Peak-to-peak, peak, magnitude, amplitude?

[Chumly]

Hello,

I teach at a local technical institute and when it comes to both AC and DC versions of sine waves, square waves, triangle waves and saw tooth waves the terms "magnitude" and/or "amplitude" seem inconsistent. There also seems to be inconsistency in defining "magnitude" and/or "amplitude" as it relates to half wave or full wave pulsating DC.

This causes confusion as the technical institute's publications use both "magnitude" and "amplitude" but fails to provide in-context explanations. However it seems to me there is no need to use "magnitude" and/or "amplitude" to describe the aforementioned waves because "peak" and "peak-to-peak" are just fine!

Do you'all have a consistent way to consider both "magnitude" and "amplitude" as it relates to all the aforementioned waves?

 

[crutschow]

Magnitude and amplitude are rather generic and somewhat interchangeable terms that do not refer to a specific value definition. That is added when you show specific values. RMS, average, peak, peak-to-peak, db, etc. are some of the value definitions used for the magnitude and/or amplitude of electrical signals. Thus you can say the magnitude of the signal is 20dBm or the amplitude is 20V RMS, for example.

 

[Chumly]

I would agree yet I am stuck with the institute supplied texts and exams that reference both "magnitude" and "amplitude" in a non-generic way that requires specific value definition but does not provide proper case by case in-context clarity!

By that I mean they do not give an in-context description of what they intend the word to mean, so one is left taking an educated guess. This not the end of the world for me because I understand most inferences, but it's understandably frustrating for the students!

Now in the lab we use true RMS meters all the time to read more complex waveforms so as to get their effective equivalence. However this is not something that can be done on an exam or when studying texts.

The institute supplied texts and exams not only use the word "magnitude" in the context of vectors, but use both "magnitude" and "amplitude" when referencing any number of waveforms including both DC and AC waveforms, and DC offset waveforms!

In fact the institute supplied texts and exams sometimes use the word "magnitude" when simply referring to the meter reading on an AC voltmeter...is that annoying or what!

Consider that when looking at a scope either "magnitude" or "amplitude" could mean the entire size of the waveform (Peak to Peak), or only the size of the waveform from the horizontal X axis (Peak). Then consider looking at a scope and trying to apply either "magnitude" or "amplitude" to any number of waveforms including both DC and AC waveforms, and DC offset waveforms!

Then consider the number of ways you might define a DC offset square wave in terms of either "magnitude" or "amplitude"!

Further, consider this problem in the context of a student looking at various waveforms drawn against a scope grid on an exam paper, and you see it can be a dog's breakfast!

 

[MrAl]

Hi,

I was going to say the same thing everyone else did here already i think...

Magnitude and amplitude are more general terms that refer to the size of something, while peak and peak to peak (and others) are more specific terms that are more descriptive.
For example, we can have the magnitude or the amplitude given in either peak units or peak to peak units, or even rms or average.

"The amplitude of the signal was very close to five volts peak to peak".

Magnitude is also a bit more general than amplitude. Magnitude can refer to something that doesnt really have what we might call an amplitude, but then again we might anyway.

"The magnitude of the vector was 5" (we didnt actually say what we were talking about yet so it's a little hard to call it an amplitude yet).

We can probably always say, "The magnitude of the vector was 5", but we can not always say, "The amplitude of the vector was 5",because it may not be describing an amplitude of something. It would sound a little peculiar to say something like, "The diagonal of the box had an amplitude of 5".

 

[Gary B]

May I suggest that while amplitude refers to the instantaneous voltage at any time, magnitude implies either the maximum amplitude or the area under the curve depending upon context? Of course, to make things difficult, I have seen articles and books over the years using the two terms exactly reversed and even a few that used them interchangeably in the same article.

For the sake of your students, I would stick to amplitude since that appears to be the most popular term recently and define if you are referring to peak, peak to peak, or RMS in any given sentence.

 

[BrownOut]

I understand the frustration over terminology. As a kid trying to make sense of all this electronic goobly-goop, I too was often frustrated. In my many years of engineering, I've rarely used those terms, prefering to simpy use "voltage" and "current." Unfortunately, those terms turn up in places where they should not! Magnitute describes the measure of a quantity in a complex system, as in "magnitute and phase angle." Amplitutde is, I think, just another way of saying the same thing. But in systems where the resulting quantity is only real, then magnitute isn't an appropriate way to describe the quantity, despite being technically correct, it's use can be confusing. Sorry, I don't have a solution. Text authors should just be mindful or thier use of the terminology. But I'm happy to hear that at least someone is trying to find a way to clear the fog.

 

[Hero999]

I'm surprised the students don't write snotty comments on their exam papers - I certainly would!


Question:what's the magnitude of the waveform on the oscilloscope plot?

My answer:
What do you mean? Please use the correct terminology.

It's a sine wave.

The peak to peak voltage is 1V.

The peak voltage is 0.5V

The RMS voltage 1.707V.

There's a DC bias of 1V

I shouldn't have to go through all that crap, if you'd used the correct terminology in the first place, I'd know which figure to give.

I suppose I'd only do that once, perhaps after I've answered all the questions (I'd leave some space), just to make my point, I wouldn't want to waste my time and not be able to complete the exam.

 

[Chumly]

Thanks most kindly for the thoughtful responses. I have to wade through a whole series of needlessly florid technical terms and present the results to my students. However magnitude versus amplitude particularly sticks in my craw!

More annoying examples:
1) true power = effective power = RMS power = W
2) reactive power = volt-amps reactive = quadrature power = VARs
3) apparent power = volt-amps = VA

I could go on...and on...and on...but...

Tautology (rhetoric), using different words to say the same thing twice where the additional words fail to provide additional clarity when repeating a meaning

https://en.wikipedia.org/wiki/Tautology