1. Prof. David R. Jackson ECE Dept. Fall 2014 Notes 30 ECE 2317 Applied Electricity and Magnetism 1

2. Magnetic Materials Because of electron spin, atoms tend to acts as little current loops, and hence as electromagnetics, or bar magnets. NS NS NS NS When a magnetic field is applied, the little atomic magnets tend to line up. This effect is what causes the material to have a relative permeability. B 2

3. Magnetic Materials 3 Type of MaterialProperty Nonmagnetic  r = 1 Diamagnetic  r < 1 Paramagnetic  r > 1 Ferrimagnetic  r >> 1 Ferromagnetic  r >> 1 http://en.wikipedia.org/wiki/Magnetism For more information: Please see the textbooks to learn more about magnetic materials and permeability.

4. Magnetic Materials 4 Material Relative Permeability  r Vacuum 1 Air 1.0000004 Water 0.999992 Copper 0.999994 Aluminum 1.00002 Silver 0.99998 Nickel 600 Iron 5000 Carbon Steel 100 Transformer Steel 2000 Mumetal 50,000 Supermalloy 1,000,000 Note: Values can often vary depending on purity and processing. http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

5. Boundary Conditions The unit normal vector points towards region 1. Note: If there is no surface current: JsJs Ht1Ht1 Ht2Ht2 Bn1Bn1 Bn2Bn2 5 Region 1 Region 2

6. Boundary Conditions (cont.) JsJs PEC Assume zero magnetic field inside the PEC (This is true for any f > 0, please see the note at the end on skin depth). HtHt Note: The surface current is directly related to the tangential magnetic field at the surface. 6

7. Boundary Conditions (cont.) Magnetic field lines must bend around a perfect electric conductor (PEC). This is the opposite behavior of electric field lines. B PEC E 7

8. Boundary Conditions (cont.) 8 Note: The “skin depth”  of a metal determines how the magnetic field will vary within the conductor (discussed in ECE 3317). The field penetrates with distance z into the conductor as z ,  Conductor DC limit: Finite conductivity and zero frequency:  =  (uniform field inside conductor) PEC limit: Infinite conductivity and nonzero frequency:  = 0 (zero fields inside conductor)

9. Magnetic Stored Energy Hence we can write We also have 9

10. Example z I a N turns LsLs Find U H inside solenoid 10 Assume infinite solenoid approximation.

11. Finite-Length Solenoid 11 In a practical finite-length solenoid, the magnetic flux density immediately outside the solenoid is the same as that inside the solenoid. From BCs: Note: It is the strength of the B field immediately outside the solenoid that determines the lifting strength of the magnet. I z a N turns LsLs