To understand electricity and how it functions, it is important to understand common electrical terms and their definitions.

Current: Current is the movement of electrons through a conductor. The amount of current moving is determined by the voltage divided by the resistance.

Alternating current (AC): One of two types of electric current, alternating current does not exhibit constant polarity or a constant voltage. The voltage builds to a maximum value in one polarity, declines to zero, builds to a maximum value in the other polarity, and declines to zero again, Figure 25.1. This sequence is called one cycle. In the United States the common current is 60 cycle; that is, 60 complete cycles occur every second.

Direct current (DC): This current flows in one direction only and at a constant voltage, Figure 25.2. The polarity will be either positive or negative.

Amperage (amp): A unit of measure for the amount of electricity flowing in a circuit. One ampere of current is equivalent to 6.28 x 1018 electrons per second.

Circuit: A continuous path for electricity from the source to the load (machine or appliance using the electricity) and back to the source.

Conductor: Any material that has a relatively low resistance to the flow of electricity. Such materials allow electrons to flow easily from one atom to another. Most metals are good conductors.

Insulator: Any material that has a relatively high resistance to the flow of electricity. Such materials do not allow easy movement of electrons from one atom to another. Glass, rubber, and many plastics are insulators.

Resistance (Ohm): The characteristic of materials that impedes the flow of electricity. All materials have varying amounts of electrical resistance. Two

90 180 270360 4B0 540 630 720 810 900 990 1080 Electrical degrees

FIGURE 25.1. Three complete cycles of 60 cycle alternating current.

90 180 270360 4B0 540 630 720 810 900 990 1080 Electrical degrees

FIGURE 25.1. Three complete cycles of 60 cycle alternating current.

characteristics of resistance are important: (1) when electricity flows through a resistant material, heat is generated; (2) when electricity flows through a resistant material, the voltage is decreased. Resistance is measured in units of ohms and designated by the Greek letter ^ (omega). Voltage (V): The electromotive force that causes electrons to flow through a conductor. Voltage is a measure of the potential for current flow. A voltage potential may exist between objects without a flow of current. In the United States the two standard voltages are nominally 120 V and 240 V. Watts (W): Unit of electrical power. Watts are determined by multiplying the voltage times the amperage. Ohm's law: Ohm's law explains the relationship between current, voltage, and resistance. Ohm's law states that the voltage in a circuit is equal to the amperage times the resistance. Expressed as an equation:

where E = Voltage (V); I = Current (amp); R = Resistance (ohms).

It is common to rearrange this equation to solve for any one of the three variables.

For example:

Problem: What is the current flow (amp) when the source supplies 120 V and the circuit has 20.0 ohms of resistance?

Solution:

E 120 V

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