EEPS Lesson | Part 1

Electricity: Basic Principles

Matter - anything that occupies space and has weight.

Element - a substance that cannot be decomposed any further by chemical action.

Compound - a combination of two or more elements.

Molecule - smallest particle that a compound can be reduced to before it breaks down into elements.

Atom - smallest part that an element can be reduced to and still be keeping the properties of the elements.

 Parts of an Atom:

Nucleus of an atom - center of the atom, it is where the protons and neutrons are located.

Electrons - revolve around in orbits or shells around the nucleus.

 

FORMULA:  N = 2n²

    

where: 

N = total number of electrons on a given shell

n = nth shell of the atom

Valence electrons - electrons found in the outermost shell or orbit of an atom.

Atomic Number - represents the number of electrons or protons of an atom.

Atomic Mass - represents the sum of protons and neutrons of an atom.

Elements - No. of electrons - No. of protons - No. of neutrons - No. of valence electron(s)

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Copper - 29 - 29 - 34 - 1

Aluminum - 13 - 13 - 14 - 3

Germanium - 32 - 32 - 41 - 4

Electric Charge

A body is said to be charge, if it has either excess or deficit of electrons from its normal values due to sharing.

Coulomb (C) - unit of electric charge, which is equivalent to 6.25x10^18 electrons or protons. Named after the French physicist, Charles A. Coulomb (1736-1806).

Note: 1 C = 6.25x10^18 e

Potential Difference

Potential - the capability of doing work.

Any charge has the capability of doing work of moving another charge either by attraction or repulsion.

a. 6 electrons will be attracted by the +2 C plate and 3 electrons will be attracted by the +1 C plate, making a resultant motion of 3 electrons going towards the +2 C plate.

b. 3 electrons will be attracted by the +1 C plate and 3 electrons will be repelled by the -1 C plate, making a resultant motion of 6 electrons going towards the +1 C plate.

The net number of electrons moved in the direction of the positive charge plate depends upon the potential difference between the two charges.

Volt (V) - unit of potential difference, which is equal to one joule of work done per one coulomb of charge. Named after Italian physicist, Alessandro C. Volta (1754-1827) who invented the first electric battery.

Electric Current

When a potential difference between two charges forces a third charge to move, the charge in motion is called an electric current.

Ampre (A) - unit of charge flow equal to one coulomb of charge past a given point in one second. Named after the French physicist and mathematician. Andre M. Ampere (1775-1836).

Resistance

The fact that a wire carrying a current can become hot, it is evident that the work done by the applied force in producing the current must be accomplished against some opposition or resistance.

Ohm (Ω) - practical unit of resistance, Named after the German physicist, Georg S. Ohm (1787-1854).

Formula:    R = ρL/A,    R = ρV/A²,    R = ρL²/V

where: 

R = resistance (ohm)

A = cross-sectional area (square meter)

ρ = resistivity (ohm-meter)

L = length (meter)

V = volume (cubic meter)

Specific resistance (resistivity) - resistance offered by a unit cube of the material.

Circular mil (CM) - area of a circle having a diameter (d) of one mil.

Conversion factor: 

CM = d²

1000 mil = 1 inch

1 MCM = 1000 CM

Effect of Temperature in Resistance

Experiments have shown that the resistance of all wires generally used in practice in electrical systems, increases as the temperature increases.

Formula:

R₁/R₂ = (T+t₁)/ (T+t ₂),    R₂/R₁ = 1 + α∆t

α = 1 / (T+t₁),    ∆t = t₂ - t₁

where:

R₁ = initial resistance (ohm)

R₂ = final resistance (ohm)

T = inferred absolute temperature, temperature when resistance of a given material is zero.

t₁ = initial temperature

t₂ = final temperature

∆t = change in temperature

α = temperature coefficient of resistance

Temperature coefficient of resistance (α) - ohmic change per degree per ohm at some specified temperature.

Insulation Resistance of Cables

where: 

ρ = resistivity of the insulating material (ohm-meter)

L = length of cable (meter)

R1 = radius of the cable

R2 = radius of the outer surface of the insulation

Note:  loge = ln

Conductance

Conductance - is a measure of the material's ability to conduct electric current. It is equal to the reciprocal of resistance.

Siemens (formerly mho) - unit of conductance. Named after the German engineer, Ernst Werner von Siemens (1816-1892).

Formula: G = 1/R,    G = δA/L,    δ = 1/ρ

where:

G = conductance (siemens)

δ = conductivity (siemens per meter)

L = length (meter)

A = cross-sectional area (square meter)

ρ = specific resistance (ohm-meter)

R = resistance (ohm)

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