Electricity: Charge and Current

• Electricity fundamental concepts lecture slides by Engr. Van Vesper J. Dulliyao

Electric Charge

• Electric charge is the physical property of matter that causes attraction or repulsion when near electrically charged matter.
• The basic unit of electric charge is the coulomb (C).
• The charge of an electron (qe) is -1.6021 x 10^-19 C.
• 1 coulomb is approximately the charge of 6.21 x 10^18 electrons.

Electric Current

• Electric current is the rate of change of charge passing through a predetermined area.
• The unit of current is amperes (A).
• 1 ampere is equal to 1 coulomb per second (1 A = 1 C/s).
• Figures including arrows are the proper way to represent current.
• 3A in one direction is equivalent to -3A in the reverse direction in terms of electrical effects.

Kinds of Current

• Direct Current (DC) has two variations: unidirectional, continuous, and pulsating.
• Alternating Current (AC) has two variations: alternating and oscillating.

Typical Current Magnitude

• Lightning bolts have a current magnitude of around 10^6 amperes.
• Large industrial motors have a current magnitude of around 10^4 amperes.
• Typical household appliances draw around 10^2 amperes.
• Ventricular fibrillation in humans can be caused by currents around 10^0 amperes.
• The human threshold of sensation is around 10^-2 amperes.
• Integrated circuit memory cells operate at currents around 10^-6 amperes.
• Synaptic currents (brain cell) are around 10^-12 amperes.

Voltage

• Voltage is the work done in moving a unit charge through an element from one terminal to the other.
• The unit of voltage is volts (V).
• Moving a unit charge from point B to point A requires W joules of work, where V = W/q.

Polarity

• Voltage can take positive (+) or negative (-) values.
• If terminal A is V volts positive with respect to terminal B, terminal A is at a potential of V volts higher than terminal B.
• Both representations are equivalent in their electrical effects.
• The plus-minus sign pair is part of the definition of any voltage.

Typical Voltage Magnitude

• Lightning bolts have a voltage magnitude of around 10^8 volts.
• High voltage transmission lines operate at around 10^6 volts.
• Voltage on a TV picture tube is around 10^4 volts.
• Large industrial motors and AC outlet plugs operate at around 10^2 volts.
• Car batteries have a voltage of around 10^1 volts.
• Integrated circuits operate at around 10^-1 volts.
• Voltage across the human chest produced by heart (EKG) is around 10^-3 volts.
• Voltage between two points on human scalp (EEG) is around 10^-4 volts.
• Antennas of a radio receiver operates at around 10^-6 volts.

Power

• Power is an indication of how much work can be accomplished in a specified amount of time, or simply the rate of doing work.
• The unit of power is watts.
• Electric power can be supplied by electric generators or chemical sources.
• 1 horsepower is equal to 33,000 foot-pounds per minute.

Absorbed/Supplied

• In an absorbing element, positive current enters the positive terminal of positive voltage.
• In an absorbing element, positive current leaves the negative terminal of positive voltage.
• In a supplying element, positive current leaves the positive terminal of positive voltage.
• In a supplying element, positive current enters the negative terminal of positive voltage.

Voltage & Current Source

• Independent sources are elements that provide a specific voltage or current regardless of other circuit variables.

Independant Voltage Sources

• These have two terminals and maintain a specified voltage.
• The magnitude is completely independent of the current through the element.

Independant Current Sources

• These have two terminals through which a specified current flows.
• The magnitude is completely independent of the voltage across the element.

Dependant Sources

• Dependant sources are voltage or current sources whose value depends on the voltage or current at another point in the circuit.

Dependant Voltage Sources

• The value of the voltage source depends on the value of the voltage at a certain part of the circuit.

Dependant Current Sources

• The value of the current source depends on the value of the voltage at a certain part of the circuit.

Resistor

• A resistor is an electrical component or device designed to have a specific magnitude of resistance.
• There are variable and fixed Resistors.
• A "mil" is a unit of length used to measure wire diameters, where 1 mil is equal to one-thousandth of an inch (0.001 in).
• A "circular mil" is a term used to define cross-sectional area.
• A = s^2, A= 1 square mil.
• A = Id^2/4, A = I/4 square mil = 1 circular mil (CM).
• 1 CM = Π/4 mil².
• 1 inch = 1000 mil.

Resistor Color Codes

• Resistors are often marked with color bands to indicate their resistance value and tolerance.

Resistance

• Resistance is a measure of the opposition to the flow of current in a resistor.
• It is typically measured in ohms (Ω).
• The larger a resistor's resistance, the greater the barrier against the flow of current.
• 1Ω = 1 Volt / Ampere

Resistivity

• Resistivity is a material property that determines how strongly the material opposes the flow of electric current.
• It is measured in ohm-cm/ft.

Inductor

• Inductors are circuit elements that store energy in a magnetic field.

Inductance

• Inductance is a measure of an inductor's ability to store magnetic energy.
• It is typically measured in henries (H).
• The inductance of an inductor depends on the number of turns in the coil, the area enclosed by the coil, the permeability of the core material, and the shape of the coil.

Capacitor

• A capacitor is a passive electronic component that stores electrical energy in an electric field between two conductive plates separated by a dielectric material.
• It consists of two conductive plates typically made of metal, and a non-conductive material, the dielectric, placed between them.
• When a voltage is applied across the terminals of a capacitor, electric charge accumulates on the plates, creating an electric field between them.

Capacitance

• Capacitance is a measure of the ability of the capacitor to store charge.
• It is typically measured in farads (F).
• The capacitance of a capacitor depends on several factors, including the area of the plates, the distance between the plates, and the dielectric constant of the material between the plates.