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

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

3. 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

4. Moving Creates a Potential– B v +
induced potential

5. 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

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

7. Rotating Loop Creates CircuitB V V

8. Rotating Loop Creates CircuitB V V

9. Rotation Powers a Generatorv
Motion B
through the field V
induces a potential
which generates a current
that charges the battery

10. 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.

11. Current in a Field Feels a ForceB F I

12. Loop in a Field Has TorqueB I I

13. Torque Turns a Motor I
current I
force F B

14. 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

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

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