Electric current (or conventional current) is the movement of free electrons within an electrical conductor caused by a difference of electric potential (voltage) between the ends of the conductor. When charges flow, they can be positive, negative, or both. It is a convention to choose the direction of current to be in the direction of flow of positive charges. However, in a conductor, such as copper, the current is due to the motion of the negatively charged electrons. Therefore, with an ordinary conductor, such as copper wire, it is customary to refer to the direction of current as being opposite the flow of electrons. However, this convention is not always used. In some cases, current is considered to be in the direction of electron flow. The direction of the flow of electric current (motion of positive charges) is always from positive to negative, while the physical motion of electrons (negative charges) is from negative to positive.

Ampere (I), or amp, is the unit of measure of electric current. It is the quantity of 6.24 x 1018 electrons flowing past a point per second. Ampacity is the current-carrying capacity of conductors. The Coulomb is the quantity of electricity transported in one second by a current of one ampere. Sources such as the National Electrical Code provide tables indicating the maximum allowable current for any given size conductor and insulation system.

Electromotive force (EMF) is the force that causes current to flow within a conductor. This force, which can be considered electric pressure, is commonly referred to as electric potential.

Volt (V) is the unit of measure of EMF. Voltage is the potential between two charges, or two points in a circuit, or the electrical pressure in the electric system. The greater the voltage (pressure), the greater the flow.

Watt (W) is the common measurement of electrical power. One horsepower (hp) of mechanical work is equal to 745.7 W of electric power. Electric power, measured in watts, is the product of volts times amperes (rate of motion). For single-phase circuits, the relationship is expressed as:


P = Power in watts

I = Current flow in amperes cos 0 = Power factor

Conductor is any material, such as silver, copper, or aluminum, that has a very low resistance to the flow of electric current. It can refer to a wire, coil, cable, bus bar, or any other object used to carry electric current.

Circuit is a conductor or a series of conductors through which an electric current flows. A circuit may have one or more electric components, known as circuit elements. Electric current will only flow in a closed or continuous circuit, on a continuous, non-interrupted path back to the source.

Cycle is the complete pattern of a single wave form of alternating current (see Figure 24-5).

Frequency is the number of cycles per second, specified in Hertz (Hz), where one Hz equals one cycle per second. The most typical frequency for alternating current in the United States is 60 Hz, although many other countries use 50 Hz.

Harmonics are wave forms (in both voltage and current) with frequencies that are multiples of the fundamental wave (60 Hz or 50 Hz). Harmonic distortion is a measure of distortion caused by devices such as inverters, rectifiers, transformers, and arc furnaces that distort the sinusoidal wave form, a wave in the form of a sine wave. A sine wave, which is shown in Figure 24-5, is a wave form that represents periodic oscillations.

Ohm (Q) is the unit of measure of the resistance to flow of electric current in a circuit. One ohm is the value of resistance through which a potential difference of 1 volt will maintain a current flow of 1 ampere.

Ohm's law states that voltage in a circuit is equal to the current flow times the resistance in the circuit:


E = EMF in volts I = Current in amperes R = Resistance in ohms

Figure 24-1 illustrates relationships developed using Ohm's law. Included are relationships between power in watts, current flow in amps, electric potential in volts, and resistance in ohms.

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