Motional emf

Intermediate

A rod moving in a field generates emf.

Key relationε = BLv

A rod of length L moving with speed v perpendicular to B has ε = BLv, from the magnetic force on charges in the rod.

Worked example

L = 0.5 m, v = 4 m/s, B = 0.2 T: 0.4 V.

Common mistake

Only the component of velocity perpendicular to B and L counts.

In more detail

A conductor moving through a magnetic field has an emf ε = Blv across its ends. This motional emf is the origin of power generation in many systems.

Moving rails in a magnetic field generate emf in some experiments.
Electric guitar pickups use motion and changing fields.
A metal bar sliding along rails in a magnetic field generates a voltage, which is used in simple demonstrations of induction.

Which direction is the emf?

Given by the right-hand rule, perpendicular to the velocity and field.

Does this need a closed circuit?

A current flows only in a closed loop, but the emf exists across the moving conductor.

Which way does the current flow?

Lenz's law determines it: the current creates a force that opposes the motion of the rod.