Direction of motional emf reverses with wire velocity Steps:

• Original setup: Wire XY moves downward in stationary magnetic field, inducing emf via motional emf ε = Bℓv, with direction set by right-hand rule (force on positive charges F = q(v × B)).
• Emf polarity across X and Y depends on downward velocity direction relative to B field.
• To reverse emf, reverse the velocity vector of the wire while keeping B stationary.
• Upward wire motion flips v direction, reversing F and thus emf polarity.

Why D is correct:

• Moving wire upward reverses velocity relative to stationary B, flipping Lorentz force direction per F = q(v × B), producing opposite emf.

Why the others are wrong:

• A: Both moving upward at same speed gives zero relative velocity, inducing no emf.
• B: Stationary wire and upward-moving magnet equivalent to downward wire motion relative to field, same emf direction.
• C: Downward wire motion with stationary magnet matches original, producing same emf direction.

Final answer: D