Instantaneous value of a sine wave

Discussion in 'Homework Help' started by Frano, Jul 23, 2008.

  1. Frano

    Thread Starter New Member

    Jun 9, 2008
    7
    0
    Hi,

    I posted a homework question on the 9th June called "Please Help. Sine waves". Anyway, I have since received my assignment back on still got the answer wrong...If there is anyone out there who could help, it would be greatly appreciated as my exam in tomorrow (cutting it fine I know)...

    I need to find out the "instantaneous value of e @ 12 milliseconds after passing through zero positively"

    I know the answer is 57.3, but don't know how to get there.

    e = 100 sin 314,28 t

    Thanks,

    Frano
     
  2. mrmount

    Active Member

    Dec 5, 2007
    59
    7
    If you took the sine value using a calculator, the setting for sine angle should be in radians; not degrees. Double check on that one.
     
  3. Frano

    Thread Starter New Member

    Jun 9, 2008
    7
    0
    Ok, if I use RAD I get 58.9 which is closer to the answer I got. However I went a long way round...I worked out the frequency as 50Hz; therefore 50 full cycles in 1 second = 360 degrees x 50 = 18000 degrees over 1 second. Worked back from there to degrees after 0.012 seconds.

    Why RAD? I need to understand.

    Thanks
     
  4. m4yh3m

    Senior Member

    Apr 28, 2004
    186
    42
    If you go to this thread:

    Electronics Cheat Sheets

    And look at either the image file or download the acrobat file for the Circuit Formulas, you'll see the formulas for calculating the instantaneous value you need under the heading Phase/Time
     
  5. Mark44

    Well-Known Member

    Nov 26, 2007
    626
    1
    The sine function takes a real number (i.e., radians, which are real numbers) as input, and evaluates to a real number. If you are using a calculator and are asked to find the sine of 56 degrees, then your calculator needs to be in degree mode. If you are asked to find the sine of an arbitrary real number, as you are in this problem, the calculator needs to be in radian mode.

    If I evaluate 100 sin(100\pi t) at t = 0.012 (I think this is the function you're dealing with), I get approximately - 58.8, not + 57.3 as you show. How is it that you know 57.3 is the right answer?
     
  6. Frano

    Thread Starter New Member

    Jun 9, 2008
    7
    0
    57.3 is the answer the guy who marked my assignment put down...so it could very well be wrong.

    Going to check out the link.

    Thanks
     
  7. m4yh3m

    Senior Member

    Apr 28, 2004
    186
    42
    I have 2 or so legal pads of notes I took during AC and Digital electronics... I remember jotting down when you need to use Rads and Degs.. I'll have to see if I can find it. It had a formula attached with it :/
     
  8. Frano

    Thread Starter New Member

    Jun 9, 2008
    7
    0
    Instantaneous time (t) = Cycle time (t) x ?/360

    0.012 = 0.02 x ?/360
    ? = 216
    sin 216 (using Deg on calculator) = -0.5878

    from given formula:

    e = 100 sin 216
    so e = 58.78

    Or you it seems I could have just used the Rad function and ended up with 58.89 :D
     
  9. Mark44

    Well-Known Member

    Nov 26, 2007
    626
    1
    So e = - 58.78. You lost the sign.
    Also, you should explicitly write the units you're using ("degrees" or "deg"). Otherwise you won't know when you're working with degrees or with real numbers (radians). For example, the formula you show above would be
    e = 100 sin (216 deg.)

    By the way, the Rad button on a calculator isn't a function.
     
Loading...