ELECTROSTATICS

All matter contains vast numbers of electrons. When a few of them get transferred from the surface of one to the surface of another, they produce an accumulation of charge causing what is known as static electricity.

Electrostatics is the study of electric charges at rest.

To demonstrate electric charges at rest;

a) blow up a baloon, tie its mouth and then rub it vigorously with a clean dry duster. Place the baloon against a wall or ceiling.

b) Rub a second baloon with a duster and then slowly lower it over very tiny pieces of paper on a desk.

USES AND HAZARDS OF STATIC ELECTRICITY

If a rubber baloon is rubbed with a duster, it will attract dust particles. Perspex, cellulose acetate and the vinyl compounds which were used for gramophone records show the same attraction. This is because the materials have become charged with static electricity. Charging by friction is sometimes associated with a cracking sound. (friction is a force that opposes objects sliding over or past each other).this is often heard when combing dry hair or when a nylon shirt is being removed from the body. The crackling sound is caused by some electric sparks which may be seen if the room is in darkness.

KINDS OF CHARGE

If a rod is charged by rubbing and then touched on a small pith ball, the ball becomes charged. If two pith balls are suspended close to each other on nylon threads and charged from the same rod, they repel. However if the balls are charged from two rods made from different materials, they may attract each other. This behavior indicates the presence of two types of static charge referred to as positive and negative charge. It is known that like charges repel whereas unlike charges attract.

In order to obtain positive charge, rub glass or cellulose acetate with silk while a negative charge is obtained on an ebonite rod by rubbing it with a duster.

INSULATORS

Glass, cellulose acetate, ebonite and polythene are examples of materials which are electrical insulators, i.e the charges produced on their surfaces do not move along or through the material, but remain at the spot where they are produced. If a piece of one of these materials is placed on an electric circuit, no current flows as the charge cannot pass through the piece of material.

CONDUCTORS

Metals are examples of electric conductors. Conductors can be charged by rubbing e.g flicking the duster across the cap of a gold leaf electroscope. However if the metal rod is held in the hand and rubbed, the charge formed flows through the metal and is lost to the earth via the hand.

MECHANISM OF A PHOTOCOPYING MACHINE

A photocopier contains a plate whose surface is positively charged when in the dark but uncharged when in light. An image of the object to be copied is projected onto the plate. The dark parts of the plate become charged. Now the plate is covered with a dark powder(toner), the particles of which have become negatively charged. Thus the toner sticks to the dark parts of the plate leaving a dark image. Next, a piece of paper which has been positively charged, is placed onto the plate and the toner is attracted to it. Lastly, the paper is heated so that the toner melts and sticks to it, thus forming a permanent copy of the document.

On board a tanker, great care has to be taken to avoid making sparks which might easily ignite any oil fumes, e.g the crew wear ‘anti-static’ clothing and shoes which do not produce and store static electricity. Sparks are more likely to ignite the oil when it is being unloaded. To avoid this, the oil is covered with a ‘blanket’ of nitrogen, a gas that does not burn so that a spark will not cause an explosion.

How objects become charged:

Electrons which are quite loosely attached to the atoms at the surface of on material are removed by the friction process and deposited on the surface of another material. As the rubbing transfers electrons from one material surface to another, the negative charge of the electrons is transferred with them.

N.B: Rub polythene rod with a cloth or a duster;

i) polythene gains extra electrons on its surface hence becoming negatively charged. The cloth duster used to rub the polythene must have lost these electrons and consequently gained an equal and opposite positive charge.

i.e excess of electrons = negative

deficiency of electrons = positive

ii) materials such as cellulose acetate and glass have electrons removed from their surfaces when they are rubbed, hence becoming positively charged.

iii) Charge is neither made nor removed by friction. It is only transferred from one material to another.

THE GOLD LEAF ELECTROSCOPE

Insulator(stem) metal cap

Wooden/

Metal case

Brass rod gold/

alminium leaf

glass plate ground

metal plate(brass)

TESTING FOR CONDUCTORS

Rub the polythene strip with a duster to give it a negative charge. Sroke the metal cap of an electroscope with the charged strip until the strip is removed, the gold leaf stays raised(the electroscope is now charged) when you touch with a finger, the leaf falls off showing that the body has conducted away the charge. This process is called earthing. Any other material held in the hand which allows the leaf to go down is also a conductor, but one which leaves the leaf raised must be an insulator.

Testing the sign of the charge on an object

1. Give a negative charge to a gold leaf electroscope by stroking the cap with a negatively charged polythene strip(ebonite rod)

2. bring a negatively charged polythene strip and then a positively charged cellulose acetate strip slowly up to the electroscope cap

3. now bring your hand slowly towards the cap

4. repeat these tests with a positively charged electroscope.

A) a charged electroscope

electrons repelled down to the gold leaf

(like charges repel)

divergence of leaf increases

electrons attracted up to the cap

(unlike charges attract, leaf falls)

Induced the charge on hand

c) hand brought near(an uncharged conductor)

* * *

* *

electrons repelled away to earth

electrons attracted upto the cap

divergence increases

CHARGING BY INDUCTION: Separating conductors

1. place two metal objects A and B in contact.

2. Ensure that the spheres are uncharged by touching them with your hand.

3. Bring a charged polythene strip S near to(but not touching) one of the metal sphere A.

A B

Insulating stands

b) s electron flow c) s

A B

Move sphere B away strip

S stays near sphere B

Strip S is removed

This is the process of charging by induction and the charges are induced(it involves action at a distance).

B: Earthing a conductor

electron flow

Insulating stand

Start with an uncharged insulator conductor A and bring a charged strip S near to it.

Electron flow to earth

Touch conductor A with a finger, leave it for a moment, then remove the finger.

Finally remove the charged strip S.

CHARGING A GOLD-LEAF ELECTROSCOPE BY INDUCTION

+ + + + cap start with an uncharged

electron flow electroscope and bring a

charged strip near to its cap.

+ + + +

electron flow to the earth and the leaf falls

Touch the cap with a finger, leave it for a moment and then remove the finger.

c) + + +

finally remove the charged strip S. the positive

charge on the cap spreads over the whole rod

+ + and the leaf rises.

+ + Repeat with a positively charged strip.

THE CHARGE DISTRIBUTION ON THE SURFACE OF A CONDUCTOR.

Proof plane

x x x x

x x x x x

Results show that the charge density is greatest where a surface is most sharply curved.(this is tested by a proof plane which is used to obtain charge from various points on the surface and then test the charge with an electroscope).

INVESTIGATING HOLLOW CONDUCTORS

Proof plane

+ +

There is no charge on the inside of a hollow conductor.

Electrophorus and proof plane

The electrophorus is a charge generator which, by charing, is able to supply almost unlimited charge in small doses.

Perspex handle

Polthene

tiles metal disc

An electrophorus is a metal disk with a perspex handle(insulator) resting on a polythene tile.

How to charge the electrophorus

a) rub a polythene tile with a duster.

b) rest the metal disc on the surface of the tile

c) touch disc D with a finger

Electron flow

c) remove the finger

d) remove disc

induced(opposite) charge

carried away on disc.

Negative charge remains on the tile.

A proof plane is similar to the electrophorus metal disc. It is used for transfering charge from charged conductors to uncharged conductors by sharing.

LIGHTENING CONDUCTOR

The lightening conductor also applies the same principle to protect houses from being destroyed by lightning. It is composed of a sharp pointed metal rod fixed at the highest point of a building and connected with a thick copper cable to a plate(copper) buried in the ground.

+ ++ +

+ + +

+ + + sharp spikes

electron flow

copper strip

electrons spread negative

charge around in the earth.

The steady leakage of positive charge towards the clouds and electron flow to the earth helps to prevent the large build up of charge on the highest parts of the building. The alternative to a steady discharge from the points and through the conductor is a sudden discharge in the form of lightning strike. Considerable heat is generated by the passage of the current and the area around tends to split open through the sudden expansion.

VAN DE GRAAFF GENERATOR

+ Metal dome

+ Pulley

+ +

spray comb +

+ +

HT +

rubber belt

source of high voltage

The sharp point from the high voltage source ‘sprays’ a charge on the belt on the lower part. This charge is collected at the top by the sharp point(brush) on the inside of the dome.

Since the dome is a hollow conductor, this charge is transferred to the outer surface. The process continues until the great charge is deposited.

If a wind mill is connected to it as the generator runs, the wind mill

rotates and can blow out a candle.

rotation Sharp points

To van de

graaf generator

The sharp points of the windmill cause molecules of air to ionise. The positive ions are quickly repelled from the sharp point resulting in a wind which blows out the candle. The sharp point is forced to move in the opposite direction(due to Newton’s 3^rd law)

ELECTRIC FIELD

We define the direction of the electric field at a particular place as being the direction of the force it produces on a positively charged object. An electric charge sets up an electric field in the space surrounding it and an electric force is exerted on any charged body placed in the field.

Electric fields may be represented by electric field lines. An electric field line is a line drawn in an electric field such that its direction at any point gives the direction of the electric field.

a) electric field of a point charge

Field lines

b) two point charges

c) point charge and plate

d) parallel plates

+ + + +

Ordinary air contains a number of positive and negative ions.

+ +

Discharging an electroscope through a sharp point;

1. Tape a sewing needle(steel) to the cap of an electroscopewith the sharp point protruding from the cap. Charge the electroscope so that the leaf diverges considerably. Now take a metal rod in your hand and bring it as close as possible (without touching it) to the tip of the needle.

Needle

Metal rod

Observe: You may hear some noise as a spark jumps across the gap between the needle and the rod. Then the leaf collapses showing that the electroscope has lost charge.(this shows that charge concentrates at sharp points.)

Why? The charge exerts enough force on the air molecules around to remove some electrons from them. The molecules then become positively charged and cause the electroscope to discharge.

POSITIVE AND NEGATIVE IONS IN THE ATMOSPHERE

In the atmosphere, the molecules of air are continuously moving up and down, colliding with each other. In the process, some molecules lose electrons and become positive ions while others gain electrons hence becoming negative ions.

In the presence of a sharp point whose charge density is very high, there builds a strong force around the point thus attracting the ions of opposite charge and repelling those of the same charge. The repelled ions move off at high speeds colliding with other molecules, creating new ions.

Diagram: conductor (ionised air)

Wing of a plane +ve ions repelled creating

new ions by collision and

+ + + dragging air molecules with

negative them, hence a current of air

ions attracted towards known as an electric wind

point and nuetralised. Streams away from the point.

MECHANICAL PROPERTIES OF MATERIALS

Mechanical properties refer to behaviour of materials under action of forces. Forces can bend, role or stretch, compress or even break materials.

When we say that a material is strong, we mean that a large a large force is needed to break it. It needs a bigger force to break a thick rope than is needed to break a thin string. So the diameter of a material affects its strength.

But if we have a steel cord and a sisal string, both of the same diameter, the steel cord will be harder to break than the sisal string. Steel is a stronger material than sisal.

Experiment 1:

Two threads were cut from the same reel. One thread was 1m long and the other was 2m long. Each thread is fixed at one end and the other end is pulled with a spring balance. Which thread will break more easily?

4. One class found that the force needed to break a bar of soap apart was 400N. when they experimented with a piece of chocolate, it needed 25N to tear it in half. Which is the stronger material, soap or chocolate?( the stronger material can be determined by finding the force it can withstand before breaking).

BREAKING STRESS: This is the force that would break a piece of material of cross sectional area 1m².

Breaking stress = force

Area

Some examples of breaking stresses

1. Aluminium 1 x 10⁸N/m²

2. Lead 0.1 x 10⁸N/m²

3. Steel 5 x 10⁸N/m²

4. Rope 0.5 x 10⁸N/m²

5. spider’s web 2.0 x 10⁸N/m²

STIFFNESS

Stiff materials resist being bent. Rubber is not stiff, a rubber band can easily be stretched. Wood is stiffer than rubber(though wood can also be bent). A ruler is easy to bend and trees lean over in strong wind.

If you push on a piece of string. It bends immediately since it has no stiffness.

DUCTILE MATERIALS

A stapling machine is filled with short bent lengths of wire. In order to hold many papers together, the staples are bent considerably but they don’t break. This is because metals are ductile and can be bent without breaking.

When two strips of metal have to be joined together, rivets are used. The rivet is passed through a hole in the two pieces of metal and is then hammered so as to make the end bend over. The metal doesn’t break off during the hammering but spreads out and finally holds the pieces of metal tightly together.

BRITTLE MATERIALS

Brittle materials are the opposite of ductile materials. If you try to bend a piece of chalk, it bends little as it is stiff, then suddenly without warning, it cracks.

Plasticine is ductile. If you pull it, it will stretch a lot before it breaks. But the brittle chalk breaks with a little stretching. The two parts can be fitted together again and the chalk appears just as it was before.

Glass is also brittle. If you drop a drinking glass on the floor and it breaks, the two parts can be fitted together accurately afterwards.

Questions:

1. There are two paper clips. The first A is made from metal. B is made from brittle plastic. How do you think A was manufactured?, Was B made in the same way?

2. If you want to join two things with a glass rivet, what would you have to do first before the glass rivet could be hammered into the right shape without breaking?

BEAMS

If two people pull outwards on a piece of rubber tubing, then the tubing stretches, the particles of rubber are pulled further apart from one another. The material is said to be in tension.

If two people push on a piece of wood, then the particles are pressed more closely together and the material is said to be in compression.

Questions:

1. If you bend a rubber too much, it will crack; where are the cracks most likely to start – in the region of tension or of compression?

2. Here is a lorry crossing a bridge made of plank. What parts of the bridge are in tension and what parts are in compression.

STIFFNESS OF BEAMS

1. If you have to make a beam out of three rulers, one of these arrangements will be much stiffer than the other.

2. Kapere had an I-beam which wasn’t quite stiff enough for the job that needed to be done. Fortunately there was some extra metal available and Kapere suggested welding this to the I-beam in the position shown. Why is this not the best place to add the extra metal? Where would be the best position?

3. A butcher wants to hang heavy pieces of meat from a piece of wood in his shop. Which is the best way for him to fix the wood?

a) b)

4. This picture shows, enlarged, the appearance of a wing bone of a vulture. The bone has been cut through to show the internal structure. The shaded parts are solid bone while the white parts are empty.

i) What advantage is there for the vulture in this bone being hollow?

ii) What advantage is there in the way the bone is arranged?

STRUCTURES

A structure like this one is not good at holding up a weight. It soon collapses as seen above.

Question:

i) How would you make this structure much stiffer and able to support the load?

ii) Draw two diagrams showing the two methods of doing this.

· Triangles give make strong structures but rectangles collapse easily.

STRUTS AND TIES

In a framework, some of the girders will be in tension and some in compression. The ones in tension are called ties whereas the ones in compression are called struts.

If you think that a certain girder is in tension, one way to test it is to remove the girder and to replace it with a string. If the string is pulled tight, then the girder is in tension but if it buckles, then it is in compression.

Questions :

1. Which of the girders, if any, in this structure could be replaced by a string?

A B

2. In this hanging structure a heavy rod DE is supported by a nail at A. Which of the structure’s girders are struts and which are ties?

B C

D E

3. Identify the struts and the ties.

strut

Strut F G strut

H K

BRIDGES AND BEAMS

If a solid beam was placed over a river to act as a bridge, then when a man walks over it, the top part will be in compression and the bottom part in tension.

Questions:

1. In the bridge below, girders LM and MN form the road. When a lorry drives over the bridge, which girders are in compression and which are in tension?

L N

P Q

2. The diagram opposite shows a crane raising a heavy load. Which of the girders could be replaced by steel cords and which could not?

R T

3. In the water tank below, the wind is blowing from the side. Which of the girders are struts and which are ties?

Wind

A B

C D

E F

CONCRETE

Concrete is strong in compression and a 1:2:4 misc (by volume cement : sand : gravel) might support a compressive of upto 20 x 10⁶N/m², though it would fail in tension under a stress of only 2 x 10⁶N/m².

If a slab of concrete is used for a bridge, then it is likely to crack.

Cracks

When the lorry drives over it, the underside of the bridge is put into tension and since the concrete cannot support much tension, it cracks.

Steel bars are placed on the underside of the concrete so as to be useful. The steel bars are put into the concrete while it is still wet in its wooden mould. As the concrete sets, it grips the steel tightly.

Another way of helping the reinforcing of bars to grip well is to bend the ends round as seen below.

Question:

The diagram below shows the edge of the road which is raised above the surroundings. The earth is held in place by a concrete wall to prevent it from sliding down. Where should steel reinforcing be added to strengthen the concrete wall?

NOTCHES

When a load is hang from a length of material, the forces from the load travel through the material to the support and the stress is set up.

If the material is thin, then the lines representing the forces are bunched closely together. The stress is greater than before and the material is more likely to break.

If the material has a notch in it, then the lines representing the forces will look like this below.

The forces cannot be transmitted through the crack but must go round it. The lines are not equally spaced near the tip of the crack but are very bunched together there. The stress just near the tip of the crack is very great. It may be 200 times what the stress would have been without a crack.

We say that there is stress concentration at the tip of the crack.

APPLICATION:

1) If we want to break a piece of glass tubing in the laboratory, we make a small notch in the glass and then bend the glass sharply so that the notch is on the side of the glass that is put under tension.

notch

The top side of the glass is stressed but the stress is concentrated at the place with the little notch. If the tube breaks, it will break first at the notch.

2) Get a new rubber, bend it a bit and then cut a small notch with a razor blade on the tension side of the rubber; what will happen?

Removing a Stress Concentration;

If a material has a notch in it and it’s likely to break at the tip, then drill a hole at the tip of the notch. This makes it blunter and so relieves the stress concentration. Then the material is much less likely to break.

REVISION QUESTIONS

Assume g=10ms-2 where necessary

1.a) i) State the law of electrostatics.

ii) Draw a labelled diagram of a gold leaf electroscope and explain how it can be charged positively by induction.

b) Draw diagrams to show the electric field around the following;

i) Isolated positive point charge.

ii) Positively charged point placed near a negatively charged plate.

iii) Two parallel plates with opposite charges.

2.a) i) Distinguish between a conductor and an insulator.

ii) Describe an experiment to show that two equal and opposite charges are produced by friction.

b) You are given a charged body in shape of a cone. Explain, with aid of a diagram, how you would investigate the distribution of charge outside the body.

c) Sketch the electric field between two positive point charges of unequal magnitude.

3.a) State the law of electrostatics.

b) A highly positively charged sharp rod is placed near a candle

Rod candle

State and explain what will be observed.

4.a) Draw a labelled diagram of a gold-leaf electroscope.

b) Draw the electric field pattern for;

i) the positively charged bodies a small distance apart.

ii) an isolated negative charge.

iii) two parallel plates with opposite charges at a small distance apart.

5.a) You are provided with two metal spheres. Describe a method which can be used to charge them positively and simultaneously with a charge of equal magnitude.

b) Given a metallic box with a highly charged conductor in it, describe how the electric field inside it can prevented from affecting the surrounding.

c) Draw the electric field of two positive charges of equal magnitude placed 4cm apart.

When a relatively charged electroscope is earthed, the leaf is seen to fall. Why does this occur?

6.a) What is an insulator?

b) Describe how two identical metal balls maybe charged positively and simultaneously by induction.

c) Draw a labelled diagram of a gold leaf electroscope.

d) i) Explain what happens when a negatively charged rod is brought near the cap of an uncharged electroscope and slowly taken away.

ii) How can an electroscope be used to test whether a material is a conductor or an insulator?

7.a) State Coulomb’s law of electrostatics.

b) Two metal spheres on insulating stands are provided. Describe an experiment which can be carried out to charge them negatively with a charge of equal magnitude.

c) You are provided with a rod which you suspect to be charged. Explain how you would investigate the sign of charge.

d) explain how a lightning conductor safeguards a building against lightning.

e) Why is the earth said to be at zero potential?

8.a) Draw a labelled diagram of a gold leaf electroscope.

b) Describe an experiment to test the sign of charge on a charged body using a gold-leaf electroscope.

c) Draw the electric field pattern for two positively charged bodies a small distance apart.

9. The diagram below shows a Van de Graaf generator.

E₂

E₁

a) Name parts labelled B and C.

b) Give the use of parts A and B.

c) Briefly explain the action of E₁ and E₂.

10.a) Sketch an electric field pattern for;

i) an isolated point charge.

ii) two opposite point charges near each other.

iii) a negatively charged hollow cylindrical conductor.

b) Explain;

i) how lightning is caused.

ii) How a lightning conductor safeguards a building against lightning.

c) What would happen if a beam of electrons were directed between;

i) two oppositely charged parallel plates?

ii) opposite magnetic poles?

11. A gold-leaf electroscope is to be charged by induction using a negatively charged rod.

a) With the help of diagrams, describe how this is done. Explain what happens at each stage of the process.

b) What will be observed if a positively charged rod is then placed near the cap of the charged electroscope without making contact? Explain what happens in what you observe.

12. By use of simple diagrams, show the electric fields between;

a) a positively charged point and a negatively charged plate.

b) two oppositely charged points.

13.a) Describe how you would charge a gold-leaf electroscope positively by induction stating any observations at each stage and the movement of charge.

b) When a strong negatively charged rod is brought near the cap of a positively charged electroscope, the leaf is seen to fall and rise again. By use of diagrams, explain what takes place.

14.a) i) Distinguish between an “insulator” and a “conductor”.

ii) Which of the following substances are conductors and which ones are insulators:- glass, water, ebonite, copper and the human body?

b) A glass rod rubbed against woolen cloth is said to be positively charged and the woolen cloth negatively charged. Explain how they acquire these charges.

In the diagram above, a positively charged glass-rod is brought near to a pear-shaped conductor;

i) Explain how the pear-shaped conductor may be charged using the positively charged glass rod and state the type of charge it will acquire.

ii) Show the distribution of charge on this conductor.

iii) The pear-shaped conductor loses charge faster when a needle is attached at point P. Explain how this happens.

15.a) What is an insulator?

b) Describe how two identical metal balls may be charged positively and simultaneously by induction.

c) Draw a labelled diagram of a gold-leaf electroscope.

d) i) Explain what happens when a negatively charged rod is brought near the cap of an uncharged electroscope and slowly taken away.

ii) How can an electroscope be used to test whether a material is a conductor or an insulator?