Rod in a Magnetic FieldSt Peter's Science Portal
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Electromagnetism

Charge Separation in a Moving Rod

Drag the conducting rod through the magnetic field. The free electrons feel a magnetic force F = qv × B and pile toward one end, leaving the other end positive — a motional EMF. Stop moving and they spread out evenly again.

Controls

Sign sets direction: positive = into the screen (⊗), negative = out (⊙).
Longer rods develop a larger EMF for the same motion.
Rod: horizontal
Field
⊗ in
Pile-up
even

Practice question

Press “New question” to begin.
Free electrons (−), mobile + Fixed positive ions (in rod) ⊗ field into screen   ⊙ field out of screen
Drag the rod through the field. As electrons pile to one end, the far end's bare + ions are left exposed — that end becomes positive. Press space to pause and read the frozen values; grab the rod to resume.
SPEED 0.00 m/s V 0.00 V
The physics behind it

A conductor holds a sea of free electrons. When the rod moves with velocity v through a magnetic field B, each electron feels the magnetic force F = qv × B (with q = −e). That force acts along the rod, sweeping electrons to one end so it becomes negative while the far end is left positive. The separated charges set up an internal electric field that grows until it balances the magnetic force — a steady motional EMF of size ε = BLv, where L is the rod length. The build-up is proportional to speed and field strength, and reverses if you reverse either the motion or the field. Stop moving (v = 0) and there is no magnetic force, so the electrons spread back out evenly. Here the field points into or out of the screen and the rod is dragged in the screen plane, so only motion perpendicular to the rod produces separation.