Magnetism

1. Magnetism A Whole New Topic March 5, 2007

2. This Week • Exam #2 Returned • Today we begin magnetism – a topic that will occupy us for most of the remainder of the semester. • Quiz on Friday • Watch for WebAssignment • Next week: Spring Break – Enjoy!

3. SEE JBB Average Grade = 54%

4. What did you think about the exam last Friday? All answers = 3 points. • It was too difficult. • It was fair. • It was easy. • I should have studied more.

5. Magnetism was known long ago.

6. Lodestone (Mineral) • Lodestones attracted iron filings. • Lodestones seemed to attract each other. • Lodestone is a natural magnet.

7. New Concept The Magnetic Field • We give it the symbol B. • A compass will line up with it. • It has Magnitude and direction so it is a VECTOR. • There are some similarities with the Electric Field but also some significant differences.

8. Magnetism • Refrigerators are attracted to magnets!

9. Where is Magnetism Used?? • Motors • Navigation – Compass • Magnetic Tapes • Music, Data • Television • Beam deflection Coil • Magnetic Resonance Imaging • High Energy Physics Research

10. Magnet Demo – Compare to Electrostatics N S Magnet What Happens?? Pivot

11. Results - Magnets • Like Poles Repel • Opposite Poles Attract • Magnetic Poles are only found in pairs. • No magnetic monopoles have ever been observed. S N Shaded End is NORTH Pole Shaded End of a compass points to the NORTH.

12. Observations • Bring a magnet to an electrically charged object and the observed attraction will be a result of charge induction or polarization. • Magnetic poles do not interact with stationary electric charges. • Bring a magnet near some metals (Co, Fe, Ni …) and it will be attracted to the magnet. • The metal will be attracted to both the N and S poles independently. • Some metals are not attracted at all. (Al, Cu, Ag, Au) • Wood is NOT attracted to a magnet. • Neither is water. • A magnet will force a compass needle to align with it. (No big Surprise.)

13. Magnets Magnetic Field Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles.

14. N S Consider a Permanent Magnet The magnetic Field B goes from North to South.

15. N N S S Introduce Another Permanent Magnet pivot The bar magnet (a magnetic dipole) wants to align with the B-field.

16. N N S S Field of a Permanent Magnet The south pole of the small bar magnet is attracted towards the north pole of the big magnet. The North pole of the small magnet is repelled by the north pole of the large magnet. The South pole pf the large magnet creates a smaller force on the small magnet than does the North pole. DISTANCE effect. The field attracts and exerts a torque on the small magnet.

17. N S N S Field of a Permanent Magnet The bar magnet (a magnetic dipole) wants to align with the B-field.

18. Convention For Magnetic Fields B X  Field INTO Paper Field OUT of Paper

19. Typical Representation

20. Experiments with Magnets Show • Current carrying wire produces a circular magnetic field around it. • Force (actually torque) on a Compass Needle (or magnet) increases with current.

21. B Current Carrying Wire Current into the page. Right hand Rule- Thumb in direction of the current Fingers curl in the direction of B

22. Current Carrying Wire • B field is created at ALL POINTS in space surrounding the wire. • The B field has magnitude and direction. • Force on a magnet increases with the current. • Force is found to vary as ~(1/d) from the wire.

23. Compass and B Field • Observations • North Pole of magnets tend to move toward the direction of B while S pole goes the other way. • Field exerts a TORQUE on a compass needle. • Compass needle is a magnetic dipole. • North Pole of compass points toward the NORTH.

24. Planet Earth

25. Inside it all. 8000 Miles

26. On the surface it looks like this..

27. Inside: Warmer than Floriduh

28. Much Warmer than Floriduh

29. Finally Hot Hot Hot

30. In Between • The molten iron core exists in a magnetic field that had been created from other sources (sun…). • The fluid is rotating in this field. • This motion causes a current in the molten metal. • The current causes a magnetic field. • The process is self-sustaining. • The driving force is the heat (energy) that is generated in the core of the planet.

31. After molten lava emerges from a volcano, it solidifies to a rock. In most cases it is a black rock known as basalt, which is faintly magnetic, like iron emerging from a melt. Its magnetization is in the direction of the local magnetic force at the time when it cools down. Instruments can measure the magnetization of basalt. Therefore, if a volcano has produced many lava flows over a past period, scientists can analyze the magnetizations of the various flows and from them get an idea on how the direction of the local Earth's field varied in the past. Surprisingly, this procedure suggested that times existed when the magnetization had the opposite direction from today's. All sorts of explanation were proposed, but in the end the only one which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was sometimes reversed.

32. This planet is really screwed up! NORTH POLE SOUTH POLE

33. Navigation DIRECTION S N Compass Direction Repeat Navigation DIRECTION N S If N direction is pointed to by the NORTH pole of the Compass Needle, then the pole at the NORTH of our planet must be a SOUTH MAGNETIC POLE! And it REVERSES from time to time.

34. Rowland’s Experiment Field is created by any moving charge. Increases with charge on the disk. Increases with angular velocity of the disk. Electrical curent is a moving charge. Rotating INSULATING Disk which is CHARGED + or – on exterior. xxx xxx B xxx ++ + + ++

35. q • If the charge is moving, there • is a force on the charge, • perpendicularto both v and B. • F = q vxB q A Look at the Physics There is NO force on a charge placed into a magnetic field if the charge is NOT moving. There is no force if the charge moves parallel to the field.

36. WHAT THE HECK IS THAT??? • A WHAT PRODUCT? • A CROSS PRODUCT – Like an angry one?? • Alas, yes …. • F=qv X B

37. The Lorentz Force This can be summarized as: F or: v q m B q is the angle between B and V

38. Nicer Picture

39. Another Picture

40. VECTOR CALCULATIONS

41. Practice B and v are parallel. Crossproduct is zero. So is the force. Which way is the Force???

42. Units

43. teslas are HUGE!

44. The Magnetic Force is Different From the Electric Force. Whereas the electric force acts in the same direction as the field: The magnetic force acts in a direction orthogonal to the field: (Use “Right-Hand” Rule to determine direction of F) And --- the charge must be moving !!

45. F Wires • A wire with a current contains moving charges. • A magnetic field will apply a force to those moving charges. • This results in a force on the wire itself. • The electron’s sort of PUSH on the side of the wire. Remember: Electrons go the “other way”.

46. The Wire in More Detail Assume all electrons are moving with the same velocity vd. L B out of plane of the paper

47. Magnetic Force Current = i Magnetic Levitation mg Where does B point???? Into the paper.

48. MagLev

49. A conductor suspended by two flexible wires as shown in the diagram has a mass per unit length of 0.040 0 kg/m. What current must exist in the conductor in order for the tension in the supporting wires to be zero when the magnetic field is 3.60 T into the page? What is the required direction for the current? Concrete Insulator

50. There was a crooked man who lived in a crooked house that was wired with crooked wires