1. Magnetic Force on Freely Moving Charges
ELECTROMAGNETISM
Magnetic Force on Freely Moving Charges

2. Important Facts about Fm
Fm acts on the e¯ in wire, not on the wire itself
∴ the wire is not required for Fm to exist
Fm acts on any moving charge.

3. Fm can act on any moving charge, even if it is not in a wire

4. Equation: q = magnitude of charge (C)
v⊥ = speed of the charge, ⊥ to B field (m/s)
B = external magnetic field (T)

5. perpendicular
parallel

11. Ex. 2 A proton is moving 8. 60 × 104 m/s North through a 1
Ex. 2 A proton is moving 8.60 × 104 m/s North through a 1.20 T perpendicular magnetic field directed vertically upward. Find its acceleration (mag and dir).

12. Direction of a:

13. Magnitude of F:

14. Acceleration

15. Uniform Circular Motion
If Fm is the only force on the charge

16. If Fm is the only force, then, Fm is Fnet

17. What is the direction of the acceleration? How do you know?

18. Newton's 2nd Law: a is in the same direction as Fnet

19. Notice that
This is called centripetal acceleration.

20. Centripetal acceleration
Change in direction, but no change in speed. Thus it is uniform circular motion.

21. Notice that both Fm and ac act toward the center of the circle
Fm is called the centripetal force since it is responsible for moving the charge in a circle
Notice that both Fm and ac act toward the center of the circle

22. Equation: