Peak or RMS

[Dave Simpson]

Hello Mike,
        I would just stick with the coursework as it progresses. Pointless trying to leap ahead of your fellow course students. As your teacher/instructor will be working to guidelines/course notes which ensure that you've fully grasped the RMS principals now, before going on to more complicated coursework. Peak to Peak studies, rectification, & the formulae involved will no doubt be soon revealed. 
        From a vintage radio hobby/pursuit point of view - don't even think of getting involved with valve testers & testing for a long while yet. Its a minefield of confusion RMS/P-P wise. 

                                                                           Regards,   David

[BusyBee]

In many ways I would agree David. Over the years I learnt that the primary objective was to pass the course, not to understand the subject. It sounds contrary but, from experience, often true. This is often a regular quote for driving tests - Pass the test then learn to drive!

My suggestion would always be to do what you feel and investigate what interests but don't try unduly to confuse the issue in the classroom. Training staff don't handle that well! Also from experience I found this separate study helped greatly with understanding. Learning is about investigating. I have found, over the years, that teaching staff can often be at the limits of their understanding. It is good reseach to find out the background of your tutor. You would only have to do a specialised project of your choice to find out if they are limited so it's good to know before that may happen.

Tracy

[Murphyv310]

I agree that many trainers are rigid on the classwork and often get stroppy if one class member starts asking irrelevant questions or tries to be smart.
They to have guidelines to follow and if they get sidetracked then it means things can get rushed.
Tracy's comment about the driving test sums it up in a nutshell.
I'd just do the course work then after passing do your own research.

[Kalee20]

Oddly enough there is no traceable definition of RMS voltage. The only way that NPL/PTB/NIST etc do it is using a thermal transfer standard. Because DC voltage is defined in terms of the Josephson effect to umpteen places of decimals, you compare the heating effect of the AC voltage being calibrated with the heating effect of a DC voltage derived from the Josephson standard.

I'd disagree here on two counts.

Firstly, the volt isn't a basic unit defined in terms of Josephson effect to umpteen places of decimals (it may numerically equal this, but that's not its definition).

A volt is the potential difference which exists if one joule of energy is produced or absorbed, by moving one coulomb of charge.

One joule is a derived unit, from metre, second, and kilogram (actually 1 kg m² / sec²) and one coulomb is a derived unit, from ampere and second (actually 1 A sec).

Metres, seconds, kilograms, amps, are all fundamental units (the amp was actually redefined last year in terms of the coulomb! Up to then, it was defined in terms of the force between two current-carrying wires).

So the volt is a derived unit.

Secondly, once a volt is defined, then RMS volts are easy to define. For a voltage constant with time, then RMS volts = actual volts. For a voltage which varies periodically with time, the RMS volts = square root of the average over one cycle, of the square of the voltage.

So to measure it, just take loads of samples over a cycle, square each sample, find the average of this, and take the square root. The error is relatively straightforward to calculate from the mathematical exact figure, by knowing how much the voltage varies between samples.

Coming back to the OP's question, yes, a voltage is assumed RMS by convention, unless otherwise stated.

[Crackle]

Hello Mike,
        I would just stick with the coursework as it progresses. Pointless trying to leap ahead of your fellow course students. As your teacher/instructor will be working to guidelines/course notes which ensure that you've fully grasped the RMS principals now, before going on to more complicated coursework. Peak to Peak studies, rectification, & the formulae involved will no doubt be soon revealed. 
        From a vintage radio hobby/pursuit point of view - don't even think of getting involved with valve testers & testing for a long while yet. Its a minefield of confusion RMS/P-P wise. 

                                                                           Regards,   David

Dave
I think you missed the point, it is needed to know for the calculations on AC, if for instance an AC voltage figure is given as 20V are they expecting me to change it to the RMS value by dividing by .707 or not in order to answer the question.

Mike

[Mike Watterson]

The exams and RSGB are broken if voltages are not ALREADY RMS unless stated otherwise. That is the absolutely standard approach. Unless it says Peak to Peak, or something else, 20 V unqualfied is assumed to be already an RMS value.

[Murphyv310]

If its given as 20v Mike its the RMS figure.
If its Peak or any other classification that should be stated. Without any other classification it is always RMS.


Crossed with Michael. Same answer but written differently.

[Crackle]

Hmm I agree that's is how it should be, but is it in the exam???

Mike

[Kalee20]

It ought to be in the exam, too.

If an AC voltage is given as 20V, and you have to calculate another voltage, just state it as a voltage. If the question happened to be peak, your answer will be peak. If the question happened to be RMS, your answer will be RMS.

[Crackle]

Not so easy to decide if you are working with power. e.g. V squared / R, hopefully there wont be a question like that.

Mike