PHY4: STATIC ELECTRICITY

This unit is about static electricity and goes on to explain insulators and conductors of electricity.

STATIC ELECTRICITY

Static electricity is the study of electric charges at rest.

Some effects of static electricity

  1. A pen made of plastic material rubbed in dry hair can attract dust particles or small pieces of paper.
  2. A nylon garment often crackles when it is taken off. The crackles are caused by tiny electric sparks that can be seen in the dark.
  3. A flash of lightning is nature’s most spectacular static electricity effect.

POSITIVE AND NEGATIVE CHARGES

There are two kinds of charges the Positive and negative charges;

From experiments, ebonite or polythene rubbed with fur leaves the ebonite or polythene negatively charged.

Glass or cellulose acetate rubbed with silk leaves the glass or acetate positively charged.

When a strip of polythene is rubbed with fur it becomes charged. If it is suspended using a string and another polythene rubbed with fur brought near it, repulsion occurs.

LAW OF ELECTROSTATICS

  1. Like charges repel, unlike charges attract.
  2. The force between electric charges decreases as their separation increases.
  3. The force between two charges depends on the product of the magnitude of the charges and on their distance of separation.

To explain electrification by friction (rubbing)

An atom is made up of a small central nucleus which contains neutrons and protons. Neutrons are uncharged particles while protons are positively charged.

The nucleus is surrounded by electrons equal to protons in number, existing in orbits. In this state an atom is said to be electrically neutral.

When rubbing takes place, for example, cellulose acetate rubbed with silk electrons go from cellulose acetate to silk leaving acetate short of electrons and silk having an excess of electrons.

Acetate becomes positively charged while silk becomes negatively charged.

NOTE: It is the electrons which move the protons remain in the nucleus.

THIS VIDEO IS ABOUT SOME OF THE EXPERIMENTS OF STATIC ELECTRICITY

INSULATORS AND CONDUCTORS

An insulator is a substance which becomes electrified by rubbing e.g. glass, amber and sulphur.

In an insulator all electrons are bound firmly to their atoms.

A conductor is a substance which cannot be electrified by rubbing e.g. metals

In a conductor electrons are able to move freely from one atom to atom.

For successful results all apparatus used in electrical experiments must be thoroughly dry because water is a conductor.

METHODS OF CHARGING A BODY

A body can either be charged by contact or by induction.

When a body is charged by contact it acquires the same charge as the charging body.

When a body is charged by induction it acquires charge opposite to the one on the charging body.

The two bodies are placed near each other but not touching.

The gold leaf electroscope

It is an instrument used for detecting and testing of small electric charges.

STRUCTURE OF A GOLD LEAF ELECTROSCOPE

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It consists of a brass cap supported by a brass rod at the foot of which is a brass plate having a leaf of gold foil attached. The rod is held by an insulating plastic plug in a case with glass sides.

USES OF A GOLD LEAF ELECTROSCOPE

  1. To detect the presence of charge on a body

If a rod e.g. glass rod is charged by friction and then brought near the cap of a gold leaf electroscope, the leaf is seen to diverge from the plate. A charge has been induced on the leaf and plate and hence repulsion occurs between them. When the charged rod is withdrawn the leaf falls indicating that the charge on the electroscope is only temporary.

  1. To test for the sign of charge on a body

We use a charged gold leaf electroscope (g.l.e) to test for the sign of charge on a body. Suppose the electroscope is negatively charged and an ebonite rod (previously rubbed with fur) is then brought near the cap of the g.l.e. an increase in the leaf divergence is noted.

A glass rod rubbed with silk is then brought down towards the cap. A decrease in divergence is noted.

An increase in divergence occurs when the charge on the electroscope and the test charge are of the same kind.

  1. To test the insulating properties of various materials

Hold a sample of the substance in the hand and then bring it into contact with the cap of a charged gold leaf electroscope.

If the substance is a good insulator there will be no leakage of charge through it and the leaf divergence does not change.

If the leaf falls instantly, it shows that the substance is a good conductor.

Between the two extremes there are certain substances which produce slow collapse of the leaf.

These are called poor insulators or poor conductor’s .e.g. Paper, wool, cotton, the human body and wood. However when these substances are very dry they become good insulators.

Electrostatic induction

Two insulated brass spheres A and B are placed together so that they touch one another and thus form a single conductor.

The spheres are supported on insulating material to prevent conduction of charge away from them to the earth.

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A positively charged rod is brought near to A.

As a result a negative charge is induced on A and a positive charge on B.

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With the charged rod still in position sphere B is moved a short distance from A.

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The charged rod is now removed and A and B are tasted for charge.

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To charge a single body by induction

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A negatively charged rod is brought near to the end A of a conductor AB.

Electrons in the conductor are repelled towards B by the electrons on the rod.

Positive charges are attracted to end A.

Earth the conductor by touching it with the finger.

Electrons are repelled to the earth. The finger is then removed from the conductor and the charged rod is withdrawn. It is found that the conductor is charged positively.

Attraction of uncharged objects

The attraction of an uncharged object by a charged object is due to electrostatic induction.

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A small piece of aluminium foil is attracted to a negatively charged rod held just above it.

The charge on the rod pushes free electrons to the bottom of the foil, leaving the top of the foil with a net positive charge and the bottom negatively charged.

The top of the foil is nearer the rod than the bottom. Hence the force of attraction between the negative charge on the rod and the positive charge on the top of the foil is greater than the force of repulsion between the negative charge on the rod and the foil.

The foil is therefore pulled by the rod.

A slow stream of water can be bent by electrostatic attraction to a charged polythene rod.

THIS VIDEO IS ABOUT THE DIFFERENCE BETWEEN CONDUCTORS AND INSULATORS

THE PROOF PLANE

A proof plane is a metal disc attached to an insulating handle. It is used to test for charge on a conductor by transferring charge onto a g.l.e.

It is placed in contact with the surface of the conductor so that it becomes charged and the charge is then transferred to the electroscope by touching the brass cap with the proof plane. When the leaf diverges this indicates that the proof plane was charged (by the conductor).

Transferring electric charge If a positively charged rod is placed near a conductor AB standing on an insulator, negative charge will be induced at end A and positive charge at end B.

When a proof plane is placed at end B it acquires positive charge by conduction. When the proof plane is transferred to the cap of a gold leaf electroscope, it diverges. The sign of charge on the proof plane is tested by bringing a positively charged rod towards the cap of the electroscope.
An increase in divergence will show that the charge taken from B is positive.

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The electroscope and the proof plane are both discharged, and the experiment repeated using charge taken from end A of the conductor. This time an increase in divergence is obtained when a negatively charged ebonite rod is brought near the cap, showing that charge taken from A is negative.

Charging a gold — leaf electroscope by induction

A rod is used, charged with electricity opposite to that required on the electroscope.

The rod is brought near the cap so that the leaf diverges and the electroscope is earthed by touching with the finger. The rod is then removed leaving the electroscope charged.

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Distribution of charge over the surface of a conductor

A proof plane and gold leaf electroscope are used to investigate charge distribution on the surface of a conductor.

The proof plane is placed in contact with the surface at various places in turn and then transferred to the electroscope.

The divergence of the leaf gives a measure of the amount of charge transferred.

Surface density is defined as the quantity of charge per unit area of surface of a conductor.

A set of proof planes are used in these investigations having the same area but different shapes designed to fit the curvature if the surface at a specific place on the conductor.

Below are some of the shapes where surface density may be investigated.

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It is noticed that charge is mostly concentrated at places where the surface is sharply curved. This is particularly noticeable at the pointed end of a pear shaped conductor.

ACTION OF HIGHLY CHARGED POINTS

Ordinary air contains a certain number of positive and negative ions.

In the neighborhood of a highly charged point conductor, ions of the same sign will be strongly repelled.

When these fast moving ions collide with air molecules they knock electrons off them thus creating more ions.

If the conductor is positively charged it attracts negative ions and repels positive ions.
The conductor is therefore neutralized.

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THE LIGHTENING CONDUCTOR

A tall building is protected from lightening by a lightening conductor consisting of a thick copper strip on the outside of the building connecting metal spikes at the top to a metal plate in the ground.

When a negatively charged thunder-cloud passes overhead it induces positive charge on the spikes and negative charge on the earthed plate.

The negative charge on the plate is dissipated to the surrounding earth.

Point action occurs at the spikes. Negative ions are attracted to the spikes, become discharged by giving up their electrons.

These electrons pass down the conductor and escape to the earth. Positive ions are repelled upwards from the spikes and form a space charge.

This positive space charge however has negligible effect on neutralizing the negatively charged cloud.

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USES OF STATIC ELECTRICITY

  1. Flue-ash precipitation. An electrostatic precipitator removes dust and ash that goes up the chimneys of coal burning power stations.
  2. Photocopiers.
  3. Lightning conductors.
  4. Electrostatic paint spraying of cars.

ELECTRIC FIELDS

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 are represented by electric field lines.

An electric field line is a line drawn in an electric field such gives the direction of the electric field at that point.

The direction of the electric field is directly  at any point is the direction of the force on a small positive charges placed at the point.

ELECTRIC FIELD PATTERNS

Two positive charges

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Two negative charges

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Faraday’s ice pail experiment

A positively charged brass sphere supported by an insulating cotton thread is lowered inside a hollow can standing on the cap of a gold leaf electroscope.

Negative charges are included on the inside of the can and a positive charge is induced on the outside of it and also on the gold leaf. The leaf diverges.

No change in divergence of the leaf is noticed when the sphere is moved inside the can with out touching the sides of the can.

When the sphere is allowed to touch the bottom of the can still no change occurs in the leaf divergence.

On removing the sphere and testing it for charge, it is found to have been completely discharged, and there’s no longer any charge on the inside of the can.

It is therefore concluded that the inducing charge on the sphere has exactly neutralized the opposite negative charge on the inside of the can and hence the two are equal in magnitude.

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ASSIGNMENT : STATIC ELECTRICITY assignment MARKS : 10  DURATION : 1 week, 3 days

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