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!

Homework Help: Magnetic fields problem

  1. Nov 15, 2004 #1
    Can anyone help me with this?

    "A research lab has a magnetic facility, which can produce intense, pulsed magnetic fields. In 1 experiment, a B-field of 40 T is to be produced over a cylindrical volume, 40 cm in length and 20 cm in diameter.

    The energy required to pulse the magnet is supplied by a flywheel, constructed from a set of circular plates, 5 m in diameter and with a total mass of 10^5 kg, rotating at 500 RPM."

    Calculate how much energy is stored in the B-field

    For this, I used magnetic energy density, u = U/V = (B^2)/2μ0, where U = energy stored, V = volume in field.

    U = 8 MJ

    Estimate the reduction in angular velocity of the flywheel needed in order to supply energy necessary to produce the required B-field. Assume conversion from rotational energy to magnetic energy is 100% efficient.

    Here I'm stuck. If the conversion is 100% efficient, why can't I use KE(rot) = U, with KE(rot) = (1/2)Iω^2?
  2. jcsd
  3. Nov 15, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    It's asking for the change in the angular speed - the wheel doesn't necessarily stop:

    [tex]\frac{1}{2} I \omega_0^2= \frac{1}{2} I \omega_f^2 + E_{magnet}[/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook