies with distance.
A point charge refers to a charge that has no mass so that the influence of gravitational forces can be neglected.
The very small forces involved in Coulomb's law must be measured with a torsion dynamometer, e.g. 0.01 N, and very light conducting spheres.
31.2.1 Balloons rubbed with wool or fur repel each other
1. Fill three air balloons with air equally and use three pieces of one metre long thread to fasten their openings separately.
Grasp the ends of the threads by one hand and hang the three air balloons in the same height.
Remove one balloon without changing the position of the others and rub it against wool or fur.
Let it drop down to resume its original position.
Note the states of the other two balloons.
2. Blow up two balloons equally.
Attach them with a one metre long string.
Rub both balloons with wool or fur.
Hold the centre of the string and let the balloons drop down.
The balloons will not come together, because both became negatively charged when they took extra electrons from the wool.
The balloons fight to move away from each other, because like charges repel each other.
Suspend the balloons in a doorway where there is a slight wind.
The balloons keep spinning around each other.
Bring one hand near one balloon, but not touching.
The balloon is attracted to the hand.
Place a hand between both balloons.
The balloons come together to touch the hand.
The negative charges on the balloons have repelled the negative charges on the hand leaving the hand with net positive charge, so the balloons are attracted to the hand.
3. Blow up two balloons equally.
Attach them with a one metre long string.
Rub one balloon with wool or fur.
Rub the other balloon with Saran Wrap or a polythene plastic shopping bag.
Hold the centre of the string and let the balloons drop down.
The balloons are strongly attracted to each other, but when they touch they hang straight down.
The balloon rubbed with wool or fur has extra electrons.
The balloon rubbed with Saran wrap, or polythene plastic shopping bag, has a shortage of electrons.
So the two balloons attract each other and come together.
When the balloons touch the balloon with extra electrons, it gives them to the balloon with shortage of electrons so they become neutral, no longer attract or repel each other and hang straight down.
4. Rub two balloons against woolen fabric, then try moving the balloons together.
Note if they are attracted or unattracted to each other.
Rub one of the balloons on your hair, then slowly it pull it away while looking at your hair in a mirror.
Put an aluminium drink-can on its side on the table.
Rub the balloon on your hair again and hold it close to the aluminium drink-can.
Slowly move the balloon away from the drink-can and follows the balloon.
31.2.2 Charging by conduction and induction
See diagram 31.2.1: Charging by induction.
Objects become electrically charge through a process of conduction or induction.
1. Charging by conduction
An electrically neutral object is charged by conduction when a charged object contacts it.
When a rod with an excess of electrons touches a neutral ball, the charge distributes itself over both objects.
When the objects are separated, the ball will now be electrically charged.
2. Charging by induction
Charging an electrically neutral object by induction is a three step process.
* Move a negatively charged rod close to an electrically neutral ball.
The electrons on the ball are repelled and move to the opposite side of the ball.
* Touch the negative side of the ball.
The electrons are then "earthed" off, i.e. the electrons flow from the ball leaving it with a net positive charge.
* Remove the rod to leave a positively charged ball.
31.2.3 Coulomb's law balance
Adapt a soda straw balance to make a simple Coulomb's law balance.
31.2.4 Measure electrostatic charge
See diagram 31.2.11: Electrostatic voltmeter.
Three types of devices are used to measure charge on a surface
1. An electrostatic field meter responds to the total or net charge on a film.
It is used to measure the electrostatic field produced by a charged surface located some distance away in voltage per unit distance.
It is used to locate and measure static charge potentials on a product.
2. A Coulombmeter is used to measure charges on isolated conductors by direct contact using a point contact probe or by placing the charged object in a Faraday cup connected to the meter input.
3. Electrostatic voltmeters respond to the charge present on only one side of a film.
They measure the actual potential (voltage) at the surface of the object under test using non-contacting sensors.
1. Make a Coulombmeter
See diagram 31.2.11.1.
In the diagram (A) is a negative charged insulated rod to determine the charge.
A metal plate (B) is connected to the Coulombmeter and located near the charged rod.
The Coulombmeter consists of a capacitor (C) with a voltmeter across it.
The bottom lead of the capacitor is connected to ground.
When the rod A is brought near plate B, the negative charged rod A will attract an equal positive charge to plate B.
The charge going to plate B, is coming from the upper capacitor plate, leaving this negatively charged.
This will then attract an equal positive charge from ground to the lower capacitor plate.
Capacitor (C) is now charged, and the voltmeter can measure a voltage across it, proportional to the measured charge.
The rod (A) should be very close to the plate (B).
Only the charge of the rod facing plate B (at short distance) is measured.
The rest of the rod at some distance of plate B may also be charged, but this has little influence on the measurement, because the attraction between charges reduces with the square of the distance between them.
2. Connect an electrostatic voltmeter to the Van de Graaff generator.
Connect one plate to earth and connect the other plate to the dome of the Van de Graaff generator, by using a shorting stick.
This voltmeter works on the principle of charged plate attraction.
The voltmeter rapidly reaches full scale then the capacitor discharges.
Repeat the experiment by separating the plates to different distances.
3. Balance a metre rule on an inverted round bottom flask.
Rub a comb or an ebonite rod with wool cloth.
Bring the comb ebonite rod near the end of the metre ruler and note the movement of this sensitive balance.
31.2.531.2.9 Metal balls play music
Charge by touching can make a hanging small conductor swing.
Use two equal sized metal balls and insulated stands with same height.
Put them 4 cm apart.
Hang a screw nut by a nylon thread.
The upper end of the thread hangs on the iron bar of the stand.
Adjust the height of the hanging point to let the screw nut being just in the line connected two conductors and equal distance from them.
Charge the two conductors with opposite charges.
Incline a conduct ball to touch the screw nut gently (it cannot discharge with another ball), then back to its original position.
The screw nut will swing between two conductors.
After the screw nut touches a ball, it has the same charge as that ball.
The nut is acted on either by repulsion of that ball or by attraction of another ball, leading to it moving to another ball and touching it.
This results in the nut having the same charge with another ball and leading to it moving to the original ball.
Thus motion is repeated until the two balls complete the discharge and the energy of swing is used up.
If this discharge occurs when the ball is first inclined, charge the balls again.
If it still discharges, increase the distance between the balls.
31.2.6 Pith balls and Styrofoam balls show electric charge
See diagram 31.49.2: Pith balls.
31.2.7 Separate pepper from salt and pepper, jumping pepper
See diagram 31.2.8: Separate pepper from salt.
1. Put ground pepper in a small plastic box with a flat lid.
Rub the lid with wool, e.g. a woollen jumper.
The pepper jumps up and sticks to the inside of the lid.
Touch the lid with an opened wire paper clip.
The pepper moves sideways away from the end of the paper clip or drops down.
2. Put some coarse salt and ground pepper in a plastic box and cover the box.
Shake the box many times to mix the contents thoroughly.
Rub the cover of the plastic box with a piece of wool cloth, or the dry palm of the hand.
Turn the plastic box upside down then turn it back again.
The pepper sticks to the cover.
3. Mix salt and pepper on a dry surface then put the mixture on a piece of paper.
Hold a plastic spoon or plastic ruler rubbed by a piece of wool cloth or woollen sock above the mixture.
The pepper jumps up out of the mixture and sticks to the plastic spoon or ruler.
Lower the plastic spoon and the salt also jumps up.
The pepper jumps up first, because it is lighter.
4. Sprinkle pepper and salt on the table.
Rub a plastic spoon with wool and hold it above the mixture.
The plastic spoon becomes negatively charged, because electrons are rubbed off the wool.
The pepper is attracted to the plastic spoon.
The pepper jumps up and sticks to the plastic spoon.
The plastic spoon attracts both the uncharged pepper and salt, but the pepper rises first, because it is lighter than the salt.
Hold the plastic spoon lower to attract the salt.
The uncharged pepper particles are attracted to the charged surface of the plastic spoon, because opposite charges were induced in the pepper particles.
No charges are induced in the salt particles.
Separate the two substances by sprinkling the mixture over the water surface in a container.
The salt sinks and the pepper particles float.
31.3.1 Charge electroscope by induction
See diagram 31.5.11: Charging by induction.
See diagram 31.5.1: Charge electroscope with cats' fur.
1. Charge a rod negatively by rubbing or flicking it with the cats' fur.
Bring the charged rod near the cap of the electroscope.
The leaves of the electroscope diverge indicating potential.
Earth the cap of the electroscope by touching it while the charged rod is still nearby.
Electrons flow to earth.
Remove the earth connection then remove charged rod.
The electroscope now has a net positive charge.
2. Charge an ebonite rod negatively by rubbing it with wool and bring it near, but not touching, the lid of the electroscope.
* Still holding the ebonite rod in position, touch the lid of the electroscope with the finger.
* Remove the finger from the lid of the electroscope.
* Remove the ebonite rod.
The electroscope is now positively charged the electroscope and the leaves are apart.
The ebonite rod near the metal top of the electroscope repels some electrons that move down to the two leaves and they fly apart.
The top becomes positively charged.
When the electroscope is touched with a finger, electrons repelled by the ebonite rod move through the body to earth.
Electrons are attracted back from the leaves and the leaves collapse.
When the ebonite rod is removed, the excess positive charge is distributed through the conducting part of the electroscope.
3. Touch the plate of an electroscope while holding a charged rod nearby.
Pith balls touching both ends of a conductor are charged when a charged rod is brought towards one end.
Use another test charge to show the polarity at each end.
4. Give a charging rod a negative charge by rubbing it with cats fur using a flicking motion.
Bring the charging rod near the cap of the electroscope so that the leaves
diverge to indicate potential.
Allow electrons to flow to earth by touching the cap of the electroscope with the finger it while holding the charged rod is still near it.
First remove the earth connection and then the charged rod to leave a net positive charge on the electroscope.
31.3.2 Charge metal objects by induction
See diagram 31.5.12: Charge metal rod by induction.
1. Charge an insulated metal sphere by induction.
Bring a negatively charge rod close to the left side of the sphere.
Electrons on the sphere near the rod are repelled causing an induced positive charge on the left side of the sphere and an induced negative charge on the right side.
The metal sphere is still electrically neutral.
Connect the right side of the sphere to the earth with a grounding wire.
The electrons repelled by the negatively charged rod flow to the earth.
Remove the grounding wire to leave the metal sphere with an induced positive charge.
2. Put a metal rod or metal spoon AB on an insulating stand, e.g. a beaker.
Hang an aluminium foil metal ball from a string so that it hangs near end B of the metal rod.
Charge an ebonite rod negative by rubbing it with wool then bring it near end A of metal rod.
A positive charge is induced at end A and a negative charge is induced at end 2.
The aluminium foil ball is attracted to end B where it receives a negative charge, is repelled by end B and moves away again.
Move the aluminium foil ball far from the metal rod.
Bring the charged ebonite rod near the aluminium foil ball.
They repel each other showing that the metal rod had lost some of its electrons to the aluminium foil ball.
31.3.3 Deflection of s compass needle
Use a handkerchief to rub a small area of a magnetic pocket compass, hand-held compass, on a dry day.
The compass needle deflects towards the rubbed area of the cover glass.
The rubbed area of the cover glass has become electrically charged by induction, the compass needle has become oppositely charged by induction and has become attracted to the rubbed region.
The compass needle is insulated at its pivot.
This experiment works only if the hand-held compass is a "dry" compass, i.e. not filled with fluid.
Instructions for using a dry compass may include the warning not to position the thumb over the cover glass, because movement of the thumb over the glass may cause induction and lead to errors in navigation.
31.3.5 Faraday cage, electrostatic shielding, electric field lines
See diagram 4.45: Faraday cage.
A positive charge placed on a solid metal sphere spreads around on the surface.
Any extra +ve or -ve charge placed on a conductor spreads over the exterior surface of the conductor.
Electrons can move freely within a conductor so the electric field within a conductor is zero.
Electric field lines towards the negative charges are from the positive charges on a conductor care at right angles to the conductor.
Fragile items can be protected from an external electric field if surrounded by hollow conductors, e.g. an ice pale, metal biscuit tin or thin metal foil.
A Faraday cage is an earthed metal cage to shield a device from electric fields.
Induced currents cannot move around the cage.
An earthed conductor can also protect against an electric field, because an electric field cannot cross the conductor.
Electrostatic shielding is used in the design of television cables and to protect electronic components.
The metal hull of an aeroplane acts like a Faraday cage to protect the passengers from lightning.
Electrical shielding tape is an all-metal, flat, open-weave shielding braid, compatible with power cable insulation and all high voltage splicing and terminating materials.
31.3.6 Faraday flashlight
See diagram 31.3.6: Faraday flashlight.
Faraday flashlight, shaking flashlight, dyno torch, mechanical source of electricity
31.3.7 Franklin's bell, lightning warning device
See diagram 31.4.43: Franklin's bell.
Place the bell apparatus beneath the Van de Graaff generator dome.
Start the generator to simulate a storm passing overhead.
The high voltage generated present rings the bell.
31.3.8 Pith balls repel and attract
See diagram 31.2.6.1: Pith balls repel and attract.
1. Use a charged rod, rubbed with fur, to contact hanging pith balls.
The pith balls are repelled from the rod and from each other.
2. Use a thin thread to suspend a pith ball on
the arm of the iron stand.
Rub a plastic comb vigorously against woollen material.
When the comb approaches the suspended pith ball, the comb attracts it.
Let the comb approach the pith ball several times and observe its movement.
Do not let them make contact.
Move the comb towards the pith ball slowly until the comb contacts it.
Note the movement of the pith ball.
Move a finger towards the pith ball to contact it.
Note the movement of the pith ball.
The pith ball is attracted by the comb and moves with the comb when the
charged comb approaches the pith ball.
A charged body attracts an uncharged body.
The pith ball bounces off the comb when the comb and the pith ball contact
each other, because some charges from the comb flow to
the pith sphere and the two bodies then have like charges.
Like charges repel each other, so the two bodies repel each other.
The pith sphere moves along with the finger's direction when the finger
approaches the charged pith ball is the same as the principle that
the charged comb attracts the uncharged pith sphere.
Because the finger is uncharged and the pith ball is very light, the pith
ball closes with the finger.
However, when the finger and the pith ball contact each other, all of the
charges of the pith ball run off through the person's body and
the pith ball becomes neutral.
So the attraction between the pith ball and the finger will disappear.
31.3.9 Proof plane to test charge on an object
A glass rod, charged by being rubbed with silk, is brought close to, not touching, an insulated conductor.
The silk will be left negatively charged and the glass positively charged, so the side of the conductor nearest to the rod will be negatively charged, and the far side will being positively charged.
However, the total charge on the conductor will remain the same and be zero if not previously charged.
Verify this with a small proof plane.
It is a little conducting plane mounted on an insulating handle that can be touched on to the large conductor at different points, so that it will be at the same potential as the touched point, and have the same nature of charge.
Later, can measure the charge on the proof plane with an electroscope.
To determine whether a metal object made of copper can be charged by rubbing with silk, the copper must held in some insulating material.
Otherwise the charge produced by rubbing would pass to earth through the body of the person holding the copper.
31.3.4 Electrostatic shielding
See diagram 31.5.0: Electrostatic shielding.
A 30 cm PVC pipe has an insulating cap at one end and an insulating handle at the other end.
A thin aluminium tube can slide over the pipe.
Use a wool or Nylon cloth to charge the exposed end of the PVC pipe by friction and then tip so the aluminium tube slides down to cover the charged end of the pipe.
The state of charge on the surfaces is monitored with an electrostatic voltmeter (ESV).
Tip the apparatus so that the conductive aluminium tube slides down and covers the charged end.
The conductive aluminium tube has little effect on the externally observed field.
Touch the sliding aluminium tube with the hand or a ground wire.
Use the ESV to measure the field again.
The reading now drops close to zero.
The experiment shows that the electrostatic charge shielding ability of a conducting vessel enclosing charged material depends on the vessel being connected to ground.
If the vessel is not grounded, the shielding vessel itself will have a net charge.
3. Show that there is no electrostatic field inside a statically charged conductor.
Place an electroscope inside a large Faraday cage 2 metres from a Van de Graaff generator.
Start the Van de Graaff generator and observe that it produces no deflection of the leaves of a gold leaf electroscope.
However, when the gold leaf electroscope is placed outside the Faraday cage the leaves of the electroscope diverge.
Turn on the light and the radio.
You should hear static that the radio picks up from the fluorescent light source.
Place the Faraday cage over the radio and you hear nothing.
4. Place a transistor radio on a sheet of aluminium foil.
Enclose the transistor radio with the wire mesh cage and observe the decrease in the level of sound.
5. Bits of material suspended in oil align with an applied electric field.
Several pole arrangements are shown.
31.4.1 Conductor and non-conductor
Suspend two light pieces of conducting material from a thread (nylon, rayon or silk).
The conducting material could be aluminium bottle tops, cooking foil or tinsel (metalled yarn), rolled into a ball.
Charge a rod, e.g. a plastic ruler, by rubbing with wool and bring it near the suspended conducting objects.
They will be attracted towards the charged ruler.
Immediately on touching the ruler, however, they will be repelled both from the ruler and from each other.
Repeat the experiment with a non-conductor, e.g. two balls of ordinary writing paper.
Note how the behaviour differs from that of the conductor?
31.4.2 Test whether an object is a conductor or insulator
See diagram 31.5.3: Electroscope.
Touch a charged electroscope with the object held in the hand.
The better the insulator, the longer it will take for the leaves to collapse.