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AC vs DCby psparky
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#1
Apr312, 02:23 PM

PF Gold
P: 752

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!!! 


#2
Apr312, 02:55 PM

PF Gold
P: 7,363

Have you tried explaining to your student that there can be DC with overlying AC components, and viceversa? (sometimes just a matter of degree) Things are not all blackandwhite in electronics.



#3
Apr312, 03:20 PM

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P: 752

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 


#4
Apr312, 03:31 PM

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P: 7,363

AC vs DC
True, but your student may be freaked out with even that (minimal) level of detail. How about a few Powerpoint slides showing the waveforms 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
Apr312, 03:45 PM

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P: 752

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
Apr312, 03:50 PM

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#7
Apr312, 04:29 PM

P: 834

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 nonlinear 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. 


#9
Apr312, 04:47 PM

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:) 


#10
Apr312, 08:59 PM

Sci Advisor
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P: 3,648

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 umpteenterm 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 


#11
Apr412, 06:34 AM

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P: 752

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
Apr412, 11:21 AM

Sci Advisor
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P: 3,648

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 dont mind. Thanks for your trolerance . old jim 


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