Hello,
Thanks in advance! Ok I, really should probably already long ago understood this. Even a tad ashamed
that I do not. However, here is the deal.
A/C Sinusoidal.

Vs.

D.C. Waveform

D.C. very easy for me to understand a positive voltage over time = Current flow.

In no way do I understand the top graph (sinusoidal) as far as where is the current flow.

The problem I face is when I see the voltage drop into the negative. I mean does the phase
push forward? I do not own a O scope (working on a cheepo) However when I see them used.
The phase appears to stand still. I do not know, I could rationalize it all day boils down to I have
No idea How this works. I hope its a simple explanation. Perhaps I am not visualizing it correctly?

 

The AC wave is always changing in value, sometimes positive, sometimes negative. Do note that the X or horizontal axis is time.

The funny symbol for X is not the best choice here, it is being used to indicate a general case for any frequency of the AC wave. For example, In the US where the AC power is 60 hearts this means the full waveform from zero to two pi happens 60 times a second.

For a resistive load (a simple resistor) the current varies just as the voltage does, sometimes positive, sometimes negative. You could just copy the same graph, reliable the Y axis to the appropriate ampers, and be done.

Things get more complicated when your load also has an inductor or capacitor, but don't worry about that for now.

Seriously, the only dumb question is the one you do not ask.

 

The top graph simply shows the voltage at any instant in time.
If that voltage is connected to a resistive load, then the current through the load will follow the voltage from Ohm's law, I = V/R.

If the AC frequency was 50 Hz then the voltage would go from 0V to the positive peak value in the first 5ms and back to zero in the next 5ms.
Then it goes to the negative peak (meaning the current in the load reverses) in the third 5ms, and then back to 0V in the last 5ms, for a total period of 20ms (1/50Hz).

The reason an oscilloscope display of the waveform looks steady is because the scope is triggered on the same point in the AC waveform for each horizontal sweep of the oscilloscope (which sweeps left to right at a rate given by the horizontal sweep speed).
If you looked at a really low frequency AC signal (say 1Hz) on an analog scope (digital scopes generally accumulate the data before displaying it all at once) then you would see the trace rising and falling with the voltage as the trace dot slowly moved from left to right.

 

Things you didn't ask but probably should have...

AC actually means alternating current, which essentally answers your question, the current alternates, as stated above.
AC waveforms are not always sinusoid and can have a DC offset, although neither is the case for distribution networks, mains.

When you are talking about an AC supply the voltage is usually measured an an RMS value. That is Root Mean Square, the root of the average square under the waveform.
Using RMS as a value for calculation allows ohms law to be used when considdering current and more imprtently power.
Ohms law still works on any instantainious value, peek voltage for example will produce peek current but that would make steady state analysis impossible and everything else exponentally harder to deal with.
Imagine trying to calculate the power dissipated in a resistor, I^2R when I is continually changing. I RMS works just fine though.

Why use AC in the first place...
It can be generated easily in fact it is a Sine wave because it is genereted by swinging a magnet past a coil, well two coils to be more accurate.
More importently because V constantly changes I constantly changes in a load. That means transformers will work and voltage can be stepped up and down easily, OK relativly easily. Since high voltage at a low current is the only practical way to didtribute large amounts of electrical energy AC is effectivly the only reasonable choice.

Before I see a storm of folk telling me there are other ways....
I am not saying AC is the only way to distribute electricity over network, just that untill we get Star Trek style plasma conduites or room temperature super conductors high voltage is the only reasonable option and as high voltage applyances could be somewhat hazardous, 33000V toaster anyone, transformers are still the best option.

Lastly if you are desigining anything for AC you will need to understand the diference between resistsance and impedence...Not a short answer but essentally anything even a humble resistor is not going to be entirly passive in practice. That means that there will be capacitence and inductance to consider and the frequency becomes important.

Its an interesting subject, worthy of some reading, even if you only deal with the basics it a good idea to have a handle on the concepts so yoiu know when you can safely ignore the complexities and simply go with RMS values.

Google is your friend... Or Bing, whatever..

Al

 

I am sorry to say not much of this includes things I do not already know.

What I need is like the version you would explain to a 12 year old.
I had voltage, amperage and, resistance explained to me much like
water contained in pipes. That gave me the visual beyond the math.

In my two graph's using water as the visual.

A/C graph. The water is being added and then taken away.
I do realize a push pull motion can create momentum. However
when I visualize A/C over lets say a nichrome strip from a heater.
Its not as simple as positive to negative. or a device that has an alternating
load that is compatible. Push pull of electrons heating the material?

D.C. graph. The water is only being added at the rate of consumption.

 

I seem to be a simple person. Let's look at my model for today.
If I push on the plunger, I force water into the tank on the right and when I then pull on the plunger, water from the tank on the right moves back into the tank on the left. My pushing and pulling is the power supply. The movement through the center tube causes friction and that is the work I am doing. I am doing work during both the push and the pull.

The fact of our world is that electrons (represented by water) are laying around all over the place and I can force more of them into one place or less of them into one place. Pressure exists and suction exists. Suction is just pressure going the other direction. Are you beginning to see a relationship?

 

ok! ok great! thank you so much yes that I was only making guesses about. so you confirm that part
it seems. Based on that it leads me to believe that it is my lack of understanding of what exactly am I
seeing on the scope. obviously its not a standing wave hitting the A/C motor or w/e. I really need to still
understand A/C on a heating coil or a nichrome strip. It would be reassuring to see that all loads that are
A/C work on this push pull factor like we both mentioned. Simple friction?

 

The interesting thing about AC is that even though the arithmetically average values of sinusoidal voltages and currents are zero, work gets done. An analog is a two-man tree saw. The blade goes back and forth, but it's net movement over time is zero inches. Yet, the tree falls.

ak

 

The heat generated by a voltage across a resistor is simply V²/ R.
Note that the polarity of the voltage has no effect on the results of this equation since a number squared is always positive.
Thus the voltage polarity and the direction of the current have no effect on the amount of power dissipated.

Below is a simple Ltspice simulation of this:
Note that the current follows the voltage, and the instantaneous power dissipated in the resistor (red line) peaks at each peak of the sinewave with the power peaks being equal for both positive and negative voltage peaks.

Normally the thermal mass of the resistor is sufficient to average this power so the resistor's temperature will rise to what the same average power from a DC current would cause.

For the peak power below of 1W the average (in small window) is 0.5W.
This is where RMS values come from. The AC RMS value voltage and current (.707 of the peak value) give the identical heating in a resistor as the same DC value voltage and current would.

 

We define power dissipated as positive. so 4 MW power plants ate minus 4 MW when you apply math to it.

In any event, with an AC waveform and a purely resistive load the current will have the identical shape at every value of t.

When v(t) is positive and I(t) is positive, v(t)*i(t) is also positive.
When v(t) is negative and I(t) is negative, v(t)*i(t) is also positive.
So power is dissipated in the resistor no matter what the polarity.

In any event, if you flip the stuff below the x axis and mathematically average it, you get something called the RMS value of voltage and the RMS value of current. This would be the same value of a DC voltage that would need to be applied to get the same power dissipated.

You do have to worry about multimeters. Many assume a sine wave and multiply by a fudge factor. Unless they are TRMS voltmeters, it won't read the equivalent DC value. e.g. suppose the negative half-cycle is missing?
A TRMS meter will get the correct result. A non-TRMS meter will not.

 

well in my water pipe circuit the pipes tighten at the resistors. thus resistance.
obviously this is oversimplified at the point we are at now.

Seems the next thing I should try to understand is what is physically happening
inside the resistor allowing it to dissipate this excess current. Why does this process
generate heat. Again some form of friction perhaps?

 

Have you ever started a fire with a bow and a string? That's AC.

 

what is physically happening
inside the resistor allowing it to dissipate this excess current.

Resistors don't dissipate current. All the current that goes in one end comes out the other end. What doesn't come out the other end is the energy it took to shove the electrons through the resistor.

The definition of more heat is atoms vibrating faster than they used to. Shoving electrons through a resistance actually causes some friction and some jostling. Bingo! That is the definition of heat.

 

You guys did great! Now I am happy to say I have a solid basic understand of A/C.
I can see In my mind what is taking place. Thanks very much for the understanding.

 

You guys did great! Now I am happy to say I have a solid basic understand of A/C.
I can see In my mind what is taking place. Thanks very much for the understanding.

Keep reading, you're just at the beginning of understanding AC. It's very likely your current mental image is either very incomplete or wrong because this field of science is not very intuitive.

 

You guys did great! Now I am happy to say I have a solid basic understand of A/C.
I can see In my mind what is taking place. Thanks very much for the understanding.

Everybody tries. Some of us usually connect. Come back when the next puzzle pops up in your head.

 

We all know that it's enough to chew on for now.

You just need to remember that we discussed the special case of Alternating voltage and current that alternates as v(t) = A*Sin(ωt+θ) and i(t) = B*Sin(ωt+θ) where θ = 0 and ω=2π*f where f is the frequency. The load is purely resistive and A is the peak value of voltage and B is the peak value of current. Ohms law is obeyed for this special case. i.e. R=RMS(v(t) / RMS(i(t)) where t varies from 0 to 2*π and the function repeats. The sin() function has a maximum value of 1 and a minimum value of -1.

θ is known as the phase angle and ω is called the radian frequency. The frequency f is in the unit Hertz or cycles/second is just ω / (2*π); f= ω / (2*π)

 

jgreene44......the main basic thing to realize about ac circuits is.............the peek voltage and the peek current are almost always separated in time. That's why you need all that highfalutin math.

But it also allows us the vary voltage and current without generating heat. (or very little)

 

thumbs up to everyone I finally have a good basic understanding that I can visualize!
Much thanks!