Help with Circuit problem

**bnk0430** · May 12th 2010, 10:58 PM

I didn't learn to do this properly so please help me and explain it clearly.

The current in amps in an ac circuit containing a resistance is given by i = 1.5 sin ωt where ω = angular velocity, t = time in seconds, and ωt = angle in radians. If the angular velocity ω = 300 rad/s

(a) Find i in milliamps when t = 2.0 ms
(b) Find the angle ωt in radians between 0 and 2π rad when i = mA

How do I go about doing this problem? Please explain clearly, thanks.

**ebaines** · May 13th 2010, 08:43 AM

This is a sine wave of magnitude 1.5. The value of i for any value of t is found by straight-forward sunstitution intoteh formula you gave us. Thus at t = 20 milliseconds you have:

$ i = 1.5 \sin(300 /s \cdot 0.020 s) = 1.5 \sin(3) = 1.5 \times 0.1411 = 0.212 $

However, you didn't tell us whether this formula yields i in amps, or milliamps. Presumbaly you can convert if need be.

The second queston involves finding the inverse of the sine function. Your post is missing an important part of the question - namely precisely what value of i you were given. So I will give an example where i = 0.75. You have:

$ i = 0.75 = 1.5 \sin(300t) $

$ \frac 1 2 = sin(300t) $

For what value of $\omega t$ does $sin(\omega t) = \frac 1 2$ ? From basic triginometry you know that $\sin(\pi/6) = \frac 1 2$ and $\sin(\frac {5\pi}{6}) = \frac 1 2$ . Hence:

$\omega t = \frac {\pi} 6$ or $\omega t = \frac {5 \pi} 6 $

Hope this helps.

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