PHYSICS FORM TWO NOTES -TOPIC 0NE -STATIC ELECTRICITY

PHYSICS FORM TWO

1.0         STATIC ELECTRICITY

1.1            Concept of static electricity

Ø  Rub a plastic comb on your sleeve and hold it near some pieces of paper. What happens?

Answer:

v  When you rub the plastic comb on your sleeve, it becomes electrically charged with static electricity. The plastic comb able to pick up pieces of paper. So pieces of paper are said to be attracted by the charged comb.

Ø  A plastic pen rubbed on the hair and holds it near some pieces of paper. What happens?

Answer:

v  Plastic pen rubbed on the hair attract small pieces of paper

Ø  A glass rod rubbed on the hair and holds it near some pieces of paper. What happens?

Answer:

v  A glass rod rubbed on the hair attract small pieces of paper

Ø  An ebonite rod rubbed with fur and holds it near to aluminium foil. What happens?

Answer:

v  An ebonite rod rubbed with fur attracts aluminium foil.

Definition

Static electricity –is the accumulation of excess electric charge in the region which does not conduct electricity such that the charge accumulation continues.

Electrostatics –is the study of stationary electric charges

Types of charges

Ø  There are two types of charges

1.      Positive charges

2.      Negative charges

The origin of charges

Ø  The charges originated from the atoms, the atom of any matter has a nucleus.

1.      Positive charges

Characteristics of positive charge

        i.            It is protons inside the nucleus.

      ii.            It is found at the centre part of nucleus.

    iii.            The sign of charge of proton is

2.      Negative charges

Characteristics of negative charge

        i.            It is electrons

      ii.            It moves around the nucleus in shells

    iii.            Are not mobile (in free to move) in materials such as wood and plastic.

     iv.            Attracted to positively charged nucleus

       v.            They are free to move in metals produces charge transfer results into electric current

The SI – unit of electrical charge is coulomb (C)

        Now, C

                   Charge on the proton

                   Charge on the electron

Note: C

Ø  The table below show the acquisition of charges by different materials

Material	Rubbed with	Charge acquired by the material
Ebonite rod	Fur / cloth	Negative charge
Glass rod	Silk	Positive charge
Polythene	Fur / cloth	Negative charge
Polystyrene	Fur / cloth	Negative charge
Perspex	Woollen cloth	Positive charge
Cellulose	Woollen cloth	Positive charge

Fundamental law of static electricity

Ø  Fundamental law of static electricity sometimes referred to as thefirst law of electrostatics Or law of charges.;

State that” Like charges repel, unlike charges attract.”

Methods of charging a body

        i.            Friction method

      ii.            Contact method

    iii.            Induction method

        i.            Charging by friction –is the electrification by rubbing.

Ø  When the ebonite rod is rubbed with fur, some electrons transfer from fur to the ebonite. Hence ebonite rod becomes negatively charged due to excess of concentration of electrons.

      ii.            Charging by contact –is the process of charging body when charged plate brought together with the uncharged plate, so electrons immediately transfer to uncharged one until equal distribution of charge.

    iii.            Charging by induction is the transfer of opposite effects from one body to another without contact.

1.2            Detection of charges

The gold-leaf electroscope

A gold-leaf electroscope –is an instrument used to identify the presence of electric charge on an object.

Determining the sign of charge

An electrophorus –is the instrument used to determine the presence of charge

Ø  The sign of charge on a body can be determined by the electrophorus

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

v  An electrophorus works by electrostatic induction and hence generate positive charges from a single negative charge

Charging an electrophorus using a charged polythene base

Ø  The polythene base (insulating slab) is rubbed with fur cause an electrostatic charge called negative charge

Ø  The top disc (brass disc) placed on it. The positive charge induced on lower surface and also negative charge are produced on top surface

Ø  When the top surface touched with finger, negative charge repel toward the ground (earth) leaves net positive charge on top of disc after separation

Ø  Note that the disc does not get the negatively charged by contact because it is not flat and makes contact with a slab at a few point.

Ø  The spark  (electric energy) is produced upon the separation

Ø  The spark can be used for lighting gas burners in the laboratory

Ø  The electrophorus used to charge a gold-leaf electroscope

Charging a gold-leaf electroscope using a positively charged electrophorus

Ø  A positively charged electrophorus charge gold-leaf electroscope by

                                            i.            Contact

                                          ii.            Induction

                                            i.            By contact –the positively charged electrophorus made to touch with brass cap of gold-leaf electroscope, hence the leaf of the gold-leaf electroscope diverges. It acquires negative charge

·         Testing for charge by using glass rod when brought near brass cap the leaf collapse

·         Testing for charge by using ebonite rod when brought near the brass cap the leaf stay diverged

                                          ii.            By induction - is the transfer of opposite effects from one body to another without contact.

Ø  When charging a gold-leaf electroscope by induction touches the brass cap, electrons will leave the electroscope through the hand and into the group. If the charged metal rod is removed, the electroscope will remain charged.

Ø  The charge will be opposite to the charge of metal rod

Discharging a gold-leaf electroscope

v  Can be discharged through induction.

Ø  If negatively charged body is brought near the brass cap, electrons in the brass cap are repelled and move down to the leaves. This cancels the positive charge. With no net charge, the leaves collapse back together.

Ø  If the object is removed, the electrons return to the metal cap and the leaves separate.

Functions of an electroscope

        i.            Testing for the design of the charge on a body

      ii.            Identification of insulating properties of materials

    iii.            Detecting the presence of charge on a body

1.3            Conductors and Insulators

Conductors –are materials that allow electrons to flow freely from atom to atom within materials. Examples copper, steel, iron, silver and gold are good conductors

Insulators –are materials that do not allow electrons to flow freely from atom to atom. Examples rubber, plastic and glass

1.4            Capacitors

Capacitors –are devices that store charge

Ø  The devices that use capacitors

·         Computers

·         Televisions

·         Radio or stereo, and others electronic devices

Potential difference –is the work done to move charge from one point to another.

            Hence:  

                                                                         

Electrons flow from points of low potential to the points of high potential

Mode of action of capacitors

Ø  The capacitors store energy by keeping electrical charges on their plates.

Ø  When power is switched off in all electrical devices their lamp fades out slowly and other like stereo has its sound playing for some seconds after the power is switched off.

Capacitance –is the ratio between the charge on a conductor (capacitor) and the potential difference (Pd) across the plates of the conductor

                     

Also,       

The SI-unit of capacitance, C is farads, F

Other SI-unit of capacitance is

·         Microfarads

·         Pico farads

·         Nano farads

Their equivalent is given by

F

F

F

A farad is the capacitance of a conductor that a charge of 1 coulomb is able to change its potential difference by 1volt.

Types of capacitors

1.      Paper or plastic capacitors –are capacitors have metal foil strips as their conductors. Waxed paper or plastic forms are insulating materials

2.      Mica capacitors –are capacitors that insulating material between the plates is strips of mica

3.      Electrolytic capacitors –are capacitors between plates separated by paper soaked in a chemical.

4.      Variable capacitors –are capacitors which insulating materials is air.

Charging a capacitor

Ø  Consider the circuit below:

·         An uncharged capacitor has no potential difference.

·         Due to the presence of insulation between the plates of the capacitor, the electrons tend to accumulate to the negative side of plate

·         This occur due to attraction of positive side of the cell or repulsion of negative side of the cell

·         Current stop to flow until the potential difference between the plates is equal to the voltage of cell, hence capacitor are said to be charged.

Discharging a capacitor

Ø  The charge in each plate equal in size but opposite signs.

Ø  Due to the leakages between the plates, the capacitor discharged

Ø  Also capacitor discharged by connecting its plates together via a resistor.

              Note:             

Construction of an air-filled capacitor

Dielectric material is an insulating medium used between the plates of a capacitor

Combination of capacitors

Ø  There two combination of capacitors

                                i.            Capacitors in series

                              ii.            Capacitors in parallel

        i.            Capacitor in series

Ø  Characteristics of capacitor in series

a)     The potential differences varies in all capacitors through the circuit

b)    It has equal distribution of charge on the plates

Recall,   

From,

       

 

, Charge (Q) cancels out since are the same

If capacitors are connected in series more than two capacitors then:

      ii.            Capacitors in parallel

Ø  Have the same potential difference across the capacitor

Ø  The charge are different across the capacitors

Recall,     

From,

 , Potential difference (V) cancels out since are the same

If capacitors are connected in parallel more than two capacitors then:

Factors affect capacitance

        i.            The area of the plates –the increase of area of the plate cause decrease in potential difference between the plates, hence  an increase in capacitance

      ii.            The dielectric –the replace of dielectric materials between the plate reduce potential difference between the plates, hence an increase in capacitance.

    iii.            The distance between the plates –the closer plates of capacitor the decrease of potential difference between the plates, hence an increase in capacitance.

Capacitance,

   Or

1.5            Charge distribution along the surface of a conductor

Charge distribution on a conductor

Ø  Charge distribution on the surface of the conductor

                                i.            Spherical conductor

                              ii.            Pear-shaped conductor

                                i.            Spherical conductor

Spherical conductor has equal distribution of charges so leaf divergence is the same throughout

                              ii.            Pear-shaped conductor

Pear-shaped conductor has the distribution of charge concentration at the point at the sharpest part of a conductor

 The leaf deflection is maximum at the sharpest part of a conductor

Note: This means that charge distribution on a conductor depends on the shape of the conductor surface.

1.6            Lightning conductor

Lightning

Lightning –is a huge discharge of static electric charges between two clouds or a cloud and the ground.

Radar –it occur when the wind passes across clouds, a few electrons from the negative cloud move across the air towards the positive cloud but in a zigzag path.

Thunderstorm

Thunder –is an audible sound often heard after lightning.

Ø  It is caused by the rapid expansion of air that has been heated by lightning

Lightning conductor

A lightning conductor –is a metal rod attached to a building and connected to a thick copper strip that leads into the ground.

Ø  Its tip has sharp spikes.

Ø  Help to protect the building and other structure from lightning strikes

Mode of action of lightning conductor

Ø  A negatively charged cloud passing overhead causes the sharp spikes of the conductor to become positively charged.

Ø  The intense field around the spikes causes air to be ionised. Electrons on the cloud are then attracted to the spike and are safely conducted to the ground while the positive ions are repelled to the cloud. Since the negative charge is earthed, the lightning does no harm to the building.