✔ 最佳答案
the answer is A, remain stationary. since steel is normally cannot be magnetized. or, it is not sensitive to magnetic field.
there are some special steel than can be magnetized. in that case, the anwser is D. since the magnetic field is oscillating at 50Hz. and the inducted magnetic field is also 50 Hz but has a 90 degree phase lag. the multiple of this 2 will produce 100 Hz motion.
a mathematical description is :
B-field from the magnetic on the strip is:
Bo= {0,0,Sin[ 2 pi 50 t ]}, assume the strength is 1.
the inducted E-field on the strip is :
E = 100 pi Cos[ 2 pi 50 t ] {-y , x, 0 }
this circulating E field will create a current.
I = 100 pi / R Cos[ 2 pi 50 t ] {- y , x , 0 }
where R is the resistance of the strip.
this circulating current will produce a B - field.
B = A { 0,0, Cos[ 2 pi 50 t ] }
where A is a constant.
the force on the strip is then,
F = B . Bo = Cos [ 2 pi 50 t ] Sin[ 2 pi 50 t] = 1/2 Sin[ 2 pi 100 t ]
therefore, the frequency is doubled.
2011-03-19 23:02:01 補充：
the force is proportional to B and Bo, sorry for my lazy.
