(2013?Pudong New District three model) in the inclined angle of θ in the inclined plane fixed two long enough smooth parallel metal guide rail PQ, MN, distance of L, the guide rail is in the magnetic

b bar at rest that b bar force balance, that is, amperometric force and gravity along the slope down the force balance, a bar moving upward at uniform speed, that a bar by the tension of the rope and gravity along the slope down the size of the force and the amperometric force along the slope down the three force balance, c uniformly down the c by gravity and the rope tension size balance. From the b balance can be known, ampere force size F an = mgsinθ, from the a balance can be known F rope = F an + mgsinθ = 2mgsinθ, from the c balance can be known F rope = mcg

A, because the tension in the rope is equal in magnitude, so the 2mgsinθ = mcg, that is, the mass of the block c is 2msinθ, so the A is correct;

B, b put on before. According to the conservation of energy know a increase in gravitational potential energy is also due to c reduce the gravitational potential energy, so B error;

C, a rise in gravitational potential energy in the increase, so according to the conservation of energy know C error;

D, according to the equilibrium of the b-bar can be known as Fan=mgsinθ and because of the Fan=BIL∴I=

mgsinθ

BL

, thus D is correct;

Therefore choose AD.