Induced Current and the Force That Fights You

Pull a loop out of a magnetic field and flux through it drops. That induces a current. But that current sits in a magnetic field too, so the field exerts a force on it — F = IL x B. Work out the direction with Lenz's law: the induced current always flows the way that opposes the change causing it. Here that means the force on the current-carrying loop points back, against the motion pulling it out. You feel this as a drag, a stiffness in your hand, even though nothing is touching the loop.

This is why a generator gets harder to turn the instant you connect a load. With no circuit closed, no current flows, so there's no opposing force — just friction. Close the circuit and current flows, and now every bit of that current fights the rotation. The more current you draw, the harder you have to push or turn. You can't get something for nothing: the electrical energy delivered to the load has to come from somewhere, and that somewhere is the mechanical work you put in against the opposing force.

This connects directly to the energy bookkeeping for the whole unit. The retarding force times the distance moved equals the work you do, and that work converts exactly into the electrical energy dissipated in the resistance. Lenz's law isn't just a direction rule — it's energy conservation wearing a magnetic disguise, and it sets up the motional EMF and power calculations that follow.