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# PHYSICS FORM 2 TOPIC 1: STATIC ELECTRICITY

## Concept of Static Electricity

Static electricity refers to the accumulation of electric charges on the surface of an object. These charges are in a stationary state (at rest). The study of electric chargers at rest is known as electrostatics. It studies about the charges at stationary state. These charges can be moved away from the object y means of electric current when connected to the circuit.

### The Orign of Charges

The knowledge of static electricity came as early as the 6th century BC (600 BC). It was first discovered by the Greek ancients who rubbed amber and observed that it was able to carry attract dusts and leaves. During those times they believed that there are charges which make two things repel or attract each other.

Therefore, people knew about charges as result of rubbing process between two objects or materials.

When a piece of amber, plastic, polythene, or hard rubber is rubbed with fur, electrons are transferred from fur to the other material. Fur acquires net positive charge, since it has fewer electrons than protons. Similarly, the amber, plastic, or hard rubber acquires a net negative charge since they have excess electrons.

Combing hair charges the plastic comb by transferring charges from hair onto it. If placed near small pieces of paper, the plastic comb will attract the pieces of paper. This is because the comb has been charge with negative charges.

Rubbing glass (perspex) with silk causes the glass to acquire a net positive charge.

#### The two Types of Charges

There are two types of charges; –

1. Positive charges – they are protons in nature and originate from the nucleus of an atom. A positive charge is represented by the ‘+’ sign. It has charge of +1.6 x 10-19 C.
2. Negative charges – they are electrons in nature and they move around the nucleus of an atom in shells. Electrons are lighter compared to protons. Electron charge is represented by the ‘-‘sign. It has a charge of -1.6 x 10-19.

### Identification of charge

Suspend a polythene rod A rubbed with fur. Bring another polythene rod B rubbed with fur up to the rod A. Take a plastic rod and rub it with fur. Bring the plastic rod to up to the suspended rod A. Repeat the exercise with acetate and glass rod rubbed with silk cloth.

Observation

An electrified polythene rod repels another electrified polythene rod. An acetate rod rubbed with silk repels another acetate rod rubbed with silk cloth but it attracts a plastic rod rubbed with fur.

Explanations

Polythene and plastic when rubbed with fur becomes electrified with the same kind of electricity known as negative electricity (charge).

Acetate and glass when rubbed with silk cloth becomes electrified with the same kind of electricity called positive electricity(charge).

Charging is the process of electrifying a body.

A positively charged body carries positive charges and a negatively charged body carries negative charges.The symbols used for positive and negative charges are + and – respectively.

The Fundamental Law of Static Electricity

The Fundamental law of electrostatic charges states that:“Like charges repel each other while unlike charges attract each other”

That means, a positive charge will attract a negative charge but two positive charges or two negative charges cannot attract rather repel.

### Charging Bodies Using Different Methods

In order to understand the process of charging we have to understand the structure of bodies or things. All bodies are made up of extremely small, indestructible bits of matter called atoms.

An atom consists of a nucleus surrounded by electrons. The nucleus consists of proton and neutron.The protons are positively charged while electrons are negatively charged and the neutrons are neutral.

The whole atom is electrically neutral because it contain equal number of protons and electrons.

The following are the methods of charging;

1. Rubbing
2. Induction
3. Contact

Charging by rubbing

A polythene rod rubbed with fur becomes negatively charged.Rubbing results in the transfer of electrons from fur to the polythene rod.

Fur becomes positively charged because some of its electrons are transferred to the polythene rod.The polythene gains excess electrons and hence it becomes negatively charged.

Note:It is only the electrons in matter which can be transferred by rubbing.

Charging by induction

A charged polythene rod is held near uncharged copper rod suspended from a cotton thread.

The electrons of the copper rod are repelled by the negatively charged polythene rod.Hence the electrons move to the far side of the copper leaving behind a net positive charge on the side facing the polythene rod.

Touch the copper rod with your finger when the charged rod is still in position. The electrons from copper rod flow through your body to the earth. Leaving it with a net positive charge. Remove the finger from the copper rod and finally remove the charged polythene rod.

The rod has therefore been positively charged by electrostatic induction.The charges that appear on the copper rod are called induced charges.

Charging by contact

A charged body (e. g; positively charged metal can) is brought in contact with uncharged body B. When in contact, negative charges from the uncharged body are attracted by the positive charges of the metal can. Finally, the body B will remain with a net positive charge. Therefore, body B has been charged with positive charges.

Detection of Charges

The Structure of a Gold-leaf Electroscope

The instrument used to detect the presence of electric charges is called gold leaf electroscope. It consists of an insulated brass rod with two pieces of thin gold foil at one end and a brass cap at the other end.

When the brass cap is touched with a charged object the leaves of the electroscope spread out. This is because the charge on the object is conducted through the brass cap and the brass rod to the leaves.

As they received the same kind of charge, the leaves repel each other and thus spread apart, this is charging by contact.

If you touch the brass cap with your finger, the charge is transferred through your body to the earth and the leaves of the electroscope then collapse together.

Function of an electroscope

1. Testing for the sign of the charge on the body.
2. Identifying the insulating properties of materials.
3. Detecting the presence of charge on a body.

The Sign of Charges

The true sign on a body has to be determined before use; the instrument that can be used to determine the presence of charge is called an electrophorus.

An electrophorus consists of a circular slab of insulating material (polythene) together with a brass disc (conductor) on an insulating handle.

An electrophorus works by electrostatic insulation and hence can be used to generate positive charges from single negative charges. The charge produced on the insulating slab is negative. The top disc is then placed on it. Since the surface is only in contact at relatively few points, a positive charge is induced on the lower surface and corresponding negative charge is produced on its top surface.

The top of the upper disc is then touched briefly using a finger, hereby carrying away the negative charge to the earth; this is called EARTHING.

Steps of Charging and Discharging of a Gold-leaf Electroscope

The polythene slab is charged negative by rubbing it with fur. The brass disc is then placed on top of the slab so that the two charges become induced onto respective materials.

Note:Contact does not negatively charge the disc because it is not flat and makes contact with the slab at a few points only. When the brass disc is touched with a finger, electrons on the upper surface are repelled to the earth.

There is a force of attraction between the metal disc and the base. A spark (electric energy) is normally produced upon their separation. This spark can be used for lighting gas burners in laboratory.

The electrophorus can now be used to charge a gold leaf electroscope.

It can be used to charge a gold leaf electroscope by:

1. Contact
2. Induction

By contact

Here a positively charged electrophorus is made to touch the brass cap of the gold-leaf electroscope. The leaf of the gold-leaf electroscope diverges.

When a charged electrophorus is brought into contact with the electroscope, the latter gets charged and the leaves diverge. It acquires a negative charge. This is determined using the charged rods. When a positively charged glass rod is brought near the cap. It causes the leaf to collapse.

By induction

Induction- is the transfer of opposite effects from one body to another without contact.

In order to obtain a charge of a given sign, the inducing charge must be of an opposite charge. If charge is placed on an insulator at a given location the excess charge will remain at the initial location. The particles of the insulator do not permit the free flow of electrons. Charge present in an insulator or conductor.

Discharging a gold leaf electroscope

Having charged a gold leaf electroscope by contact and induction, the same can be discharged effectively through induction.

If while the electroscope is being charged by induction you touch the brass cap, electrons will leave the electroscope through your hand and onto the ground. If the charged metal rod is removed, the electroscope will remain charged. The charge remaining on the electroscope will be the opposite of the charge on the rod.

If a negatively charged object is now brought near the brass cap electrons in the brass cap are repelled and moved down to the leaves. This cancels the positive charge. With no net charge, the leave collapse back together.

If the object is removed, the electrons return to the metal cap leaving the leaves of the electroscope with a net positive charge again and they separate.

Conductors and Insulators

Difference between a Conductor and Insulator

Conductors

Are bodies, which readily allow electric charge in motion to flow through them

OR

Are materials that permit some electrons to flow freely from atom to atom within the materials examples are copper, steel, iron, silver and gold.

When there is excess of positive or negative charge on an object made of a conducting material, the conduction electrons will move to minimise the repulsive force.

Insulators

These are bodies, which do not allow electric charges to flow through it. Insulators on the other hand do not allow their electrons to flow freely from at atom to atom; this is because the electrons in their atoms move around their nuclei in various equal magnitudes to the charge on the protons. The electrons are also firmly attracted to the nucleus hence bound to these atoms.

Capacitors

Capacitor is a device which is used for the storage of charges consisting of two conductors, parallel-nearly separated by air or any other dielectric.Dielectric is an insulating medium used between plates of a capacitor.

Mode of Action of a Capacitance

The capacitor store energy by keeping electrical charges on its plates. Capacitors are used in radio circuits, television circuits and other electronic devices.

When the power switches off, the energy stored in the plates of the capacitor will be released to flow in the circuit for sometimes. This will keep the device functioning until all the energy is worn out.

That means, when the electric power is available, the capacitor is charged and store electric energy on its plates but when the power in the circuit switches OFF, the capacitor continues to supply the electrical power in the circuit. This process through which the capacitor releases its charges to the circuit is known as discharging.

The Action of a Capacitor

A fully charged capacitor has a net positive charge on one of its plates and a net negative charge on the other plate. The potential difference between its plates can be measured by connecting the voltmeter across its plates.

The ability of a capacitor to store charges is known as the capacitance. Capacitance is the ratio between the quantity of charge stored and the potential difference (p.d) across the plates of the capacitor.

That means, the quantity of charges Q increases with the increase in the potential difference (p.d) across the plates.

A farad is the capacitance of a conductor that its potential difference can be changed by 1 volt by a charge of 1 coulomb.

However, 1 Farad capacitance is very large to be reached thus most of the times the smaller units are used to simplify measurements.

Example 1

A 3µF capacitor has a 18V of potential difference. What will be its total charge?

Example 2

Calculate the capacitance of the capacitor if the cell connected to it has 1.5V when the charge is 120 coulombs.

Construction of an Air-filled Capacitor

This constitute two parallel metal plates with air band between them.A flat metal A is set up vertically on insulating legs and is connected to a gold leaf electroscope by means of a wire.

The plate is then given a positive charge by induction with a negatively charged ebonite rod. The divergence of the leaf indicates the potential of the plate.A second insulated plate B is now brought up slowly into a position parallel to A.

When B is very close to A but not touching it, it will be noticed that the leaf divergence decreases very slightly.We conclude from this that the potential of A has been decreased by the presence of B, and hence its capacitance has increased slightly.

Equivalence Capacitance of a Combination of Capacitors

Factors affecting the capacitance of a parallel-plate capacitor.

There are three factors which affect the capacitance of a parallel-plate capacitor, namely;

1. Area of plates
2. Distance apart of the plates.
3. Dielectric between the plates.

Relative permeability (dielectric constant) of a medium

Relative permeability is the ratio of the capacitance of a given capacitor with the medium as dielectric to the capacitance of the capacitor with a vacuum as the dielectric.

It has no units since it is a ration of similar quantities.Paraffin wax has a relative permeability of about 2 while that of mica is about 8.

Combination of capacitors

Capacitors can be combined in series or in parallel so as to prevent overheating by being continuously overcharged.

Capacitors in Series.

When capacitors are in series, charge distribution Q is equal to all capacitors but p.d, V and capacitance are different.

Therefore, the total p.d, VT

Where, CT is the equivalent capacitance (combined capacitance) for the capacitors in series.

Capacitors in Parallel.

When capacitors are in parallel, potential difference V is equal to all capacitors but charge distribution, Q and capacitance are different.

Therefore, the total charge, QT

Where by, CT is the equivalent capacitance (combined capacitance) for capacitors in parallel.

Charge Distribution Along the Surface of a Conductor

Charge on a Conductor Reside on its Outer Surface

Usually, charges are distributed on the outer surface of conductors of different shapes.

Investigating surface distribution of a charge on conductors

• A proof plane is pressed into contact with the surface at various places of the conductor.
• The charges on the proof plane are then transferred to the electroscope.
• The divergence of the leaf will give a rough measure of the amount of charge transferred and hence surface density of the charge.

Charge on a Conductor is Concentrated on Sharply Curved Surfaces

So far we have considered excess charges on a smooth, symmetrical conductor surface. What happens if a conductor has sharp corners or is pointed? Excess charges on a nonuniform conductor become concentrated at the sharpest points. Additionally, excess charge may move on or off the conductor at the sharpest points.

To see how and why this happens, consider the charged conductor. The electrostatic repulsion of like charges is most effective in moving them apart on the flattest surface, and so they become least concentrated there. This is because the forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surfaces are different. The component parallel to the surface is greatest on the flattest surface and, hence, more effective in moving the charge.

The same effect is produced on a conductor by an externally applied electric field, as seen inFigure(c). Since the field lines must be perpendicular to the surface, more of them are concentrated on the most curved parts.

Excess charge on a nonuniform conductor becomes most concentrated at the location of greatest curvature. (a) The forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surface are different. It isF∥that moves the charges apart once they have reached the surface. (b)F∥is smallest at the more pointed end, the charges are left closer together, producing the electric field shown. (c) An uncharged conductor in an originally uniform electric field is polarized, with the most concentrated charge at its most pointed end.

Lightning Conductor

The Phenomenon of Lightning Conductor

Lightning is a gigantic electric spark discharge occurring between two charged clouds or between a cloud and the earth.

Ligthning conductor is a long pointed iron rod with its lower end buried in the earth and the other above the highest part of the building which is used to protect the building from lightning damage.

Test Yourself

The Structure and Mode of Action of Lightning Conductor

Structure of a lightning conductor

It consists of a long thick pointed copper rod with its lower end buried in the earth(earth plate) and the other end reaching above the highest part of the building and ending in several sharp spikes. -It is fixed to the side of the building.

Mode of action of lightning conductor

When a negatively charged thunder-cloudpasses overhead it acts inductively on the conductor,charging the points positively and the earth plate negatively.

The negative charge on the plate is, of course, immediately dissipated into the surrounding earth. At the same time point action occurs at the spikes. Negative ions are attracted to the spikes and becomes discharged by giving up their electrons. These electrons then pass down the conductor and escape to earth.

At the same time positive ions are repelled upwards from the spikes and spread out to form what is called a space charge. This positive space charge, however, has a negligible effect in neutralizing the negative charge on the cloud.

Note:Without the protection of a lightning conductor the lightning usually strikes the highest point, generally a chimney, and the current passes to earth through the path of least resistance. Considerable heat is generated by the passage of the current and sometimes it may set into fire.

A Simple Lighting Conductor

A simple lightning conductor

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