How does AC compare to DC in terms of power and energy flow in a circuit?

[Femme_physics]

Is that right to see AC as current that flows like that? (marked in red)


http://img864.imageshack.us/img864/5056/10vac.jpg

Whereas DC would just be a straight line.


Is that right?

 

[dlgoff]

No. Not really.

Here's some animations though.

http://www.animations.physics.unsw.edu.au/jw/AC.html"

And this one.

http://www.absorblearning.com/media/attachment.action?quick=oy&att=1787"

 

[dlgoff]

I'm not to fond of animations but here's a Wikipedia one for three phase "power flow".

[URL]http://upload.wikimedia.org/wikipedia/commons/4/48/3-phase_flow.gif[/URL]

 

[sophiecentaur]

One misleading thing about that animation is that the electrons move at a few mm/second maximum and they end up moving a microscopic distance each cycle.
Also, it's Current flow that they are showing - not Power flow. The Power goes steadily from generator to load, not back and forth; there's no generator in your house!

 

[Jiggy-Ninja]

I'm not to fond of animations...

Why? They are the most natural way to demonstrate phenomena that varies by time. Like...AC waveforms.

One misleading thing about that animation is that the electrons move at a few mm/second maximum and they end up moving a microscopic distance each cycle.
Also, it's Current flow that they are showing - not Power flow. The Power goes steadily from generator to load, not back and forth; there's no generator in your house!

EVERY illustration and animation is going to have something drawn out of scale so that it's easier to see. Most of the illustrations used to begin an ADC/DAC topic show only 2-3 bit conversion, despite the fact that most ADC/DACs are at least 8 bit. For conceptual understanding, 2 bit is much easier to see than 8 bit.

For a lot of difficult to understand things, drawing them exactly to scale would be useless.

 

[sophiecentaur]

You are right except that people tend to think that the electrons go racing up and down the supply cables from generator to load and back. A caveat is needed, I think.

And how about my comment about Current and not Power? You can't disagree with that, can you?

 

[dlgoff]

One misleading thing about that animation is that the electrons move at a few mm/second maximum and they end up moving a microscopic distance each cycle.

Exactly. Thanks for this very important fact.

Not an animation but...

Read more about this here:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html"

and here:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html"

 

[Jiggy-Ninja]

You are right except that people tend to think that the electrons go racing up and down the supply cables from generator to load and back. A caveat is needed, I think.

Drift velocity is a rather unimportant value for most electronics work, isn't it? Though I do agree that the exaggerations should be mentioned along with the illustration.

And how about my comment about Current and not Power? You can't disagree with that, can you?

I can't, that's why I didn't.

 

[sophiecentaur]

So I can't get stroppy can I?

 

[Jiggy-Ninja]

So I can't get stroppy can I?

Um...yes? If I knew what that meant...

*checks dictionary*

No, it looks like you can't.

 

[MisterX]

Hi,

Are you asking if the voltage and current are in a wave pattern around the circuit at a fixed time?

It depends on the wavelength and the size of the circuit. Voltage waves waves travel through the circuit at the speed of light in the conductor. If the circuit size is significantly close to the wavelength, or is the size is bigger than the wavelength, then the voltage and current may have significant wave patterns along the length of the circuit.

This is covered by transmission line theory, which is taught to people after they learn basic circuits.

When people first learn to solve AC problems, this theory is usually not considered.

Instead, for a certain time, the current will be the same at everywhere along a path that does not branch.

I made this picture to explain AC voltage in this circuit, using the basic method:

 

[Femme_physics]

I'm quite confused by the chain of replies. The links confused me even more. Was I "right" or "wrong"?

In this case, does it flow the same way as in DC, just that it tends to go forth and back in the circuit?

Are you asking if the voltage and current are in a wave pattern around the circuit at a fixed time?

It depends on the wavelength and the size of the circuit. Voltage waves waves travel through the circuit at the speed of light in the conductor. If the circuit size is significantly close to the wavelength, or is the size is bigger than the wavelength, then the voltage and current may have significant wave patterns along the length of the circuit.

I'm not sure how to relate wavelength and AC right now. I originally thought that AC is wave-like current, and DC is just a straightforward current. I am aware of calculations such as Vpeak-to-peak, but maybe it's a bit beyond me right now. We've only started AC.

This is covered by transmission line theory, which is taught to people after they learn basic circuits.

With respects to all the pics and imgs I've seen, they all show completely different stuff, which just confused me further!

When people first learn to solve AC problems, this theory is usually not considered.

Solving problems is more important to me, for sure :) But right now, I'm not at the HW section

 

[sophiecentaur]

I could say that your initial picture is wrong as waves do not wiggle from side to side as you have drawn them.
In DC, there is a steady, very slow, drift of electrons in one direction. In AC, there is no net movement of electrons - just bacwards and forwards. BUT the electrons are also moving about at a range of speeds and in all directions - much more than the drift speeds due to the 'electrical current'.

The only 'wave' associated with 50Hz AC has a wavelength of 3e6/50m. Very big, you'll agree, and much larger than any circuit you are likely to build at home.

 

[MacLaddy]

As the electrons move forward in the wire you have a positive charge, and they move backwards you have a negative charge. The current is alternating back and forth.

If you were to graph that as a function of time you would end up with an oscillating sine wave. Up-down related to forward-back.

That's probably an oversimplification, but it's how I understand it.

 

[sophiecentaur]

You may have to re-think that bit about a charge. There are always the same number of electrons in a wire - as one enters one end, another leaves at the other end. (That is when the frequency of the signal is low, like the mains, or DC.)

The "up-down" you refer to is just for the purposes of the current / time graph - not reality.

 

[MacLaddy]

You may have to re-think that bit about a charge. There are always the same number of electrons in a wire - as one enters one end, another leaves at the other end. (That is when the frequency of the signal is low, like the mains, or DC.)

The "up-down" you refer to is just for the purposes of the current / time graph - not reality.

I understand that there is always an equal amount of electrons. Not sure how you think I conveyed that they are lost.

I was simply expressing the relationship of the alternating current to a sine wave, as I think that is where the initial confusion comes from. People see a/c described as a wave and assume that's how it flows. I know, because I used to be one of em.

 

[sophiecentaur]

I understand that there is always an equal amount of electrons. Not sure how you think I conveyed that they are lost.

.

You wrote " As the electrons move forward in the wire you have a positive charge, and they move backwards you have a negative charge."

 

[Femme_physics]

So if the electrons are moving forwards as well as backwards, does this mean AC can't be used as a power source? Because, as they move in towards a consumer, they suddenly move back...and never get to reach it.

 

[turbo]

So if the electrons are moving forwards as well as backwards, does this mean AC can't be used as a power source? Because, as they move in towards a consumer, they suddenly move back...and never get to reach it.

Devices that run on AC are designed to exploit this oscillation (60 hz in the US), and AC definitely is a power source. Electric lights, refrigerator compressors, electric ranges, fans, etc, all consume power.

 

[Jiggy-Ninja]

So if the electrons are moving forwards as well as backwards, does this mean AC can't be used as a power source? Because, as they move in towards a consumer, they suddenly move back...and never get to reach it.

Positive Voltage * Positive Current = Positive Power

Negative Voltage * Negative Current = Positive Power

It's when you have Voltage and Current of opposite sines that you get negative power, but that only happens when there is reactance in the line.

 

[sophiecentaur]

So if the electrons are moving forwards as well as backwards, does this mean AC can't be used as a power source? Because, as they move in towards a consumer, they suddenly move back...and never get to reach it.

Think of a mechanical hand crank. Your hand may go backwards and forwards, not 'getting anywhere' but you still input work into the system you are Powering.
It's the Power that is transferred in AC circuits - not the Current.

 

[Femme_physics]

@ Jiggy & Ninja - you'd have to excuse me as I only understand this reply best so far:

Think of a mechanical hand crank. Your hand may go backwards and forwards, not 'getting anywhere' but you still input work into the system you are Powering.
It's the Power that is transferred in AC circuits - not the Current.

I see. So, it's a different kind of electrical energy.

Is ohm law still applied in AC? Are the gamerules changed drastically?

 

[I like Serena]

I see. So, it's a different kind of electrical energy.

Is ohm law still applied in AC? Are the gamerules changed drastically?

Ohm still applies in AC.

Capacitors show more interesting behaviour. ;)

 

[dlgoff]

Is ohm law still applied in AC?

Yes. But instead of the resistance R of a d.c. circuit, you use the impedance Z of a a.c. circuit.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html"

 

[sophiecentaur]

It's exactly the same stuff as DC and the game rules are exactly the same except that the Reactances of circuit components need to be considered also, when dealing with AC. Look upon DC as and AC with a very low frequency - after all, unless a DC current has been flowing for all time, it's actually varying - just very slowly.

Power still = V * I (as Jiggy-Ninja wrote)
I think that you are wanting a far too 'pictorial' view of all this and this is letting you down.

 

[Jiggy-Ninja]

Consider a purely resistive element with an AC voltage applied to it. You can see more on this website: http://www.play-hookey.com/ac_theory/ac_resistors.html

For the first half of the wave, the voltage is positive, say 10 V_RMS. The current is also positive, say 1 A_RMS.

The power is given by P = VI. 10V * 1A = 10 W. That's positive power, representing power taken from the source.

At the second half of the wave, the voltage goes negative, and the current reverses with it.

-10V * -1A = 10W, a positive value of power again.

With inductors and capacitors, the voltage and current have a 90 degree phase shift, to the power relationship is a little more complicated. View this page now: http://www.play-hookey.com/ac_theory/ac_capacitors.html and look at the graph with red and blue lines. Red line is voltage, blue line current.

During the first quarter of the voltage wave, both voltage and current are positive. That's positive power, representing energy being taken from the source.

During the second quarter, voltage is still positive, but the current has reversed and become negative. This is negative power, representing energy being returned to the source.

During the third quarter, the voltage becomes negative too. Both current and voltage re negative, so the power is now positive, and energy is being taken from the source.

At the final quarter, voltage remains negative, but current swings up to positive, resulting in negative power, and energy is returned back to the source. Then the cycle repeats again. The situation is similar for inductors.

In a perfectly reactive circuit, the periods of positive power and negative power exactly cancel each other, so no energy is lost. No circuit is purely reactive though, so any practical circuit will have resistance in it, resulting in less negative power and energy dissipation.