May 12th, 2017, 04:37 AM  #1 
Newbie Joined: May 2017 From: alabama Posts: 6 Thanks: 0  differentiation
Can anybody tell me where I've gone wrong on this question? An alternating current, i amperes, is given by i = 15 cos (2pi f t) Calculate the rate of change of current when t = 2 ms and f = 75 Hz. dy/dx = dy/du × du/dx y = 15 cos du/dt = 15sin = (15) (2pi f)  sin 2 pi f t = (15) (2 pi × 75)  sin (2pi × 75 × 2×103) = 7067.77 a/s Last edited by skipjack; May 12th, 2017 at 06:53 AM. 
May 12th, 2017, 04:56 AM  #2  
Math Team Joined: Jan 2015 From: Alabama Posts: 2,875 Thanks: 766  Quote:
Quote:
Last edited by skipjack; May 12th, 2017 at 06:52 AM.  
May 12th, 2017, 05:56 AM  #3 
Math Team Joined: Jul 2011 From: Texas Posts: 2,678 Thanks: 1339 
Assuming frequency is constant since no information is given if or how it changes w/respect to time ... $i=15\cos(150\pi t)$ $\dfrac{di}{dt} = 2250\pi \sin(150\pi t)$ At $t=0.002 \, sec$ ... $\dfrac{di}{dt}=2250\pi \sin\left(\dfrac{3\pi}{10}\right) \approx 5718.6 \, amps/sec$ 
May 12th, 2017, 06:10 AM  #4 
Newbie Joined: May 2017 From: alabama Posts: 6 Thanks: 0 
Well I redid the question and I have the same answer as you skeeter. Do you know where country boy got the answer 3250 from?
Last edited by skipjack; May 12th, 2017 at 06:54 AM. 
May 12th, 2017, 06:58 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 18,245 Thanks: 1439 
He didn't convert 2 ms to seconds.


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