For this example, the voltage and current are said to be in phase, as seen in Figure 1 b. If the resistor is a fluorescent light bulb, for example, it brightens and dims times per second as the current repeatedly goes through zero. A Hz flicker is too rapid for your eyes to detect, but if you wave your hand back and forth between your face and a fluorescent light, you will see a stroboscopic effect evidencing AC.
The fact that the light output fluctuates means that the power is fluctuating. Wave your hand back and forth between your face and a fluorescent light bulb. Do you observe the same thing with the headlights on your car?
Explain what you observe. Warning: Do not look directly at very bright light. Figure 3. AC power as a function of time. Since the voltage and current are in phase here, their product is non-negative and fluctuates between zero and I 0 V 0. We are most often concerned with average power rather than its fluctuations—that W light bulb in your desk lamp has an average power consumption of 60 W, for example.
As illustrated in Figure 3, the average power P ave is. Similarly, we define an average or rms current I rms and average or rms voltage V rms to be, respectively,. In general, to obtain a root mean square, the particular quantity is squared, its mean or average is found, and the square root is taken.
This is useful for AC, since the average value is zero. It is standard practice to quote I rms , V rms , and P ave rather than the peak values. The common A circuit breaker will interrupt a sustained I rms greater than 10 A. Your 1. You can think of these rms and average values as the equivalent DC values for a simple resistive circuit. We are told that V rms is V and P ave is This means that the AC voltage swings from V to — V and back 60 times every second.
An equivalent DC voltage is a constant V. So the power swings from zero to W one hundred twenty times per second twice each cycle , and the power averages 60 W.
Most large power-distribution systems are AC. Moreover, the power is transmitted at much higher voltages than the V AC V in most parts of the world we use in homes and on the job. Economies of scale make it cheaper to build a few very large electric power-generation plants than to build numerous small ones.
This necessitates sending power long distances, and it is obviously important that energy losses en route be minimized. High voltages can be transmitted with much smaller power losses than low voltages, as we shall see. See Figure 4. For safety reasons, the voltage at the user is reduced to familiar values. The crucial factor is that it is much easier to increase and decrease AC voltages than DC, so AC is used in most large power distribution systems.
Figure 4. Power is distributed over large distances at high voltage to reduce power loss in the transmission lines. Waves of alternating current are made when the wire moves into areas of different magnetic polarity—for example, the current changes direction when the wire spins from one of the magnetic field's poles to the other.
This wave-like motion means that AC power can travel farther than DC power, a huge advantage when it comes to delivering power to consumers via power outlets. Direct current DC power, as you may suss from the name, is a linear electrical current—it moves in a straight line. Direct current can come from multiple sources, including batteries, solar cells, fuel cells, and some modified alternators.
DC power is far more consistent in terms of voltage delivery, meaning that most electronics rely on it and use DC power sources such as batteries.
Electronic devices can also convert AC power from outlets to DC power by using a rectifier, often built into a device's power supply. A transformer will also be used to raise or lower the voltage to a level appropriate for the device in question. Not all electrical devices use DC power, though. Many devices, household appliances, especially, such as lamps, washing machines, and refrigerators, all use AC power, which is delivered directly from the power grid via power outlets.
Although many of today's electronics and electrical devices prefer DC power because of its smooth flow and even voltage, we could not get by without AC. Both types of power are essential; one is not "better" than the other.
In fact, AC dominates the electricity market; all power outlets bring power into buildings in the form of AC, even where the current may need to be immediately converted into DC power. Inverters change DC to AC. For example, for your car an inverter would change the 12 volt DC to Volt AC to run a small device.
While DC can be stored in batteries, you cannot store AC. Share this comparison:. If you read this far, you should follow us:. Diffen LLC, n. Comparison chart Alternating Current versus Direct Current comparison chart Alternating Current Direct Current Amount of energy that can be carried Safe to transfer over longer city distances and can provide more power. Voltage of DC cannot travel very far until it begins to lose energy. Cause of the direction of flow of electrons Rotating magnet along the wire.
Steady magnetism along the wire. Frequency The frequency of alternating current is 50Hz or 60Hz depending upon the country. The frequency of direct current is zero. Direction It reverses its direction while flowing in a circuit. It flows in one direction in the circuit. Current It is the current of magnitude varying with time It is the current of constant magnitude. Flow of Electrons Electrons keep switching directions - forward and backward.
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