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 AAC Fanatic!

Apr 28, 2004
186
43
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 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 AAC Fanatic!

Apr 28, 2004
186
43
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 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.