[quote author=Fu-Kwun Hwang link=topic=1281.msg4947#msg4947 date=1257256703]<br />I think the magnetic field is assumed to be uniform B=B[sub]0[/sub].<br />The magnetic flux pass through the coil depends on the angle <img class='tex' src="/demolab/smf/teximages/a/f/8/af8640990e102117952b5734d439dada.png" alt="\theta=\omega t" />, <br />assume the coil rotate at constant angular velocity <img class='tex' src="/demolab/smf/teximages/4/d/1/4d1b7b74aba3cfabd624e898d86b4602.png" alt="\omega" />.<br />i.e. <img class='tex' src="/demolab/smf/teximages/5/c/f/5cf8f867ec751de62abab061f815910b.png" alt="\Psi=\int \vec{B}\cdot d\vec{s}= B A cos(\theta)=B A cos(\omega t)" /><br />From Faraday's law, induced voltage <img class='tex' src="/demolab/smf/teximages/0/7/d/07d1f41c41e4e075511f9f86be848300.png" alt="V= -\frac{d\Psi}{dt}= BA\omega \sin(\omega t)" /><br />So the voltage is a sine wave (without commutator).<br />[/quote]<br />i agree with the above analysis that AC generator (without commutator) the emf induced in the coil is a sine wave.<br /><br />so is it that with a commutator, which flip the current in half cycle rotation, &nbsp;therefore emf induced in the coil will be a |sine| wave?<br /><br />if that is true, then i understand why the emf induced is |sine| wave.<br /><br />By the way,<br />i am certain http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/motdct.html is accurate for torque vs time is a |sine| wave.<br />[img]http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/imgmag/dcmtorv.gif[/img]<br /><br /><br />