[填空题]
A sort of “projectile launcher” uqontrljk08wy28,- q is shown is3fx1vje. y(*i,u dii n (Fig. Below). A large current moves in a closed loop composed of fixed rails, a power supply, and a very light, almost frictionless bar touching the rails. A magnetic field is perpendicular to the plane of the circuit. If the bar has a length 3v(1fujx.deisyi* i, L = 22cm, a mass of 1.5 g, and is placed in a field of 1.7 T, what constant current flow is needed to accelerate the bar from rest to 28m/s in a distance of 1.0 m? In what direction must the magnetic field point?
A
the magnetic field must be
