(2008? Wuchang district simulation) as shown in the figure, the smooth insulation inclined plane with inclination θ = 30 is in an electric field, the length of the inclined plane AB is L, the charge in the region is +q, and the mass is. A. When the ball moves from A to B, the gravitational potential energy increases and the electric potential energy decreases, so the electric potential energy of the ball at point B must be less than that of the ball at point A. Therefore, A is wrong.

B, according to the kinetic energy theorem:-mglsin θ+quab =12mv20-12mv20 = 0, UAB = mglsin θ q, so b is correct.

C, if the resultant force of electric field force, gravity and supporting force is zero, and the ball moves in a straight line at a uniform speed, the velocity of the ball is still v0 when it reaches point B, and the downward component of gravity of the ball is mgsinθ, then when the electric field force goes up the slope, the magnitude is F=mgsinθ, the electric field force is the smallest, the field strength is the smallest, and the electric field strength is F=Eq, then the minimum value of electric field strength must be MGS in θ Q.

D. If the electric field is generated by a point charge Q just above the midpoint of the inclined plane, then the potential of the ball at point A and point B is equal, and the potential is equal, and gravity does negative work. The speed of the ball to point B is less than v0, so Q must be negatively charged. So, d is wrong.

So choose B.