Why does ac alternate

Ocean Acidification. Rising Sea Level. Alternating current Alternating current AC is the type of electric current generated by the vast majority of power plants and used by most power distribution systems.

Figure 1. An animation from a PhET simulation [2] of alternating current which has been slowed down considerably. See direct current for a comparison.

Brain et al. How Electricity Works [Online]. Pigman power engineer for Tacoma Power, Sept. 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.

An alternator can also be used to purposely generate AC current. In an alternator, a loop of wire is spun rapidly inside of a magnetic field. This produces an electric current along the wire. As the wire spins and periodically enters a different magnetic polarity, the voltage and current alternate on the wire.

This current can change direction periodically, and the voltage in an AC circuit also periodically reverses because the current changes direction. AC comes in several forms, as long as the voltage and current are alternating. If an AC circuit is hooked up to an oscilloscope and its voltage is plotted over time, you are likely to see several different waveforms such as sine, square, and triangle — sine is the most common waveform and the AC in most mains-wired buildings have an oscillating voltage in the sine wave form.

AC is most commonly found in mains-wired buildings such as homes and offices. This is because generating and transporting an AC current across long distances is relatively easy. At high voltages of over kV, less energy is lost in power transmission. At higher voltages, lower currents are produced, and lower currents generate less heat in the power line due to a lower level of resistance.

The equation to the right of the equals sign describes how the voltage changes over time. V P is the amplitude. The sin function indicates that our voltage will be in the form of a periodic sine wave, which is a smooth oscillation around 0V. This is given in the form of hertz or units per second. The frequency tells how many times a particular wave form in this case, one cycle of our sine wave - a rise and a fall occurs within one second.

As time varies, our waveform varies. Phase is a measure of how shifted the waveform is with respect to time. It is often given as a number between 0 and and measured in degrees. We can turn to our trusty outlet for a good example of how an AC waveform works.

In the United States, the power provided to our homes is AC with about V zero-to-peak amplitude and 60Hz frequency. We can plug these numbers into our formula to get the equation remember that we are assuming our phase is 0 :. We can use our handy graphing calculator to graph this equation. If no graphing calculator is available we can use a free online graphing program like Desmos Note that you might have to use 'y' instead of 'v' in the equation to see the graph.

Notice that, as we predicted, the voltage rise up to V and down to V periodically. Additionally, 60 cycles of the sine wave occurs every second. If we were to measure the voltage in our outlets with an oscilloscope, this is what we would see WARNING: do not attempt to measure the voltage in an outlet with an oscilloscope!

This will likely damage the equipment. This is also correct. When talking about AC since the voltage changes constantly , it is often easier to use an average or mean. To accomplish that, we use a method called "Root mean squared. Home and office outlets are almost always AC. This is because generating and transporting AC across long distances is relatively easy. At high voltages over kV , less energy is lost in electrical power transmission.

Higher voltages mean lower currents, and lower currents mean less heat generated in the power line due to resistance. AC can be converted to and from high voltages easily using transformers. AC is also capable of powering electric motors. Motors and generators are the exact same device, but motors convert electrical energy into mechanical energy if the shaft on a motor is spun, a voltage is generated at the terminals!

This is useful for many large appliances like dishwashers, refrigerators, and so on, which run on AC. Direct current is a bit easier to understand than alternating current.

Rather than oscillating back and forth, DC provides a constant voltage or current. The tank can only push water one way: out the hose. Similar to our DC-producing battery, once the tank is empty, water no longer flows through the pipes. DC is defined as the "unidirectional" flow of current; current only flows in one direction.