A conducting rod slides to the right on two frictionless, horizontal rails that are connected by a resistor. The region contains a uniform magnetic field directed into the page. An external agent must exert a force on the rod to keep its speed constant.
Which statement best accounts for the need for this external force?
(a) Electric charges accumulate on the rod, creating an electrostatic attraction to the rails that resists motion and must be overcome.
(b) The induced current in the rod interacts with the magnetic field to produce a force opposite the motion, so the external agent must do work that becomes thermal energy in the resistor.
(c) The magnetic field pulls the rod forward faster than intended, so a backward force is required to control its speed.
(d) The external force is needed only to maintain a changing magnetic flux; once the current is established it sustains itself, so the work done is stored in the magnetic field, not dissipated as heat.