A long straight copper rod with a linear mass density of m/L is suspended using massless strings. A current is sent through the rod. A vertically upward magnetic force displaces the rod by θ degrees, measured from the vertical. What is the strength of the magnetic field? Use symbols as necessary.

• A. B = i/µgtanθ
• B. B = µgtanθ/i
• C. B = iµg/tanθ
• D. B = µg/i•tanθ

Answer: (B)