# Camara Sample Exam #15



## jdd18vm (Oct 16, 2007)

Simple series circuit with a voltage source v(t)=(169.7) sin 377t in series with an open switch t=0ms and a 50 micro Farad Cap. An electric switch closes in 0ms and opens in 6Pi ms in the circuit shown. What is the total charge on the Cap

The lines for the solutions

q=Cv

=(50x10^-6) x ((169.7V) sin ((377s^-1)( 6Pi x 10^-3s)))

=(50x10^-6) (169.7V) sin 7.106

=50x10^-6) x (169.7V) (0.733)

=6.2 x 10^-3C

Can someone walk through these steps?

How does sin 7.106=0.733 in the 4th line

Maybe im inputting something wrong

John

Can I scan this stuff in rather than type out w/o getting into trouble?


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## Dark Knight (Oct 16, 2007)

jdd18vm said:


> Simple series circuit with a voltage source v(t)=(169.7) sin 377t in series with an open switch t=0ms and a 50 micro Farad Cap. An electric switch closes in 0ms and opens in 6Pi ms in the circuit shown. What is the total charge on the Cap
> The lines for the solutions
> 
> q=Cv
> ...


John,

I am out of home and do not have the EERM with me but I think that you have to solve that in radians mode. That is the reason for the discrepancy in the numbers. I don't remember the exact reason but if I am not wrong that dog did also bite me.

About the equation to solve it I think is a formula somewhere. It must be somewhere inside EERM. C is the capacitance and for the voltgae we have that equation in terms of the sine and all that crap.

Sorry I cannot be of more help. I am sure Jim will come to the rescue soon.


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## Flyer_PE (Oct 16, 2007)

jdd18vm said:


> How does sin 7.106=0.733 in the 4th line
> Maybe im inputting something wrong


John,

sin(7.106 radians) = 0.733

The charge on the capacitor is equal to the voltage multiplied by the capacitance in farads. All they are doing is determining the voltage value at t=6*pi ms.

This is one that requires you to keep track of the zeros and make sure your calculator is set up for radians instead of degrees.

Keep plugging. It's pretty stressful now but you're only 10 days away from getting blind-ass drunk. 

Jim


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## Dark Knight (Oct 16, 2007)

^^^^Told you arty-smiley-048: and I did not know Jim was logged in. Man I picked the wrong career.


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## jdd18vm (Oct 16, 2007)

Man that was easy enough, that explains another problem too....thanks again guys.

Jim you teach this crap (oh...I guess you ARE). I am totally intimidated that I have to know this stuff as you do. Obviously Im not there....

or I better get damn lucky

John


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## Flyer_PE (Oct 16, 2007)

I don't think I'm much of a teacher. I perform and review a lot of electrical calculations for the nuclear industry. It's a very anal industry. I end up doing a lot of method and math checking for relay settings and such. I also do hand calcs to verify computer modeling programs do what they are supposed to. It keeps the skills fresh.

From the questions you have been asking, I think you'll do fine on the test. The NCEES sample test is pretty representative. I studied those problems a lot. The Camara sample test is good for practice at working problems but it's not all that representative of what you will find on the test.

Good luck.

Jim


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## chicago (Oct 17, 2007)

John,

I don't mean to state the obvious alternative as I'm sure you know this, but whenever I encounter problems where there is pi in radians, I always substitute it with 180 degrees and do the calcs in degrees. That way, I don't make the mistake of forgetting to convert in a high pressure situation we are going to face in 9 days and counting...

Just my 2 cents.


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## benbo (Oct 17, 2007)

chicago said:


> John,
> I don't mean to state the obvious alternative as I'm sure you know this, but whenever I encounter problems where there is pi in radians, I always substitute it with 180 degrees and do the calcs in degrees. That way, I don't make the mistake of forgetting to convert in a high pressure situation we are going to face in 9 days and counting...
> 
> Just my 2 cents.


Maybe I'm unclear on what your saying but I think you have to be careful. A lot of these formulas are in terms of w (omega). A simple example - what if they simply ask you to calculate the impedance of some capacitor or inductor at some frequency f. You have to convert that to w at some time to get the answer in ohms, don't you? Oh wait, never mind - you would just use the formula Z = 2pi*f*L. Carry on. My mistake.


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