This is a simulation of an electromagnetic railgun. The red projectile can slide without friction on the blue rails. The battery causes a current to flow around the loop consisting of the battery, rails, and the projectile. The current directed down through the projectile, in the uniform magnetic field directed into the page, gives a force on the projectile directed right - this accelerates the projectile to the right.
The motion of the projectile increases the magnetic flux through the loop. This gives rise to an induced current in the opposite direction of the current from the battery, and an associated resistive force that acts on the projectile. The faster the projectile goes, the larger the rate of change of flux, the larger the induced current, and the larger the resistive force. Eventually, the resistive force matches the force to the right. The forces balance, and the projectile moves at constant velocity after that. The velocity can be impressively large, however!
Simulation posted on 2-27-2018. Written by Andrew Duffy
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