AP® Physics 1 & 2

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Motional EMF: Conservation Law: Loop Entering Magnetic Field


A copper wire loop sits on the top of a rolling cart. The cart moves with constant speed and negligible friction as it enters a region of uniform magnetic field.

Which conservation law can be applied in this situation to predict the cart’s motion?


Conservation of energy. The cart will slow down because some of its kinetic energy is used to create electrical current in the wire loop.


Conservation of charge. The cart will maintain its constant speed because there is no current in the loop before entering the field and there will be no current as the wire loop moves through the field.


Conservation of momentum. There is no outside force on the cart, so it must move at constant velocity through the field.


Conservation of current. The cart will stop and reverse direction because the electric current in the wire loop must also reverse direction.