AC vs DC: Same Rules, Different Frequency

In summary: But it's not the only kind of AC or the only math we can use. We can use FOURIER SERIES to make any repeating wave shape from sinewaves. And we can use FOURIER TRANSFORMS to make any shape at all from sinewaves. You can even make a square wave from sinewaves, if you use enough of them.So, in summary, AC and DC are not as different as they may seem. They both follow the same formulas and processes, with the only difference being the presence of a reactance vector in AC. AC can also be represented as a combination of different frequencies using Fourier series and transforms. It's important for students to understand that
  • #1
psparky
Gold Member
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AC vs DC!

The intent of this post is mostly for aspiring students:

When working with one of my students recently….I notice he tends to freak out between the difference of AC and DC……Like they are two completely different subjects. He says one has a frequency and the other doesn’t. I say not true….they all have many different frequencys….just in DC that frequency (ω) happens to be zero.

They are relatively the same in my mind. Other than the fact that maybe AC must be put in RMS before finding most results. And yes, AC is an alternating current….but it means very little for problem analysis.

The following formulas and processes apply to AC and DC:
V=IR
P=IV
V^2/R=P
P=I^2*R
Votage Dividers
Current Dividers
JωL
1/JωC

The biggest hang up is the capacitor and inductor. The sight of these in AC is enough to put any aspiring student in a complete frenzy. Why? In DC you are still using JωL and 1/JωC are you not? Right? Same for AC. Again, all those formulas above work for AC and DC. How could they not?

The only difference in AC is that you will have a reactance vector with magnitude and direction if there is an inductor or capacitor in the problem. You should know basic j plane addition and basic polar vector multiplication I would hope.

In DC…..V=IR

In AC…..V(ω)=I(ω)*R(ω)……and come to think of it….this is really the formula for DC….just the ω=0!

You work all the problems the same…..and yes….they can both shock the crap out of you!

Please stop putting AC and DC into two different categories. Yes, they have some differences……….but 95% of the rules apply to both equally!
 
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  • #2


Have you tried explaining to your student that there can be DC with overlying AC components, and vice-versa? (sometimes just a matter of degree) Things are not all black-and-white in electronics.
 
  • #3


Sure...good point.

A simple circuit with two voltage supplies...a battery and an AC source is a good example of this.

Even so...all these rules still apply:

V=IR
P=IV
V^2/R=P
P=I^2*R
Votage Dividers
Current Dividers
Reactance of inductors: JωL
Reactance of capacitors: 1/JωC
 
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  • #4


True, but your student may be freaked out with even that (minimal) level of detail. How about a few Powerpoint slides showing the wave-forms of pure AC, pure DC, (mostly) AC with a DC offset, and (mostly) DC with a superimposed AC ripple? Some students seem to grasp stuff when they see graphic representations. Then you can get into detail a bit more easily. Just a suggestion.
 
  • #5


He actually solved a reallly complex circuit today...AC voltage source with an AC current source with a capacitor and resistor in a parallel and series type connection.

Asked him the watts across the capacitor. Turns out I should been asking for VARS!
I am getting schooled at this very moment by my mentor as we speak!

But anyways...I asked for vars, watts, VA, and power factor used by the capacitor.

He did really well on it...got most of it right.

But still...theres this big hesitation between AC and DC when I bring it up. I can see the fear in his eyes. I was like...dude...chill out man. V=IR...relax.

But more or less trying to shout out to all aspiring students reading this thread!
 
  • #6


psparky said:
But more or less trying to shout out to all aspiring students reading this thread!
That's a worthy endeavor. Hope your student gets over that hurdle, though.
 
  • #7


I find DC and AC analysis to be two different things.

Why do they have DC sweep and AC sweep in simulators if they're the same thing?

Yes, you can simplify it and say DC is a frequency of 0, but that oversimplifies it in a lot of practical uses.

Plus, analyzing a DC circuit in steady state is more simple than analyzing an AC circuit. Take a circuit with diodes for example. Once you introduce an AC signal, you must consider many non-linear conditions, and the equations for the circuit no longer holds under all conditions.

I think it is perfectly understandable and expected for this to cause trouble for students. Perhaps if DC isn't taught exclusively for months before AC is introduced, the transition wouldn't be so confusing/scary?

I am finding lots of instances where I say "if I had know that before I learned this, it would have made a lot more sense". Sometimes because something is easier to learn doesn't mean it should be taught first.
 
  • #8


DragonPetter said:
I find DC and AC analysis to be two different things.

Why do they have DC sweep and AC sweep in simulators if they're the same thing?

Yes, you can simplify it and say DC is a frequency of 0, but that oversimplifies it in a lot of practical uses.

Plus, analyzing a DC circuit in steady state is more simple than analyzing an AC circuit. Take a circuit with diodes for example. Once you introduce an AC signal, you must consider many non-linear conditions, and the equations for the circuit no longer holds under all conditions.

I think it is perfectly understandable and expected for this to cause trouble for students. Perhaps if DC isn't taught exclusively for months before AC is introduced, the transition wouldn't be so confusing/scary?

I am finding lots of instances where I say "if I had know that before I learned this, it would have made a lot more sense". Sometimes because something is easier to learn doesn't mean it should be taught first.

I'm with you 100% bro. Good points.
 
  • #9


Why do they have DC sweep and AC sweep in simulators if they're the same thing?

Because a frequency of "zero" is very common. So they put a special button or selector knob for that particular frequency of interest.

:)
 
  • #10


I sometimes tell them to "freeze frame" their AC problem.

To effect:
"At any instant it's DC.
Current in capacitor is slope of voltage across it Δv/Δt ,
and voltage on inductor is slope of current thru it
and a sinewave is a special case, mathematically speaking. Just the sinewave is so common in nature and the math is soooo very convenient that it's become ubiquitous. We EE's are fortunate - an awful lot of what we use comes straight from Pythagoras.
I pity Mechanicals, their Ohm's Law is that umpteen-term Bernoulli equation.

So just bite the bullet and get good with your jω = 2∏f . You'll need to be fluent in phasor notation for sanity checks on the Laplace transform methods that are coming up. "



old jim
 
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  • #11


jim hardy said:
I sometimes tell them to "freeze frame" their AC problem.

To effect:
"At any instant it's DC.
Current in capacitor is slope of voltage across it Δv/Δt ,
and voltage on inductor is slope of current thru it
and a sinewave is a special case, mathematically speaking. Just the sinewave is so common in nature and the math is soooo very convenient that it's become ubiquitous. We EE's are fortunate - an awful lot of what we use comes straight from Pythagoras.
I pity Mechanicals, their Ohm's Law is that umpteen-term Bernoulli equation.

So just bite the bullet and get good with your jω = 2∏f . You'll need to be fluent in phasor notation for sanity checks on the Laplace transform methods that are coming up. "



old jim

jω = 2∏f?

I've never seen that before. I've always seen ω = 2∏f.

...or...S=jω

A clerical error I assume? Or am I missing something?
 
  • #12


No, you're quite right.

I am terribly awkward with words and have to edit things several times. It's like polishing rocks, and i didnt do enough to that post.

I have a bit of Demosthenes in me. Participating here is in no small part for my own self improvement. Hope you guys don't mind.

Thanks for your trolerance .

old jim
 
  • #13


jim hardy said:
No, you're quite right.

I am terribly awkward with words and have to edit things several times. It's like polishing rocks, and i didnt do enough to that post.

I have a bit of Demosthenes in me. Participating here is in no small part for my own self improvement. Hope you guys don't mind.

Thanks for your trolerance .

old jim

Jim,

Seeing as clearly know more than most of us on here...

We will learn to live with your "human side".

All good.

"Because we fail...we succeed."
 

1. What is the difference between AC and DC?

AC and DC refer to the types of electrical current. AC stands for alternating current, which changes direction periodically, while DC stands for direct current, which flows in only one direction.

2. Why do we use both AC and DC?

AC is typically used for long-distance transmission of electricity because it can be converted to different voltages easily. DC, on the other hand, is used for electronic devices that require a steady current, such as computers and cell phones.

3. What are the advantages of AC over DC?

One of the main advantages of AC is that it can be easily transformed to different voltages using a transformer, making it more efficient for long-distance transmission. AC also allows for the use of electric motors, which are essential for powering large machines and appliances.

4. How does frequency affect AC and DC?

Frequency refers to the rate at which the current changes direction in an AC circuit. Higher frequencies allow for more efficient transmission of electricity, but also require more insulation and specialized equipment. DC, on the other hand, has a constant frequency of 0 Hz.

5. Can AC and DC be used interchangeably?

No, AC and DC are not interchangeable. Some electronic devices, such as light bulbs, can function with either AC or DC, but others, like computers, require a specific type of current. Additionally, the infrastructure and equipment needed to transmit and use AC and DC are different.

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