Saturday, June 25, 2011
1:10 PM | Posted by
Anonymous |
DC Voltage and Current
Direct current (DC) is the flow of electrons in a circuit that is always in the same direction. Direct current (one-direction current) occurs when the voltage is kept constant, as shown in above Figure. A battery, for example, produces DC current when connected to a circuit. The electrons leave the negative terminal of the battery and move through the circuit toward the positive terminal of the battery.
AC Voltage and Current
When the terminals of the potential energy source (voltage) alternate between positive and negative, the current flowing in the electrical circuit likewise alternates between positive and negative. Thus, alternating current
(AC) occurs when the voltage source alternates.
Above Figure shows the voltage increasing from zero to a positive peak value, then decreasing through zero to a negative value, and back through zero again, completing one cycle. In mathematical terms, this describes a sine wave. The sine wave can repeat many times in a second. The length of time it takes to complete one cycle in a second is called the period of the cycle.
Frequency
Frequency is the term used to describe the number of cycles in a second.
The number of cycles per second is also called hertz, named after Heinrich Hertz (1857–1894), a German physicist. Note: direct current (DC) has no frequency or Zero frequency.
therefore, frequency is a term used only for AC circuits.
For electric power systems in the United States, the standard frequency is 60 cycles/second or 60 hertz. The European countries frequency is 50 hertz
Comparing ac and dc Voltage and Current
Electrical loads, such as lightbulbs, toasters, and hot water heaters, can be
served by either ac or dc voltage and current.
It is important to note that there is an equivalent ac voltage and current that will produce the same heating effect in an electrical load as if it were a DC voltage and current. The equivalent voltages and currents are referred to as the root mean squared values (RMS values). The reason this concept is important is that all electric power systems are rated in rms voltages and currents.
For example, the 120 Vac wall outlet is actually the rms value. Theoretically, one could plug a 120 Vac toaster into a 120 Vdc battery source and cook the toast in the same amount of time. The AC rms value has the same heating capability as a dc value. Do you feel confused confused about this lesson? Leave your question now in a comment.
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