Need help with AC Circuits Calculations plz!

Discussion in 'Homework Help' started by Flupps, Jan 21, 2014.

  1. Flupps

    Thread Starter New Member

    Nov 14, 2013
    8
    0
    I'm having trouble with a couple calculations I'm working on for my AC Circuits homework.

    Q1: The frequency of the AC mains in the European countries 50 (Hz) and the effective value of the voltage is 220 (V). Due to turbine control disruption of one the power generation facility the frequency was fluctuating; calculate the period of the waveform when the frequency is (5.15x10^1) (Hz). Enter the answer in seconds (s) rounded to 3 significant digits.

    * so I did P = 1/f = 1/51.5Hz = 0.194s which was wrong
    * the program they're using for online quizzes requires these types of answers to be in scientific notation for some reason, so I put 1.94x10^-1

    Q2: Calculate the instantaneous value at t=(1.0400x10^1) (ms) of the current in a heating element connected to (1.000x10^2) (V) 60.0 (Hz) AC mains. The resistance of the heater is R=(2.50x10^1) (Ω) Enter the answer in Amperes rounded to 3 significant digits.

    * I tried calculating the instantaneous voltage first: v=Vmsin(2*pi*f*t) = 100 sin (2*pi*60*10.4ms) = 6.84V
    * Then divided by resistance to get current = *ans / 25 = 274 mA = 2.74x10^-1A

    Q3:
    Calculate the angle (θ) (in degrees) of the complex number
    - (8.2000x10^0) - j (2.70x10^0).
    Enter the answer rounded to three significant digits.

    * I tried doing θ = tan-1(y/x) = tan-1(-2.7/-8.2) = 18.2
    * Then I added 270 since it should be in the fourth quadrant: ans + 270 = 288.23 = 2.88x10^2

    If someone could tell me what I'm doing wrong that would be great, thanks!
    I've tried this homework 3 times, and each time I got these same 3 answers wrong.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
    497
    On Q3 I am getting -161.77º, which can also be written as +198.22º. I assume you have multiple choice of the answers. Is either number I provided in the list of multiple choices?

    I am using my calculator for the calculations. I have done complex numbers on it for years, so I know how to do it and I know it works and is right.

    Also. That complex number is in the third quadrant. So 18.2 is right answer, you add 180º and you get my answer of 198.22º.

    Just for general education:
    First quadrant: Positive Real number, Positive Imaginary number (j).
    Second quadrant: Negative Real number, Positive Imaginary number (j).
    Third quadrant: Negative Real number, Negative Imaginary number (-j).
    Fourth quadrant: Positive Real number, Negative Imaginary number (-j).
     
    Last edited: Jan 21, 2014
    Flupps likes this.
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
    497
    On Q1 I am getting 0.0194, so that would be 0.194x10^-1
     
    Flupps likes this.
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
    497
    About Q2. Should you use the rms value of the voltage?

    With voltage amplitude being rms value (70.71 volts) instead of 100 volts, I get 0.193 ampere. Is that an option in the list of answers?
     
    Flupps likes this.
  5. Flupps

    Thread Starter New Member

    Nov 14, 2013
    8
    0
    Hey shteii01 thanks for the quick reply

    Unfortunately none of the answers are multiple choice. It gives you a box for the 3 significant digits, and another box for the exponent, and each number must be to the highest possible power. There are no answers to compare to when I'm doing the quiz so I can't really tell if I'm doing the questions right until I submit the quiz, which tells me if a question is wrong but doesn't give you the correct answer.

    I just redid the first question and also got 19.4ms... it's possible the scientific notation might be screwing me up to when I'm putting in the answer

    on Q3 I think if I did +180 instead of +270 I should get the same answer you have, for some reason I was thinking it should be in the 270-360 range but I think that would mean x would have to be positive

    on Q2 I think I tried using the rms value on the last quiz attempt, but I'll try it again this time. It's possible I could have put in the wrong answer when trying to convert to scientific

    edit:

    After redoing the quiz, question 1 and 3 were correct, but still having some problems with q#2
    This was the q I got this time, and my answer


    Calculate the instantaneous value at t=(3.2000x10^0) (ms) of the current in a heating element connected to (1.24x10^2) (V) 60.0 (Hz) AC mains. The resistance of the heater is R=(2.2000x10^1) (Ω) Enter the answer in Amperes rounded to 3 significant digits.




    Answer:​
    0.839x10^-1

    I got the rms value, then did v=Vrmssin(2*pi*60*3.2ms) and divided that by resistance to get 0.839x10^-1
     
    Last edited: Jan 21, 2014
Loading...