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Alternating & Direct Current: AC DC Electricity

Most of the examples dealt with so far, and particularly those utilizing batteries, have constant voltage sources. Once the current is established, it is thus also a constant.

Direct current DC is the flow of electric charge in only one direction. It is the steady state of a constant-voltage circuit. Most well-known applications, however, use a time-varying voltage source. Alternating current AC is the flow of electric charge that periodically reverses direction.

If the source varies periodically, particularly sinusoidally, the circuit is known as an alternating current circuit. Examples include the commercial and residential power that serves so many of our needs. The AC voltages and frequencies commonly used in homes and businesses vary around the world.

Figure 1. The voltage and current are sinusoidal and are in phase for a simple resistance circuit. The frequencies and peak voltages of AC sources differ greatly.

Figure 2. The potential difference V between the terminals of an AC voltage source fluctuates as shown. The voltage between the terminals fluctuates as shown, with the AC voltage given by. 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. 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.

The voltages generated at the power plant are stepped up by passive devices called transformers see Transformers to , volts or more in some places worldwide. At the point of use, the transformers reduce the voltage transmitted for safe residential and commercial use. This gives. One-fourth of a percent is an acceptable loss. Note that if MW of power had been transmitted at 25 kV, then a current of A would have been needed. This would result in a power loss in the lines of The lower the voltage, the more current is needed, and the greater the power loss in the fixed-resistance transmission lines.

Of course, lower-resistance lines can be built, but this requires larger and more expensive wires. If superconducting lines could be economically produced, there would be no loss in the transmission lines at all.

But, as we shall see in a later chapter, there is a limit to current in superconductors, too. In short, high voltages are more economical for transmitting power, and AC voltage is much easier to raise and lower, so that AC is used in most large-scale power distribution systems.

It is widely recognized that high voltages pose greater hazards than low voltages. But, in fact, some high voltages, such as those associated with common static electricity, can be harmless. So it is not voltage alone that determines a hazard.

It is not so widely recognized that AC shocks are often more harmful than similar DC shocks. There were bitter fights, in particular between Edison and George Westinghouse and Nikola Tesla, who were advocating the use of AC in early power-distribution systems. AC has prevailed largely due to transformers and lower power losses with high-voltage transmission.

Give an example of a use of AC power other than in the household. Similarly, give an example of a use of DC power other than that supplied by batteries.

Why do voltage, current, and power go through zero times per second for Hz AC electricity? You are riding in a train, gazing into the distance through its window.

As close objects streak by, you notice that the nearby fluorescent lights make dashed streaks. Certain heavy industrial equipment uses AC power that has a peak voltage of V. What is the rms voltage? A certain circuit breaker trips when the rms current is What is the corresponding peak current? Military aircraft use Hz AC power, because it is possible to design lighter-weight equipment at this higher frequency. What is the time for one complete cycle of this power? A North American tourist takes his Assuming constant resistance, what power does the razor consume as it is ruined?

In this problem, you will verify statements made at the end of the power losses for Example 2 above. A small office-building air conditioner operates on V AC and consumes Two different electrical devices have the same power consumption, but one is meant to be operated on V AC and the other on V AC.

Nichrome wire is used in some radiative heaters. Skip to main content. Search for:. Calculate rms voltage, current, and average power. Explain why AC current is used for power transmission. Example 1. Strategy We are told that V rms is V and P ave is Solution for b Peak power is peak current times peak voltage.

We know the average power is Example 2. Discussion One-fourth of a percent is an acceptable loss. Discover the physics behind the phenomena by exploring magnets and how you can use them to make a bulb light. Click to download the simulation. Run using Java. Conceptual Questions 1. What is the peak power consumption of a V AC microwave oven that draws What is the peak current through a W room heater that operates on V AC power?

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What is Alternating Current (AC)?

Introduction 2. Generating AC Voltages 3. Values 6. Sinusoidal waveform are tricky things to draw. By using phasor, it can be represent as straight lines. By joining up such lines, we can undertake apparently difficult additions and subtractions and this simplifies the later analyses which will be considered.

Electric current is defined as the rate of flow of negative charges of the conductor. For an electric current to occur, it has to be conducted through a material. When the flow of electrons provided by electricity finds a resistance, it results in a dissipation of energy in the form of heat. If there is an excessive amount of heat generated, the tissue may be burnt. Physiologically, the difference between a normal burn and a burn caused by electricity is that electricity has the ability to burn tissue beneath the skin, even burning internal organs without apparent external evidence of it.

Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. If you continue browsing the site, you agree to the use of cookies on this website. See our User Agreement and Privacy Policy. See our Privacy Policy and User Agreement for details. Published on Sep 20, Alternatin current history and producing.


The peak value of a sine wave is the value of voltage or current at the positive or negative maximum with respect to zero. • Peak values are represented as: V p.


Chapter1 -Alternating Voltage & Current :)

DC is the kind of electricity made by a battery with definite positive and negative terminals , or the kind of charge generated by rubbing certain types of materials against each other. Certain sources of electricity most notably, rotary electromechanical generators naturally produce voltages alternating in polarity, reversing positive and negative over time. Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the circle with the wavy line inside is the generic symbol for any AC voltage source. One might wonder why anyone would bother with such a thing as AC.

Alternating Voltage and Current

Physiological effect of electric current

Most of the examples dealt with so far, and particularly those utilizing batteries, have constant voltage sources. Once the current is established, it is thus also a constant. Direct current DC is the flow of electric charge in only one direction.

Both AC and DC have their own characteristics and provide different advantages that can be used in different situations. As the name implies direct current, DC is a form of electricity that flows in one direction — it is direct and this gives it its name. The characteristic of direct current, DC can be shown on a graph.

Physiological effect of electric current

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  1. Avelaine L.

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