I am having trouble with chapters 12 and 13 for ECE 303. I went to professor alexanders office hours and still confused.

here is one problem i'm having trouble with

B(t)= te^(-5t)[0.4ux+0.2uz] , t>=0
A circular loop in the x-y plane has radius of 2. compute Vind(t) over range 0<t<1

i figured out the following

Vind= -d/dt * {surface integral}(B*dS*un)
=-d/dt * (ub*un*B[mag]*{surface integral}dS) ub*un=1*cos(theta)
=-d/dt *( cos(theta)*B[mag]*[Area of circle= pi*r^2]
plugging in numbers i have

=-d/dt *(cos(theta)*0.2te^(-5t)*12.56

This is where i'm stuck i can't figure out how to get theta, also why (according to Alexander) does the B[mag] value only take into account the 0.2 of uz. Lastly where does

Vind= 2.51(5t-1)*e^(-5t) come from after finding B[mag] and theta

4/16/2007 4:03:02 PM

Well, you only take into account the 0.2 because the B field has to be normal to the loop.

That cos(theta) = 1 because the B field is normal (orthoganal) to the loop (cos(90) = 1)

[Edited on April 16, 2007 at 4:24 PM. Reason : .]

4/16/2007 4:21:38 PM

what is the B-field was not normal to the loop kinda like

homework problem 13-4
when Ub= 0.5uz+0.866ux

4/16/2007 6:14:55 PM

4/16/2007 6:26:56 PM

^ wtf?

^^ cos only comes into play when the loop is tilted. For example sampleq 13-3.

Here was my approach.

flux = B(t)ub DOT A (Area = 6)
flux = |B(t)|*|6 uz (z because the loop is normal to Z) cos (theta)

since Vind = -d/dt of the flux
vind(0.333) = 3e-3(0.333) cos (theta) * 6

solve for theta = 53.1
B(t) = e-3t

4/16/2007 7:10:33 PM

jbl4me should take over teaching for alexander

4/16/2007 8:37:29 PM