/sci/ - Science & Math

[Anonymous No.1595731]

What is the difference between direct and alternate current, and how do they each work?

[Anonymous No.1595742]

bump because interesting topic

[Anonymous No.1595747]

>>1595742

That and Modern Marvels didn't do a good enough job.

[whiskeytrot !6Gc6E2gG86 No.1595757]

AC will kill cute animals.
DC makes you shit rainbows.

[Anonymous No.1595761]

>>1595757
Wrong.
It makes you shit DOUBLE rainbows.

[Anonymous No.1595762]

>>1595757

http://www.youtube.com/watch?v=RkBU3aYsf0Q

Euthanasia the Tesla way!

[Anonymous No.1595765]

DC is always constant. It's always 9v or whatever it's set.

AC works in push-pull kinda. It's sent in pulses, never getting below or peaking above a certain threshold. It makes it more efficient to send it over long distances.

DC is abysmal for long distances. You'll lose like 9/10 of your signal in the same distance that AC will lose 1/10 of it's signal... well not really, but it's still pretty bad.

captcha : Culture, galore

[Anonymous No.1595766]

I think AC/DC is a pretty cool guy. eh makes good music and doesn't afraid of anything

[Anonymous No.1595769]

The difference is in the flow of electrons. In direct current the electrons will always flow in the same direction, from negative to positive. In alternating current the direction of electron flow changes back and forth (always from negative to positive, relatively speaking).

If you look at a graph where the y-axis is voltage and the x-axis is time, DC would be a horizontal line at some steady voltage, and AC looks like a sine wave. When the voltage is positive current flows one way, and when the voltage is on the negative side current will flow the opposite direction.

Or something like that.

[Anonymous No.1595773]

Direct current has a constant voltage/current. Alternating current is a sinusoidal voltage/current. I'm not sure what you want for how they work. You have a power source that accelerates electrons through a wire, the size of the acceleration is the voltage difference between the terminals. With an alternating voltage the size goes up and down, once it becomes negative, it accelerates the electrons in the opposite way. DC is usually simpler, but it's easier to transfer AC over long distances.

[Anonymous No.1595776]

>>1595765
>>1595769

OP here, while C is sending a constant stream of electrons, AC sends electrons in packets (I'm not sure if that would be the correct term, basically the electrical equivalent of quanta) so that the amount of electricity coming through isn't overbearing and still retains strength?

CAPTCHA: hambery processing

[Anonymous No.1595787]

>>1595776
>>1595776

Yeah, kinda. It doesn't really cut off though. I think with electrical outlets in the US the AC is between 120-240, so it doesn't drop below a threshold.

[Anonymous No.1595794]

We could really use an electrical engineer horn here.

[Anonymous No.1595808]

My dad's an electrical engineer (and he's 51). I could ask him for you...but he's asleep.

[Anonymous No.1595815]

>>1595794
There are no EE's on /sci/.

[Anonymous No.1595834]

>>1595815
I'm studying to become one, but i'm really just doing my basics and have a ton of knowledge that i've picked up over the years. I'm contribootin what I can.

[Anonymous No.1595847]

Tesla was a fraud.

[Anonymous No.1595848]

EE here

AC = alternating current. It alternates = goes from high to low, usually periodic = sinusoidal. Think about how it is generated. In an electric turbine in a dam the water spins a wheel. This wheel will generate a voltage that varies depending on the position of the wheel. (Pic related)

Direct current is direct. It is direct = a single voltage that does not change with time -> constant. A battery is an example of this. The potential difference between two different chemicals amounts to a certain voltage, this voltage will be constant and the same magnitude for as long as the chemical reaction lasts.

[Anonymous No.1595852]

>>1595847

0/10

[Anonymous No.1595865]

Well I worked on electronics in the US Navy for 6 years, and I had to take most of the technical classes an EE would have to take.

Like people said, DC stays at a steady voltage and AC alternates directions. There is a lot more to AC though, more than I care to go into on a /sci/ post. You have to consider how many phases, RMS (root mean square, in simple terms it could be considered effective voltage) vs maximum voltage, and different frequencies for different applications, and blah blah blah.

There is a lot of information on the web if you just google alternating current.

[Anonymous No.1595867]

>>1595848
> usually periodic = sinusoidal
>direct = a single voltage that does not change with time
If you're a real EE, you can do better than that.

[Anonymous No.1595871]

>>1595765

>A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses

one of you two is wrong...

[Anonymous No.1595884]

>>1595871

HVDC is more efficient over LONG distances. Over short distances (from the power plant to your house) AC rules the day.

[Anonymous No.1595908]

>>1595871
Whoops, must have typed that out backwards.

[Anonymous No.1595910]

>>1595731
DC describes a zero-frequency flow of electrons with definite polarity.

AC describes a flow of electrons that alternates polarity. The direction of current changes periodically. Notice from the graph that AC voltage is not a constant value.

AC waveforms can have a DC component eg, it is the average value of the function.
<span class="math"> \frac{1}{2\pi}\int_0^{2\pi} v(t) dt[/spoiler]

[Anonymous No.1595927]

>>1595884
no, this is wrong on very basic levels

Both AC and DC have <span class="math">I^2R[/spoiler] losses in the conductors. AC, additionally, has capacitive losses because wire+air+earth is like an endless capacitor. If you want to minimize resistive losses, you increase the voltage, current decreases (for a given power), and <span class="math">I^2R[/spoiler] losses go down. If you want to minimize capacitive losses you decrease the frequency, since the impedance of a capacitor is inversely proportional to frequency.

Therefore, high-voltage direct current is the best means of transmitting power efficiently.

Converting voltage levels, however...

[sage No.1595941]

>>1595794
EE Here

It's been said pretty well before: "DC" is Direct Current; ideally a DC source puts out a voltage at a constant level; when plotted against time its a straight horizontal line. DC is the choice for anything that has Memory/RAM; it has no "zero volts" time, which would clear the memory every time it occurred. DC is bad for transmission though, which is why household electronics and power transmission lines use AC with DC converters where needed.

AC stands for Alternating Current. The voltage put out by an AC source varies with time and is Sinusoidal when plotted with time. AC is just as good as DC for electronics that aren't very sophisticated, like the heat coils in a toaster or hair-dryer. AC is also easily "transformed"; a 120 V AC signal can be easily transformed into a 240 V AC signal using a "Transformer", which is an arrangement of mutually inductive magnetic coils and cores; DC voltage can't be converted nearly as easily; a 9 V source can't be changed into a 18 V source (the physics behind transformers rely on the time-varying nature of AC, DC can't compete because it is by definition time-invariant). AC power can also be more easily converted to DC than the reverse, and AC is the natural way that a rotating generator produces electrical energy. Finally, AC is better for transmission because of how easy it is to transform. The power lost in a wire can be modelled as P = I²R, where I is current and R is the wire resistance. A second relationship exists for the power being transmitted as P' = VI, where V is voltage of the signal and I is again current. For a constant P', Raising V very high (24000 volts) makes I small, and I² even smaller. Thus the power lost in transmission is reduced because it only depends on I and R (this is why power in powerlines and comming from power plants to step down facilites read "high voltage"; it saves power and therefore saves money).

[Anonymous No.1595944]

>>1595927

So if this is the case, why don't power plants across the world use DC? Why not send it out in DC if it will save them that much money, even if the start up costs would be say... three times more.

[Anonymous No.1595949]

>>1595927
Not this guy but I can add...

AC can also lose power from reflected waves caused by supply/load impedance mismatch.

[Anonymous No.1595956]

>>1595941
>likes to use semi-colons.

[Anonymous No.1595966]

>>1595944
It was implied in
> converting voltage levels, however...
Practical voltages for household use would be... well, who knows, since we don't have it. But you'd never transmit at that level. And DC/DC converters are worse than transformers in terms of efficiency.

[sage No.1595969]

>>1595956
its 3 am; i warned you it might not be coherent.

;

[Anonymous No.1595971]

>>1595944
> adding from previous reply
but, as mentioned in this thread, some do use high voltage direct current

[Anonymous No.1595975]

>>1595944
you just said it

>start up costs

[Anonymous No.1595993]

little bit of trivia:

>Highest capacity system: 6.3 GW HVDC Itaipu (Brazil) (±600 kV DC)[31]

>Highest transmission voltage (AC): 1.15 MV on Powerline Ekibastuz-Kokshetau (Kazakhstan)

>Largest double-circuit transmission, Kita-Iwaki Powerline.

>Highest pylons: Yangtze River Crossing (height: 345 m/1,132 ft)

>Longest power line: Inga-Shaba (length: 1,700 kilometres / 1,056 miles)

>Longest span of power line: 5,376 m (17,638 ft) at Ameralik Span

Longest submarine cables:
>NorNed, North Sea - (length of submarine cable: 580 kilometres / 360 miles)
>Basslink, Bass Strait - (length of submarine cable: 290 kilometres / 180 miles, total length: 370.1 kilometres / 230 miles)
>Baltic-Cable, Baltic Sea - (length of submarine cable: 238 kilometres / 148 miles, HVDC length: 250 kilometres / 155 miles, total length: 262 kilometres / 163 miles)

Longest underground cables:
>Murraylink, Riverland/Sunraysia - (length of underground cable: 180 kilometres / 112 miles)

[Anonymous No.1596043]

>>1595993
Edison was right and Tesla was wrong.
HVDC > HVAC

[Anonymous No.1596053]

>>1595969
It's not even grammatically correct. Stop blaming it on time.

[Anonymous No.1596070]

build more pylons

[Anonymous No.1596097]

Regarding loss: AC can operate at a much higher voltage which is why the power lines are at tens of thousands of volts and only requiring a lower current for a significant power to be supplied. DC on the other hand must crank up a high current and this leads to significant loss of energy through thermal heating of resistive elements in a transmission line.


"These HVDC systems use electronic devices like mercury arc valves, thyristors or IGBTs that were unavailable during the War of Currents era. Power is still converted to and from alternating current at each side of the modern HVDC link. The advantages of HVDC over AC systems for bulk transmission include higher power ratings for a given line (important since installing new lines and even upgrading old ones is extremely expensive) and better control of power flows, especially in transient and emergency conditions that can often lead to blackouts. Many modern plants now use HVDC as an alternative to AC systems for long distance, high load transmission, especially in developing countries such as China, India and Brazil. (See List of HVDC projects for more details.)"

See the fact that AC is for long distance and is converted into DC....

>>1595867
It's obvious but apparently you dumbasses need it simple English.

[Anonymous No.1596111]

>>1596097
>It's obvious but apparently you dumbasses need it simple English.
Obvious fake engineer is obvious.

[Anonymous No.1596144]

>>1596111

Chill summerfag, you'll make it to college eventually.

[Anonymous No.1596159]

>>1596144
>you'll
>you

[Anonymous No.1596199]

>>1596097
retarded post is retarded

[Anonymous No.1596204]

>>1596159
right

you will make it to..

not

you make it to

what was the point of your post again?

[Anonymous No.1596220]

>>1596204
It's too bad you didn't understand my post.

[Anonymous No.1596257]

I'm not going to go into the entire thing but AC is favored in power generation and transmission for a few reasons:
1) DC motors/generators require more maintenance and fail more often
2) 3-phase power (look it up) provides a nice constant power supply for motors which are the primary electricity consumer and they can manage it with less copper than DC and therefore it is cheaper
3) Stepping up or down in voltage with DC is fuckexpensive and annoying. With AC power you just need wire wrapped around a core.
4) DC power transmission required a booster station every absurdly short unit of distance (less than a mile I think). This really killed off Edison more than anything

If you were strictly looking at transmission losses and didn't need to change voltages then DC would be superior but the real life doesn't work that way. DC is favored in scenarios such as:
1) Subsea transmission because AC has crap power factor, lots more capacitance in salt water, etc.
2) Interconnecting two major independent grids are done with a DC link because synchronizing two massive grids' waveforms and practically impossible
3) For very long distance transmission with large amounts of power high voltage DC is favored because despite the expense and pain of converting to DC, stepping up, stepping back down and back to AC the losses are lower; this is especially due to the skin effect and generally HVDC has less loss due to corona discharges than AC

[Anonymous No.1596266]

>>1596220
I think >>1596204
understands it more than you do.

[Anonymous No.1596308]

No one has mentioned that you can go from 120 to 240 with only a simple phase shift? If one lead is 180 degrees out of phase with another, you suddenly have 240V. This is how your dryer/stove/hotwater tank/baseboard heaters get their 240voltage. This is what makes AC power so useful in a residential setting; you have access to these two voltages without have to transform.

Think like this: if an AC voltage vs time is represented by a sine wave, then the voltage at anytime is the potential difference between the sine wave, and the x axis. If you now superimpose a second sine wave, phase shifted by π, the active voltage is the difference between the 2 sine waves (240v at maximum).

DC has none of these capabilities.

[Anonymous No.1596431]

>>1596308

<span class="math">sin\theta + sin(\theta + \pi) = 0 [/spoiler]