TOPIC 1 : STATIC ELECTRICITY
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, unlike charges attract”
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;
§
Rubbing
§
Induction
§ 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
§
Testing for the
sign of the charge on the body.
§ Identifying the insulating properties of
materials.
§ 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:
§ Contact
§
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.
The S.I unit of capacitance is is Farads (F).
Other units include microfarads (µF), picofarads (Pf) and nanoFarads (nF).
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.
A
3µF capacitor has a 18V of potential difference. What will be its total charge?
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;
§ Area of plates
§ Distance apart of the plates.
§
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.
Lightning 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.
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