THEORY OF BASIC CONCEPT OF CURRENT ELECTRICITY

ATOM:-

An element is defined as a substance which cannot be decomposed into other substance

The smallest particle of an element which takes part in chemical reaction is known as

atom.

• All matter is composed of atoms which are infinitesimally small.

All atoms are made of electrons, protons and neutrons. Most solid materials as

classed, from the stand point of electrical conductivity, as conductors, semi-conductors

insulators. To be conductor, the substance must contain some mobile electrons-on they

can move freely between atoms. These free electrons come only from the valence (out.

orbit of the atom. Conductivity depends on the number of electrons in the valence orbit

"The energy level of an electron increases as its distance from the nucleus increases. Thus

electron in the second orbit possesses more energy than electron in the first orbit, electrons in the tha

orbit have higher energy than in the second orbit and so on. It follows, therefore, that electrons in a

last orbit will possess very high energy. These high energy electrons are less bound to the nucleus a

hence they are more mobile. It is the mobility of last orbit electrons that they acquire the property

combining with other atoms. Further due to this combining power of last orbit electrons of an ote

they are called valence electrons".

*  Atoms with fewer than four valence electrons are good conductors.

*  Atoms with more than four valence electrons are poor conductors.

* Atoms with four valence electrons are semi-conductors.

* Important data of an atom

   Mass of an electro = 9.11 x 10^-31 kg

* mass of proton

= 1/1840 mass of proton

charge on Electron =-1.602 ×(10)^-19 columb

diameter of an electron= (10)^-15 m

PROTON :- Mass of proton  =1.67 ×(10)^-27 kg

charge on proton = +1.602 × (10)^19 coulumb

NEUTRON :- Mass of neutron = mass of proton(=1.67×(10)^-27 kg

Charge on neutron =Nil

Diameter of nucleus .........of the order of (10)-14 m

Diameter of orbits = (10)^4 times the dia of the molecule 

Normally, the atoms are electrically neutral, that, the number of electrons and proton

are the same, cancelling each other's electrical force. Atoms "stay together" because unlike

charges attract each other. The electrical force of the protons hold the electrons in the

orbits. Like electrical charges repel each other so negatively charged electrons will no

collide with each other.

Positive and Negative Ions:-

When an electron is removed from a neutral atom, this atom becomes positively charged an

is called positive ion. However if an electron is added to a neutral atom, it becomes negatively charge

and is called a negative ion. Thus an atom becomes an ion by the gain or loss of an electron.

*ELECTRIC CURRENT:-

The controlled movement of electrons (or drift) through a substance is called current.

- Current is the rate at which electrons move. One ampere (unit of current) represents 6.28 x (10)^18 electrons passing a point each second (1 coulomb past a point in one

second).

Ampere:- coulumb second

One coulomb - charge of 6.28 x 10×^18 electrons

® ELECTROMOTIVE FORCE:-

Electromative force (e.m.f.) is the force that causes a current of electricity to flow.

* The potential difference (p.d.) V, between two points in a circuit is the electrical pressure

or voltage required to drive the current between them.

* The volt is a unit of potential difference and electromotive force. It is defined as the

difference of potential across a resistance of 1 ohm carrying a current of 1 ampere.

* Electron Volt :-

Electron volt is a unit in terms of which the energies of atomic particles are expressed.

it is the work done when an electron, whose charge is e coulombs, is moved in an electric field through

potential difference of 1 volt against the force (newtons) acting on the charge.

Thus 1 electron volt =e joules.

# RESISTANCE :-

The opposition to flow of electrons (due to bands between protons and electrons, as well a

collisions) is called a electrical resistance (R)

Resistance may also be defined as "The property of the electric circuit which opposes the

flow of current".

The practical unit of electric resistance is ohm.

1 Mega-ohm =(10)^6 ohm

1 kilo=(10)^3 ohm

1 milli-ohm=(10)^-3 ohm

1 micro-ohm=(10)^-6 ohm

Laws of Resistance

The resistance of a conductor, such as a wire, of uniform cross-section depends on the following

factors:

(1)Length (l) : varies directly as its length, I

(2) Cross-section (A) : varies inversely as the cross-section, A, of the conductor

(3) Nature of the material

(4) Temperature of the conductor : It almost varies directly with the temperature

R= pl/A

where p is known as specific resistance or resistivity.

Specific resistance or resistivity of a material may be defined as "The resistance between the

opposite faces of a metre cube of that material".

The unit of resistivity is ohm-metre

Conductance (G):-

Conductance (G) is the reciprocal of resistance

* Conductivity :- The reciprocal specific resistance

of a material is called its conductivity

The reciprocal of specific resistance a =

is mho/metre.

The unit of conductivity is = mho/metre

Temperature co-efficient of resistance at 0"C may be defined as follows:-

"The change in resistance per ohm for change in temperature of from 1•C to 0•C"

Over large temperature ranges the simple formula

Temperature Co-efficient of Resistance

R, = Ro (1 + xt)

does not completely fit, but a formula of the type

R= Ro (1+ xt + B(t)^2

(where B is a smaller co-efficient)

OHM’S LAW Ohm’s law can be stated as follows :-

 

"For a fixed metal conductor, the temperature and other conditions remaining constant the

current (l) through it is proportional to the potential difference (V) between its ends"

In other words V/I =CONSTANT

where R is the resistance of the conductor between the two points considered:

- The linear relationship (I « V) does not apply to all non-metallic conductors. For example

for silicone carbide, the relationship is given by:

V=KIx where K and x are constants and x is less than unity

The following relations hold good

where P= V.I

I = √P/R

R=P/(l)^2

P=V/I =√PR

Power is expressed in terms of kW (kilowatt = 1000 W) or MW (megawatt 1000 kW or

(10)^6W).

Electrical energy is expressed in terms of kWh (kilowatt hours)

1 kWh = 1 kW x1 hour = 1000 watt-hours ( 1000 x 60 x 60 watt-sec.)

*Linear and Non-linear Resistors

A linear resistor is one which obeys Ohm's law. A circuit which contains only linear

components is called a linear circuit.

*Such elements in which the VII (volt-ampere) plots are not straight lines but curves are

called non-linear resistors or non-linear elements.

Ohm's law does not apply under  the following conditions

1. Electrolytes where enormous gasses are  produced on either electrode.

2.Non-linear resistors like vacuum radio valves, semi-conductors, gas filled tabes ete.

3. Are lamps.

4. Metals which get heated up due to flow of current through.

5. Appliances like metal rectifiers, crystal detectors, etc, in which operation dependa on the

direction of current..

* Super-conductivity:-

Equation R =R[1 + a (t-20)] holds good for temperature below 20°C. But at very low

temperature, some metal acquire zero electrical resistance and zero magnetic induction; the property

known as super-conductivity.

Super-conducting elements. Zine, cadmium, mercury, lead.trick :-ZCML

Typical superconducting compounds and alloys : PbAu, PbTl2, SnSb, CuS, NbN, NbB, ZrC.

The superconductivity will disappear if

(1) The temperature of the material is raised above its critical temperature

(1i) a sufficiently strong magnetic field ar current density is employed.

* RESISTORS

A resistor entails the following two main characteristics:

(a) Its resistance (R) in ohms. The resistors are available from a fraction of an ohm

many mega ohms.

(b) The wattage rating:-

The power rating may be as high as several hundred watts on

watts or as 1/10 watt. Power rating indicates the maximum wattage the resistor can dissipate without

excessive heat (Too much heat can make the resistor burn open).

*Classification of Resistors :-

The resistors are classified as follows:

1. Fixed resistors. The fixed resistor is the simplest type of resistor. Fixed means that the

is so constructed that its resistance value is constant and unchangeable. These are made of a co.

composition and have a cover of black or brown hard plastics.

2. Tapped resistors. A tapped resistor is a resistor which has a tap, or connection, somewhere

along the resistance material. These resistors are usually wire wound type. If they have more

one tap, they will have a separate terminal for each.

3. Variable resistors. A variable resistor has a moveable contact that is used to adjust or select

the resistance value between two or more terminals, A variable resistor is commonly called a control.

4. Special resistors:- The most common type of special resistors in the fusible type .A fusible resistor has a definite resistance value and it protect the circuit much like a fuse another special resister is the temperature compensating unit .Such resistors are used to provide special control of circuit that must be extremely stable in their operation.

To be continued ...........