1. The shadow of electricity
Magnetism
The shadow of electricity

2. Magnetic Force Magnets apply forces to each other.
Opposite poles attract, like poles repel.

3. CPS Question The magnitude of the force between magnetic poles
is the same at all distances.
becomes stronger as the poles come closer.
becomes weaker as the poles come closer.

4. Magnetic Field Lines Conventions as with electric field lines.
direction is direction of force on a north pole
strong field where lines are close together
Field lines point from North to South pole.

5. Group Work
What happens to a dipole magnet in a uniform magnetic field? B N S

6. Naming the poles
N = north-seeking
Earth’s north pole is a south pole!

7. Magnets and Charges

8. Vector Direction Conventions
Right
Left Up
Down In
Out

9. Magnetic Force on a Charge
F = qv  B
Source: Griffith, The Physics of Everyday Phenomena
F is directed out of the screen.

10. Vector Cross Product Operation symbol 
Another way to multiply two vectors
Product is a vector!
Direction of AB is perpendicular to both A and B

11. Cross Product Magnitude
A  B = ab sin q A q
Maximum for q = 90°
Zero for q = 0°, 180° B b

12. Magnitude Geometricallyq B b
AB = area of parallelogram

13. Magnetic Force on a Charge
F = 0 unless charge is moving
F = 0 if velocity is  to field
F = maximum if velocity is  to field
F  0 only if charge crosses B field lines
F = qvB sin(q) in general
If v or B reverse, direction of F reverses

14. Group Work
What are the units of magnetic field B?

15. Cross Product Direction
Curl right-hand fingers in direction of q
Right-hand thumb points in direction of cross-product
Not commutative A B a b q
AB = –(BA)

16. Force Direction F = qv  B Right-Hand Rule
Source: Griffith, The Physics of Everyday Phenomena
Right-Hand Rule

17. Origami Right Hand paper square creases: in out vectors fold over
magnetic
field
current qv
Force
Lorenz
vectors
fold over

18. Group Question
What is the direction of the force on the object moving with velocity v through magnetic field B? A. D. B + B. E. v C. F.

19. Group Question
What is the magnitude of the force on object A compared to the magnitude of the force on object B? B A B v q v 2q

20. Group Work
A particle with charge q has a velocity perpendicular to a uniform magnetic field. What will its subsequent path be? B v q

21. Group Work
A current runs through one wire of a pair of side-by-side wires. What is the direction of the resulting magnetic field at the location of the other wire? I ?

22. Group CPS Question
Two parallel wires carrying currents in the same direction
attract each other.
repel each other.
have no effect on each other.
Think of their magnetic fields and charge movements!

23. Force between parallel currents
What is the force on this current? I  B

24. Force between parallel currents