FUNDAMENTAL OF ELECTRICITY
History of Electricity
The science of electricity and magnetism has a history going back many centuries. The ancient greek new that rubbed amber would attrack small particles, and the directional properties of naturally magnetic lodestone were known to chinese in the elevent century and posibly much earlier.
investigation into the nature of electricity began around 1600. Understanding grew steadily during the following 200 years, until the discoveries of nineteenth-century scientist such as Ampere, faraday and Maxwell made posible the great advances that have since taken place.Electric Current
We can regard the electric current simply that something that flows. Like water flows into the pipe or flows along a river.Ampere
When water flows throught the pipe, the rate of flow is usually expressed in terms of the quantity that will pass through per unit time, such as 10 litres per second. Current flows through a wire is expressed in similar manner. the Unit of current is called A mpere
, abbreviated as A or I in other terminologyRange of Current
The range of current in a circuit is a very small fraction of Ampere
. that would measured in milliapere (mA) or microampere (µA) define as:
1 milliampere 1 mA = 10^-3 A
1 microampere 1 µA = 10^-6 AConductors and Insulators:Conducting materials
We learnt that electric current, flow through a solid path of suitable material. Subtances that offer little opposition to the flow of electricity known as Conductors.
Known good Conductors are; Silver, copper, gold, aluminum, zinc, brass, phospor-bronze, platinum, tin, nickel, steel, iron, mercury, carbon and water.
Silver and gold are too expensive for most purposes, copper is the most widely used, but its going to be expensisve.Insulating Materials
Subtance that offer great opposition to the flow of electricity are called Insulators.
Common materials of these type are; paraffin, wax, porcelain, bakelite, ebonite, (hard rubber), dry silk and cotton
. all of this materials keeping apart conductors to prevent unwanted current to flow (short). Usual Insulating Materials are; rubber, PVC (polyvinyl chloride) and polythene .
Insulatung materials that provide Insulation are term Insulators Resistance
Resistance define as the property of subtance varying w/ different subtance to oppose or resist the passage of electricity.
Insulating Material high resistance and conductors have low resistance. Remember: no known subtance is wholly without resistance.
Factor influencing Resistance of a conductor:
1. The resistance of a wire is directly proportional to its length.
R α L
2. The Resistance of a wire is inversely proportional to the area of its cross-section.
R α 1/A
3. The Resistance of a wire depends upon the nature of material.
R α ρ
4. The Resistance of a wire depends upon the temperature
Mathematically from the above factors (from 1 to 3)
R = ρ L/A (ohms, Ω)
ρ = resistivity or specific resistance of the wire material usually measure@ 20ºC
Specific rsistance (ρ) of a material maybe defined as the resistance offered to a current if passed between the opposite faces the unit cube of the material.It is measured in Ω-CM/ft (in English)
For most conductors, and increase in temperature corresponds to an increase in resistance due to the increased molecular movement within the conductor which hinder the flow of charge.
R2= R1 [ 1+α1(t2-t1)] Ω
where α1 = temperature coefficient of resistance t1
T=Infered absolute zero temperature Alloys
Alloys are mixture of metals.Most alloys increase very slightly in resistance with rise in temperature.Some Alloys posses practically constant resistance at all temperatures such as germanium, silver, eureka. Electrolytes/Carbon/Insulators
The valu of resistance of carbon, Electrolytes and Insulators decreases as the temperature increases. Conductance (G)
- The Reciprocal of resistance and is the measure of the ease with which the current will flow through a subtance.
G = 1/R
Conductance may be expressed in terms of length, and Area cross section of the conductor.
G = 1/ρ x A/L = δA/L
Where δ = specific conductance or conductivity of the material in mho or siemens/m Voltage or Potential Difference (p.d) or Electromotive Force (e.m.f)
The energy required to effect successive acceleration and move an electron from one p[oint to another. It is measured in terms of Work per unit Charge (force of electrical motion)
V = W/Q (volts, V)
Range of Voltage or p.d or e.m.f
Volts is sometimes is too large or too smal at the range of:
1 μV = 10 ˉ6 V
1 mV = 10ˉ3 V
1 kV = 10^3 V
1 MV= 10^6VElectrical Circuit
Circuit is a path composed of conductor,or of several conductors, joined together through which an electrical current may flow from a source at a given point along the conducting path and back to its starting point. circuit may be represented on paper by means of diagram called circuit diagram. Direct Current (DC)
that current that flows in one direction
Three forms of direct current:
1. Continous dc - derived from battery
2. unidirectional dc - derived from a dc generator
3. pulsating dc - derived from rectifier circuits (either half wave or full-wave) Electric current (I)
- the motion of electric charge in a conducting material
- the timerate at which the charge Q passes a given point