# Given i(t), Find q between t=0s and t=0.01s

Discussion in 'Homework Help' started by Eng.Mubarak, Sep 24, 2011.

1. ### Eng.Mubarak Thread Starter New Member

Oct 9, 2010
7
0
Hello

Problem :
i(t) = 10 sin(200 pi t)
Find q between t=0 s and t=0.01s

I know that it is a straightforward problem.

q=integration of i(t) from t=0 to t=0.01
The problem here is that the value of the integral equal zero!

q = 10 integration of ( sin(200 pi t) ) from t=0 to t=0.01
= ( - 10 / 200 pi ) [ cos(200 pi t) ] from t=0 to t=0.01
= ( -1 / 20 pi) [ cos( (200) (0.01) pi ) - cos(0) ]
Now the big problem is what is pi ?!!
is it 3.14 or 180 ?

Our T.A. said that if your calculator is in degrees, then pi=180
and if it is in radians, then pi = 3.14

My calculator is in degree, so i will put pi = 180
but this give me zero !

Any help?

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
0 is a valid answer. It means that the total load in coulombs that has entered your device over the given time period is zero. In other words, as much load has got in, that much current has got out. It shouldn't startle you so much.

If you want to find how much load has gone in AND out of your device, you should calculate the integral of the absolute value of the current. Then your answer won't be zero.

3. ### Eng.Mubarak Thread Starter New Member

Oct 9, 2010
7
0
Thanks.
is my T.A right?

4. ### edgetrigger Member

Dec 19, 2010
133
19
i(t) = I sin( ωt)

ω = 2∏ft ; if f is in hertz, t in secs than ∏ = 3.14

you need to substitute 3.14 not 180 for value of π

5. ### Eng.Mubarak Thread Starter New Member

Oct 9, 2010
7
0
^^^
We did not learn f or omega.
this is the first lecture in this course.
What I know is I = dq / dt
and i want to find q given i(t) by integration.
my question is while i evaluate the integral, pi is 180 or 3.14 ?

6. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
It really depends on what your calculator accepts. Use pi if the calculator says that sin(3.14)=0 or 180 if you get sin(180)=0.