1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Induced EMF on a rotating coil

  1. Dec 2, 2013 #1
    1. The problem statement, all variables and given/known data

    a coil starts perpendicular to a magnetic field of 0.365 tesla , it has a length of 0.15 cm and a height of 0.25 cm , it rotates with a frequency of 1800 rotation / min . it also has 30 turns
    calculate the mean induced Emf during quarter a rotation

    2. Relevant equations
    emf = rate of change of magnetic flux / sec


    3. The attempt at a solution
    i have already solved the problem , during quarter a rotation , the magnetic field goes from 0.365 to 0 because in a quarter rotation , the coil goes from 90 degrees to 0 degrees with the field .
    a quarter rotation takes a time of 1/30 /4 = 1/120 sec.
    but i dont really want to find that , i want to use calculus to find the exact rate of change at a particular moment
    now weber = area * field * sin theta
    where theta is the angle between the coil and the field
    now the rate of change of weber = area * field * cos theta * rate of change of theta .
    when it takes a quarter of rotation , the theta will be 0 so cos theta = 1 , but how do i exactly find the rate of change of theta ?
    if it takes 360 degrees / (1/30) seconds , is that the rate of change of theta ? , how do you exactly find dtheta / dt ?
     
  2. jcsd
  3. Dec 3, 2013 #2
  4. Dec 3, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member



    Not right. See below.

    The way you described the orientation of the field, it's weber = area x field x cos(theta). So let's be scientific and say ψ = AB cosθ. I use ψ since phi never shows up in my toolbar. θ is the angle between the B field and the normal to the coil. You have to be precise when you say " ... a coil starts perpendicular to a magnetic field ... ". That could mean that two of the four sides of the coil are perpendicular to the field. Anyway, we start at θ = 0 and go to θ = π/4.

    So, what is the emf induced in the coil at any moment? Hint: Faraday plus hint of post #2.

    And, 1800 rotations per minute is how much dθ/dt?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Induced EMF on a rotating coil
Loading...