AP Physics L14_motionalEmf Transforming energy with magnetism Magnetic Induction Transforming energy with magnetism

Lorentz Force Reminder F = qv  B Source: Griffith, The Physics of Everyday Phenomena F is directed out of the screen.

Motion Creates a Potential What is the direction of the Lorentz force on the (+) charges in this wire? B A. B. C. D. E. F. v

Moving Creates a Potential – B v + induced potential

Motional emf Moving a conductor through a magnetic field causes separation of charges. e = LvB when L, v, B all orthogonal This is how a dynamo (generator) works

Question What is the direction of the Lorentz force on the charges moving in this wire? B A. B. C. D. E. F. I

Rotating Loop Creates Circuit B V V

Rotating Loop Creates Circuit B V V

Rotation Powers a Generator v Motion B through the field V induces a potential which generates a current that charges the battery

Energy Conservation In a magnetic field: Moving a conductor induces a voltage. The voltage can drive a current. The current receives a backward force. It takes work to drive a current.

Current in a Field Feels a Force B F I

Loop in a Field Has Torque B I I

Torque Turns a Motor I current I force F B

Back-emf A current moving through a field experiences a force. The force can move the conductor. The moving conductor sets up an emf. The back-emf opposes the current

Back Emf As the motor turns, back-emf opposes m I As the motor turns, back-emf opposes m Faster rotation  more back emf

Back Emf A motor draws the most current when it is not rotating At speed its load is only resistive