like I’m an idiot.
I understand the direction of the current periodically changes. What I don’t know is how this happens. What causes it. How does it work?
Wikipedia doesn’t seem to explain this, or if it does, it’s buried in some paragraph containong enough technical terms to cause me to skip to the next paragraph.
The current doesn’t periodically change, it changes consistently and predictably with the rotation of electrical generators. If a generator with two poles revolves at 60 cycles per second, the resulting output will be a sine wave at 60 Hertz where half the time the produced electricity will be positive, and half the time it will be negative, and it is sinusoidal because the coils rotating on the armature are wound in two different directions.
Generators are manufactured this way to optimize power distribution. Direct Current has a lot more loss, and 50 Hertz and 60 Hertz are standardized on due to realistic generator rotational speeds.
Picture a wire U-shaped loop spinning between two magnets. You have yourself a simple generator. As it spins, one end of the loop is coming towards one of the magnets, passes the magnet, and then moves away from it. This is why the current changes directions. In a nutshell.
When a magnet passes near a wire, it generates a current in the wire, but only when it is moving. To keep the current flowing, keep it moving, but to avoid traveling an infinite distance, bring it back over the wire in the opposite direction. This generates a current in the opposite direction, too. Rinse, repeat. Now you have an alternating current generator. Half of the cycle is in the forward direction, half in the other direction.
Make the magnet and wire combo into a circular apparatus and you have an efficient AC generator that looks a lot like a motor. In fact, it is a motor in reverse.
Imagine a bar magnet with an axle in the middle, spinning at 60 full turns per second. Bring a coil of wire with its ends connected to a light bulb close to the ends of the spinning magnet. As the north pole of the magnet approaches and passes the coil, electrons will be pushed one way in the wire by the magnet’s magnetic field. As the south pole approaches and passes the coil, the electrons in the wire will be pushed the other way. Since the magnet is spinning at 60 revolutions per second, the current is said to be 60 hertz (Hz) or 60 cycles per second.
You can spin the magnet or the coil so that the magnetic field moves and crosses the coil of wire. A magnetic field must be changing or moving in order for the electrons to be pushed. An AC generator can be made with electromagnets instead of permanent magnets. Alternators in cars work that way.
Not quite. Direct current actually experiences slightly lower loss. The advantage of AC is that the voltage can easily be transformed up or down using a transformer. High voltage transmission is much more efficient but too dangerous, so it is transformed down before it enters your home.
The great electricity war between Westinghouse and Edison was based on the fact that Westinghouse owned the patents for transformers and for Tesla’s polyphase motors and generators. AC was without question the superior system and has prevailed to the present day.
It is difficult to make a DC generator, so alternating current is the output normally found from generators. As others have described, it’s the effect of a magnet rotating relative to a conductor. As the conductor is pushed through a magnetic field current is generated in one direction. If the magnet is rotating relative to the conductor the current will change direction with each rotation, causing the alternating direction of the current.
Interesting. That’s not the way I understood it. Is there a cite for DC distribution being more effective than AC? I suppose you could ask me the same question, in reverse. I’ll go looking too.
Thanks.
Thanks for that.
I believe you meant transmission, not distribution.
<Pedantic> How is something measured at 60 hertz considered to not change periodically? </Pendantic>
-D/a
I was also confused by this, but I’m a physicist for whom the word has a rather precise and narrow connotation, i.e. happening at regular intervals. I decided he must be using some other definition of periodically. When I looked it up, the third definition was “intermittent”, which has more of a connotation of irregular or sporadic intervals. I suppose that is what he meant.
DC transmission is more efficient than AC, but I am pretty sure AC generators are much more efficient than DC generators. So the main advantage of AC is the ease of converting to transmission voltages with transformers, but another advantage is that it is easier and more efficient to generate. (And since a huge portion of our power output is consumed by industry using induction machines, it’s more efficient to use, too.)
Except that those induction machines represent a large reactive power load, which DC motors wouldn’t.
I have another question about alternating current. What defines the direction of flow of the electrical energy?
At one end of a cable is a power station, generating electricity. At the other end is a consumer, using electricity. So the power is clearly flowing in one direction, from the power station to the consumer.
But if you look at any point on the cable, the current is continually reversing direction, with no preferred direction. There is no net movement of electrons. And yet, hundreds of megawatts of power are being transmitted in one direction along the cable.
Is it possible, by taking measurements at that intermediate point, to determine which way the power is flowing?
Think of the difference between a circular saw and a reciprocating saw. The teeth on the circular saw move in one direction, that’s DC. The reciprocating saw moves back and forth, both directions, that’s AC. The electrons in AC are moving back and forth like that, changing direction 60 times a second.
Yes, I understand that. But take a look at the specific question I asked.
You’re saying there’s no net movement, but there is constant movement. The movement of electrons in either direction is sufficient to produce electromotive force or resistance to power devices.
Think of a chain saw: The saw teeth move continuously in one direction, cutting the wood. Now think of a large two-man manual cross-cut saw: It moves back and forth, with zero net movement, yet still cuts the wood.
Electrons can do the same thing…the direction of movement does not determine the sign of the work accomplished. If you hook a light bulb to a battery with a polarity reversing switch, light is produced and the battery drained, regardless the polarity, so if you flip the switch back and forth rapidly, nothing really changes…except you could now run a high voltage bulb through a transformer. This is how old style 12VDC to 110VAC inverters worked…new ones are different, and usually don’t have a transformer.