1. Magnetism another “invisible” force Physics 100 Chapt 14

2. N-poles & S-poles NS N S Unlike poles attract

3. N-poles & S-poles NS S N like poles repel

4. N-poles & S-poles can never be isolated NS NSNS Break a magnet into two pieces: A new N-pole & S-pole are formed

5. Compass needles If left to its own accord, the N-pole of a magnet will try to point towards the North

6. The Earth is a magnet South Pole North Pole N S Earth’s geographic North pole is a Magnetic S-pole geographic South pole is a magnetic N-pole

7. Magnetic Fields N S x Imagine a small N-pole is placed at the point N Determine the magnetic force on the imaginary N-pole B-field is in the direction of the net force

8. B-field from a bar magnet NS

9. Rules N S B-field lines come out of N-poles & go into S-poles B-field lines never end Density of B-field  strength of the field

10. E-field rules (reminder) E-field lines start on + charges and end on - charges Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge) Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.

11. Horseshoe magnet

12. Earth’s B-field

13. Magnet arrays N N N S

14. Christian Oersted

15. Electric currents produce B-fields I B

16. Right-hand rule B

17. Current loop NS

18. Two current loops N S

19. Even more loops N S

20. Solenoid coil Looks like a bar magnet S N

21. Magnetic forces on electric currents I

22. Another right-hand rule I

23. Forces on two parallel wires II Current in same direction: wires attract B

24. Forces on two parallel wires I I Current in opposite directions: wires repel B

25. Force law of Biot & Savart I2I2 I1I1 F =  B I1I2 ldI1I2 ld l d  = 2 x 10 -7 NA2NA2

26. Biot & Savart example 20A F =  B I1I2 ldI1I2 ld 2m 0.01m F = 2 x 10 -7 NA2NA2 20A (20 A ) 2 2 m 0.01 m F = 2 x 10 -3 N Small, but not tiny

27. Electric motor I B F I F

28. I B

29. Speakers Permanent magnet Solenoid Electro-magnet