A conducting rod of length slides to the right at constant speed on frictionless…
PHYSICS · Single correct · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A conducting rod of length ℓ slides to the right at constant speed on frictionless rails that complete a rectangular circuit with a resistor. The region contains a uniform magnetic field directed into the page. While the rod moves at constant speed, which statement best describes the horizontal force that must be applied to the rod?
(a) A constant force directed to the left is required to counteract the induced current in the rod.
(b) A force whose magnitude is proportional to the square of the rod’s speed, F∝v2, must be applied to the right.
(c) A constant force directed to the right and equal in magnitude to the magnetic force on the rod is required.
(d) No external force is required once the rod is moving, because magnetic forces act only on stationary charges.