# Any TRICK problems on exam?



## Phatso86 (Sep 7, 2016)

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?


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## cupojoe PE PMP (Sep 7, 2016)

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).


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## Phatso86 (Sep 7, 2016)

cupojoe PE PMP said:


> 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.


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## cupojoe PE PMP (Sep 8, 2016)

Post an example question...word for word.


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## Phatso86 (Sep 10, 2016)

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


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## cupojoe PE PMP (Sep 10, 2016)

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.


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## Phatso86 (Sep 10, 2016)

cupojoe PE PMP said:


> 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


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## knight1fox3 (Sep 11, 2016)

cupojoe PE PMP said:


> 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:


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## matt267 PE (Sep 12, 2016)

Phatso86 said:


> View attachment 8591


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## Panchito (Sep 12, 2016)

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.


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## knight1fox3 (Sep 12, 2016)

Get back to work interloper!!! ldman:


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## Phatso86 (Sep 12, 2016)

Panchito said:


> 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


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## Panchito (Sep 12, 2016)

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)


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## Phatso86 (Sep 12, 2016)

Panchito said:


> 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:
> 
> ...


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

that's why it requires an assumption.


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## cupojoe PE PMP (Sep 13, 2016)

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...


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## Phatso86 (Sep 14, 2016)

cupojoe PE PMP said:


> 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?


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## matt267 PE (Sep 14, 2016)

Phatso86 said:


> What is the confusion here?


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


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## Phatso86 (Sep 14, 2016)

Phatso86 said:


> 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.
> 
> ...


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.


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## Phatso86 (Sep 16, 2016)

matt267 PE said:


> 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


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## Phatso86 (Sep 17, 2016)

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


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## Ken PE 3.1 (Sep 17, 2016)

To answer your original question, yes, there are tricky questions on the test.

They need to know that you will not fall for the trap questions and understand the question at hand.


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## Phatso86 (Sep 18, 2016)

Ken PE 3.0 said:


> To answer your original question, yes, there are tricky questions on the test.
> 
> They need to know that you will not fall for the trap questions and understand the question at hand.


well I hope they have some details. Unlike some practice problems, especially the one I posted, there is ZERO to go by. Gets pretty stressful thinking that I could fail a test due to lack of detail or making the wrong assumption

sigh...


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## Ken PE 3.1 (Sep 18, 2016)

There are assumptions made all over the test. That is what the 4 years of work experience after 4 years of education are supposed to teach you.


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## Ultrafault (Sep 18, 2016)

The two examples you gave would be obvious to an electrical engineer. The first describes a sign wave. The second is a motor rating. With the proper education and experience to qualify for the electrical PE. An engineer would know motor ratings are in rms and the relationship between a sign wave and peak to peak or rms values. You can do this, but you will have to study more.


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## Phatso86 (Sep 18, 2016)

v



Ken PE 3.0 said:


> There are assumptions made all over the test. That is what the 4 years of work experience after 4 years of education are supposed to teach you.


I like how you fail to address or acknowledge that the SAMPLE problems (emphasis on sample, meaning it's not a real world scenario) make assumptions as to whether it is RMS or Peak. Must I point out that identical problems still have different assumptions?

perhaps it is not obvious to you that work experience is not equivalent to sample problems


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## Phatso86 (Sep 18, 2016)

Ultrafault said:


> The two examples you gave would be obvious to an electrical engineer. The first describes a sign wave. The second is a motor rating. With the proper education and experience to qualify for the electrical PE. An engineer would know motor ratings are in rms and the relationship between a sign wave and peak to peak or rms values. You can do this, but you will have to study more.


i have already mentioned that some problems give me an RMS sine wave function. Must I post a problem so this incorrect explanation isn't brought up again?

"With the proper education and experience to qualify for the electrical PE."

this is also incorrect, btw. you simply need ANY engineering degree and ANY engineering experience. At least in my state.

"An engineer would know ... the relationship between a sign wave and peak to peak or rms values."

this is a math issue, not an engineering one. I suppose this wasn't obvious.


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## Ken PE 3.1 (Sep 18, 2016)

Someone is talking a lot of smack for being a structural engineer asking BASIC electrical questions.

Ok, so try this one on for size: it is assumed to be rms unless otherwise noted, unless it isn't.


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## matt267 PE (Sep 18, 2016)

opcorn:


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## Ultrafault (Sep 18, 2016)

Sorry, I meant an electrical engineer would know those things. I am not trying to give you flack only to express that with patience you can learn those things as well.


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## cupojoe PE PMP (Sep 19, 2016)

Ken PE 3.0 said:


> Someone is talking a lot of smack for being a structural engineer asking BASIC electrical questions.
> 
> Ok, so try this one on for size: it is assumed to be rms unless otherwise noted, unless it isn't.


I said it before, the OP seems to be more interested in arguing than getting help...glad I'm not the only one who was thinking this.


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