ELECTRICITY PRINCIPLES

Chapter 1: BASIC CONCEPTS OF ELECTRICITY

Static electricity
All materials are made up of tiny "building blocks" known as atoms. All naturally occurring atoms contain particles called electrons, protons, and neutrons, with the exception of the protium isotope (1H1) of hydrogen. Electrons have a negative (-) electric charge. Protons have a positive (+) electric charge. Neutrons have no electric charge. Electrons can be dislodged from atoms much easier than protons or neutrons. The number of protons in an atom's nucleus determines its identity as a unique element.

Conductors, insulators, and electron flow

In conductive materials, the outer electrons in each atom can easily come or go, and are called free electrons. In insulating materials, the outer electrons are not so free to move. All metals are electrically conductive. Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor. Static electricity is an unmoving (if on an insulator), accumulated charge formed by either an excess or deficiency of electrons in an object. It is typically formed by charge separation by contact and separation of dissimilar materials.

For electrons to flow continuously (indefinitely) through a conductor, there must be a complete, unbroken path for them to move both into and out of that conductor.

Electric circuits
A circuit is an unbroken loop of conductive material that allows electrons to flow through continuously without beginning or end. If a circuit is "broken," that means its conductive elements no longer form a complete path, and continuous electron flow cannot occur in it. The location of a break in a circuit is irrelevant to its inability to sustain continuous electron flow.Any break, anywhere in a circuit prevents electron flow throughout the circuit.

Voltage and current

Electrons can be motivated to flow through a conductor by the same force manifested in static electricity.

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Voltage is the measure of specific potential energy (potential energy per unit charge) between two locations. In layman's terms, it is the measure of "push" available to motivate electrons. Voltage, as an expression of potential energy, is always relative between two locations, or points. Sometimes it is called a voltage "drop." When a voltage source is connected to a circuit, the voltage will cause a uniform flow of electrons through that circuit called a current. In a single (one loop) circuit, the amount of current at any point is the same as the amount of current at any other point. If a circuit containing a voltage source is broken, the full voltage of that source will appear across the points of the break. The +/- orientation of a voltage drop is called the polarity. It is also relative between two points.

Resistance
Resistance is the measure of opposition to electric current. A short circuit is an electric circuit offering little or no resistance to the flow of electrons. Short circuits are dangerous with high voltage power sources because the high currents encountered can cause large amounts of heat energy to be released. An open circuit is one where the continuity has been broken by an interruption in the path for electrons to flow. A closed circuit is one that is complete, with good continuity throughout. A device designed to open or close a circuit under controlled conditions is called a switch. The terms "open" and "closed" refer to switches as well as entire circuits. An open switch is one without continuity: electrons cannot flow through it. A closed switch is one that provides a direct (low resistance) path for electrons to flow through.

Chapter 2: OHM's LAW

How voltage, current, and resistance relate

REVIEW: Voltage measured in volts, symbolized by the letters "E" or "V". Current measured in amps, symbolized by the letter "I". Resistance measured in ohms, symbolized by the letter "R". Ohm's Law: E = IR ; I = E/R ; R = E/I

An analogy for Ohm's Law

Ohm's Law also makes intuitive sense if you apply it to the water-and-pipe analogy. If we have a water pump that exerts pressure (voltage) to push water around a "circuit" (current) through a restriction (resistance), we can model how the three variables interrelate. If the resistance to water flow stays the same and the pump pressure increases, the flow rate must also increase.

ELECTRICITY PRINCIPLES

If the pressure stays the same and the resistance increases (making it more difficult for the water to flow), then the flow rate must decrease:

If the flow rate were to stay the same while the resistance to flow decreased, the required pressure from the pump would necessarily decrease:

As odd as it may seem, the actual mathematical relationship between pressure, flow, and resistance is actually more complex for fluids like water than it is for electrons. If you pursue further studies in physics, you will discover this for yourself. Thankfully for the electronics student, the mathematics of Ohm's Law is very straightforward and simple.

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REVIEW: With resistance steady, current follows voltage (an increase in voltage means an increase in current, and vice versa). With voltage steady, changes in current and resistance are opposite (an increase in current means a decrease in resistance, and vice versa). With current steady, voltage follows resistance (an increase in resistance means an increase in voltage).

Power in electric circuits

So, our 100 horsepower diesel and motorcycle engines could also be rated as "74570 watt" engines, or more properly, as "74.57 kilowatt" engines. In European engineering specifications, this rating would be the norm rather than the exception. REVIEW: Power is the measure of how much work can be done in a given amount of time. Mechanical power is commonly measured (in America) in "horsepower." Electrical power is almost always measured in "watts," and it can be calculated by the formula P = IE. Electrical power is a product of both voltage and current, not either one separately. Horsepower and watts are merely two different units for describing the same kind of physical measurement, with 1 horsepower equaling 745.7 watts.

Calculating electric power

REVIEW: Power measured in watts, symbolized by the letter "W". Joule's Law: P = I2R ; P = IE ; P = E2/R

Resistors
Devices called resistors are built to provide precise amounts of resistance in electric circuits. Resistors are rated both in terms of their resistance (ohms) and their ability to dissipate heat energy (watts). Resistor resistance ratings cannot be determined from the physical size of the resistor(s) in question, although approximate power ratings can. The larger the resistor is, the more power it can safely dissipate without suffering damage.

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Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a non-specific load, rather than an actual resistor.

Nonlinear conduction
The resistance of most conductive materials is stable over a wide range of conditions, but this is not true of all materials. Any function that can be plotted on a graph as a straight line is called a linear function. For circuits with stable resistances, the plot of current over voltage is linear (I=E/R). In circuits where resistance varies with changes in either voltage or current, the plot of current over voltage will be nonlinear (not a straight line). A varistor is a component that changes resistance with the amount of voltage impressed across it. With little voltage across it, its resistance is high. Then, at a certain "breakdown" or "firing" voltage, its resistance decreases dramatically. Negative resistance is where the current through a component actually decreases as the applied voltage across it is increased. Some electron tubes and semiconductor diodes (most notably, thetetrode tube and the Esaki, or tunnel diode, respectively) exhibit negative resistance over a certain range of voltages.

Circuit wiring
Connecting wires in a circuit are assumed to have zero resistance unless otherwise stated. Wires in a circuit can be shortened or lengthened without impacting the circuit's function -- all that matters is that the components are attached to one another in the same sequence. Points directly connected together in a circuit by zero resistance (wire) are considered to beelectrically common. Electrically common points, with zero resistance between them, will have zero voltage dropped between them, regardless of the magnitude of current (ideally). The voltage or resistance readings referenced between sets of electrically common points will be the same. These rules apply to ideal conditions, where connecting wires are assumed to possess absolutely zero resistance. In real life this will probably not be the case, but wire resistances should be low enough so that the general principles stated here still hold.

Polarity of voltage drops

The polarity of the voltage drop across any resistive component is determined by the direction of electron flow through it: negative entering, and positive exiting.

Chapter 3: ELECTRICAL SAFETY