Any TRICK problems on exam?

[Phatso86]

As i'm practicing some problems, I realize that values of V and I are sometimes given in RMS and other times in PEAK.

problem is, it is always an assumption(I realize this by looking at solutions. Has there been questions where the answers are off by a factor of sqrt(2) ? Or does the question specify either PEAK or RMS?

 

[cupojoe PE PMP]

On the power exam I'd expect to see questions that are off by a sqrt(3) more so than sqrt(2). But I suppose either are possible. It is a safe to assume that any problem requiring calculations will have wrong answers that could be calculated easily by mistake, i.e. dividing by sqrt(3) rather than multiplying by sqrt(3). Other possible errors are incorrectly placed negative signs and trigonometry tricks when converting into or out of polar notation. You will have to know your stuff.

I'm not sure what you mean by "it is always an assumption." The problem should give you enough information to know, though not always explicitly spelled out. If you are looking at a specific problem, perhaps someone can help you identify how you know (or possibly if the book you are using has a mistake).

 

[Phatso86]

On the power exam I'd expect to see questions that are off by a sqrt(3) more so than sqrt(2). But I suppose either are possible. It is a safe to assume that any problem requiring calculations will have wrong answers that could be calculated easily by mistake, i.e. dividing by sqrt(3) rather than multiplying by sqrt(3). Other possible errors are incorrectly placed negative signs and trigonometry tricks when converting into or out of polar notation. You will have to know your stuff.

I'm not sure what you mean by "it is always an assumption." The problem should give you enough information to know, though not always explicitly spelled out. If you are looking at a specific problem, perhaps someone can help you identify how you know (or possibly if the book you are using has a mistake).

the problems i'm working with do not say anything. The only possible clue is that the given value is shown as: 10*sqrt(2)

no where does it say that it is peak or rms. so aside from assuming that the sqrt(2) makes it peak, there is nothing to prove that it is.

 

[cupojoe PE PMP]

Post an example question...word for word.

 

[Phatso86]

here is another problem I found.

there is zero regarding RMS or max.

this will definitely kill me on the test if its completely ambiguous

View attachment 8591

 

[cupojoe PE PMP]

This looks like it comes from a text book rather than a study guide for the exam. In my experience the questions you should be more prepared to answer are a lot more practical than the way this question is structured. This feels like something from a second level circuits class. But either way.... the magnitude on phasors are always expressed in RMS and the angle are usually expressed in degrees. In my opinion, when you are talking about power systems you should always assume RMS and not peak to peak unless you are instructed otherwise.

 

[Phatso86]

This looks like it comes from a text book rather than a study guide for the exam. In my experience the questions you should be more prepared to answer are a lot more practical than the way this question is structured. This feels like something from a second level circuits class. But either way.... the magnitude on phasors are always expressed in RMS and the angle are usually expressed in degrees. In my opinion, when you are talking about power systems you should always assume RMS and not peak to peak unless you are instructed otherwise.

got it

I should pay attention to the practice questions then. The problem above was in peak

 

[knight1fox3]

This looks like it comes from a text book rather than a study guide for the exam. In my experience the questions you should be more prepared to answer are a lot more practical than the way this question is structured. This feels like something from a second level circuits class. But either way.... the magnitude on phasors are always expressed in RMS and the angle are usually expressed in degrees. In my opinion, when you are talking about power systems you should always assume RMS and not peak to peak unless you are instructed otherwise.

Textbook response. :thumbs:

 

[matt267 PE]

View attachment 8591

 

[Panchito]

The voltage expression that is given in that problem is neither peak nor RMS. It is the instantaneous value of the voltage as a function of time. Therefore, the peak value occurs when the cosine function is 1. So Vab_peak = 679 V.

 

[knight1fox3]

Get back to work interloper!!! ldman:

 

[Phatso86]

The voltage expression that is given in that problem is neither peak nor RMS. It is the instantaneous value of the voltage as a function of time. Therefore, the peak value occurs when the cosine function is 1. So Vab_peak = 679 V.

yes, it's peak. the issue is that one must assume this

 

[Panchito]

Phatso, you don't need to make any assumption to determine that the peak voltage is 679V. If you are given the INSTANTANEOUS voltage as a function of time, you can easily determine the peak value of the voltage without having to assume anything. If you plot the voltage expression given above, you will get a sinusoidal wave with Vmax = 679, and Vmin = -679. Therefore, the peak value is 679V. No assumption has to be made.

Remember, a sinusoidal function is expressed as follows:

V(t) = Vo*sin(2*pi*f*t + phi)
or
V(t) = Vo*cos(2*pi*f*t + phi)

Where Vo is ALWAYS the peak value of the wave.

The RMS value is:

Vrms = 0.707 * Vo

(RMS voltage is not a function of time)

 

[Phatso86]

Phatso, you don't need to make any assumption to determine that the peak voltage is 679V. If you are given the INSTANTANEOUS voltage as a function of time, you can easily determine the peak value of the voltage without having to assume anything. If you plot the voltage expression given above, you will get a sinusoidal wave with Vmax = 679, and Vmin = -679. Therefore, the peak value is 679V. No assumption has to be made.

Remember, a sinusoidal function is expressed as follows:

V(t) = Vo*sin(2*pi*f*t + phi)
or
V(t) = Vo*cos(2*pi*f*t + phi)

Where Vo is ALWAYS the peak value of the wave.

The RMS value is:

Vrms = 0.707 * Vo

(RMS voltage is not a function of time)

there are many problems where voltage or current is given as RMS as a function of time.

that's why it requires an assumption.

 

[cupojoe PE PMP]

I think you are more interested in arguing than understanding. In this case you are given a signal in time domain you are asked to find the phasors. I don't see what the issue is here...

 

[Phatso86]

I think you are more interested in arguing than understanding. In this case you are given a signal in time domain you are asked to find the phasors. I don't see what the issue is here...

Can't understand if the issue isn't addressed. I'll point out the issue again:

If you read my original question: "Any TRICK problems on exam?" and the follow up "Has there been questions where the answers are off by a factor of sqrt(2) ?" you'll see that no one indicated yay or nay.

if you read my previous response it suggests that some sample questions give sine functions with RMS value and some sample questions give sine functions with Peak value with no indication of either or. (That is the whole reason I'm posting my question)

What is the confusion here?

 

[matt267 PE]

What is the confusion here?

I'm confused. If your discipline is "structural," why are you taking the Electrical exam?

 

[Phatso86]

Can't understand if the issue isn't addressed. I'll point out the issue again:

If you read my original question: "Any TRICK problems on exam?" and the follow up "Has there been questions where the answers are off by a factor of sqrt(2) ?" you'll see that no one indicated yay or nay.

if you read my previous response it suggests that some sample questions give sine functions with RMS value and some sample questions give sine functions with Peak value with no indication of either or. (That is the whole reason I'm posting my question)

What is the confusion here?

I suppose I need to mention this again:

The sample problems do not indicate whether the given signal is peak or rms, so the solutions randomly divide by sqrt(2) and other times it does not. Sometimes the given value is divided by sqrt(2) and we get a nice number, sometimes its a nice number (such as 10) and its divided by sqrt.

So judging by the sample questions, it could be completely random if I am given peak or rms (again, its given as a sine function).

I am interested in knowing if these issues have been encountered in the PE.

 

[Phatso86]

I'm confused. If your discipline is "structural," why are you taking the Electrical exam?

two reasons:

1) it'd be nice to have that qualification on my license for when I do certain inspections in south florida. Don't need it, but I want to have it.

2) qualifies me for electrical contractor license. This is the main reason

 

[Phatso86]

Well, of course I came across a tricky problem on the Complex Imaginary sample tests

it says motor is 60Hz, 125V. what is the peak to peak?

of course I got it wrong since I assumed 125V is instantaneous. I chose 250V, but answer was sqrt(2)*250V