Basic Electrical - Part 1

Electrical Circuit

A basic electrical circuit consists of the power source, the conductors, and the load. A switch can be placed in series with the circuit conductors to control the operation of the load (turning it on or off).

According to the “electron current flow theory,” current always flows from the negative terminal of the source, through the circuit and load, to the positive terminal of the source.

Power Source

The power necessary to move electrons out of their orbit around the nucleus of an atom can be produced by chemical, magnetic, photo-voltaic, and other means. The two categories of power sources are direct current (dc) and alternating current (ac).

Direct Current

The polarity and the output voltage from a direct-current power source never change direction. One terminal is negative and the other is positive, relative to each other. Direct-current power is often produced by batteries, direct-current generators, and electronic power supplies.

Direct current is used for electroplating, street trolley and railway sys-tems, or where a smooth and wide range of speed control is required for a motor-driven application. Direct current is also used for control circuits and electronic instruments.

Alternating Current

Alternating-current power sources produce a voltage that changes polarity and magnitude. Alternating current is produced by an alternating-current power source such as an alternating-current genera-tor. The major advantage of alternating current over direct current is that voltage can be changed through the use of a transformer.

Conductance

Conductance, or conductivity, is the property of a metal that permits current to flow. The best conductors in order of their conductivity are silver, copper, gold, and aluminum. Copper is most often used for electrical applications.

Circuit Resistance

The total resistance of a circuit includes the resistance of the power supply, the circuit wiring, and the load. Appliances such as heaters and toasters use high-resistance conductors to produce the heat needed for the application. Because the resistances of the power source and conductor are so much smaller than that of the load, they’re generally ignored in circuit calculations.

Ohm’s Law

Ohm’s Law expresses the relationship between a direct-current circuit’s current intensity (I), electromotive force (E), and its resistance (R). This is expressed by the formula: I = E/R.

The German physicist Georg Simon Ohm (1787-1854) stated that current is directly proportional to volt-age, and inversely proportional to resistance.

Direct proportion means that changing one factor results in a direct change to another factor in the same direction and by the same magnitude.

If the voltage increases 25 percent, the current increases 25 per-cent—in direct proportion (for a given resistance). If the voltage decreases 25 percent, the current decreases 25 percent—in direct proportion (for a given resistance).

Inverse proportion means that increasing one factor results in a decrease in another factor by the same magnitude, or a decrease in one factor will result in an increase of the same magnitude in another factor.

If the resistance increases by 25 percent, the current decreases by 25 percent—in inverse proportion (for a given voltage), or if the resistance decreases by 25 percent, the current increases by 25 per-cent—in inverse proportion (for a given voltage).

Ohm’s Law and Alternating Current

Direct Current

In a direct-current circuit, the only opposition to current flow is the physical resistance of the material through which the current flows. This opposition is called resistance and is measured in ohms.

Alternating Current

In an alternating-current circuit, there are three factors that oppose current flow: the resistance of the material; the inductive reactance of the circuit; and the capacitive reactance of the circuit.

For now, we’ll assume that the effects of inductance and capacitance on the circuit are insignificant and they’ll be ignored.

Ohm’s Law Formula Circle

Ohm’s Law, the relationship between current, voltage, and resistance expressed in the formula, E = I x R, can be transposed to I = E/R or R = E/I. In order to use these formulas, two of the values must be known.

Place your thumb on the unknown value in Figure 1–16, and the two remaining variables will “show” you the correct formula.

Current Example

Question: 120V supplies a lamp that has a resistance of 192 ohms. What’s the current flow in the circuit?

Step 1: What’s the question? What’s “I?”

Step 2: What do you know? E = 120V, R = 192 ohms

Step 3: The formula is I = E/R

Step 4: The answer is I = 120V/192 ohms

Step 5: The answer is I = 0.625A

Voltage-Drop Example

Question: What’s the voltage drop over two 12 AWG conductors (resistance of 0.20 ohms per 100 ft) supplying a 16A load located 50 ft from the power supply?

Step 1: What’s the question? What’s “E?”

Step 2: What do you know about the conductors?

I = 16A, R = 0.20 ohms. The NEC lists the alternating-current resistance of 1,000 ft of 12 AWG as 2 ohms [Chapter 9, Table 8]. The resistance of 100 ft is equal to 0.20 ohms.

Step 3: The formula is E = I x R.

Step 4: The answer is E = 16A x 0.20 ohms

Step 5: The answer is E = 3.20V

Resistance Example

Question: What’s the resistance of the circuit conductors when the conductor voltage drop is 3V and the current flowing in the circuit is 100A?

Step 1: What’s the question? What’s “R?”

Step 2: What do you know about the conductors?

E = 3V dropped, I = 100A

Step 3: The formula is R = E/I

Step 4: The answer is R = 3V/100A

Step 5: The answer is R = 0.03 ohms