1. Basic Electronics Ninth Edition Grob Schultz
©2002 The McGraw-Hill Companies

2. Basic Electronics Magnetic Units 14 Ninth Edition CHAPTER
©2003 The McGraw-Hill Companies

3. Topics Covered in Chapter 14
Ampere-turns of Magnetomotive Force (mmf)
Field Intensity (H)
Permeability (m)
B-H Magnetization Curve

4. Topics Covered in Chapter 14 (continued)
Magnetic Hysteresis
Ohm's Law for Magnetic Circuits
Relations between Magnetic Units
Comparison of Magnetic and Electric Fields

5. Ampere -Turns Ampere-turns = I ´ N
I ´ N specifies the amount of magnetizing force or magnetic potential (mmf).
I is the amount of current flowing through N turns of wire.

6. Magnetomotive Force (A•t)
Ampere-turns = I x N.

7. Field Intensity (H) Equation: H = mmf/length
Units: A·t/m ampere-turns per meter
Shorter magnetic circuits produce a greater field intensity.

8. The field intensity in the core is inversely related to length.
Field Intensity (H)
length
The field intensity in the core is inversely related to length.
H = A·t/m
(ampere-turns per meter)

9. B-H Magnetization Curve
B meter
0.5
0.4
Saturation
B in Teslas
0.3
0.2
0.1
Soft iron
k 2k 3k 4k 5k
H in A•t/m

10. Hysteresis Loop BR is due to retentivity. (Note that H = 0
but B > 0.)
+ B
+ Bmax
HC is the coercive force.
(that needed to make B = 0)
+ BR
- HC
- H
+ H
+ HC
- BR
- Bmax
- B

11. Demagnetization (Also Called Degaussing)
BR is reduced to zero.

12. Hysteresis Curve When Retentivity is Very Low
+ B
+ Bmax
- H
+ H
- Bmax
- B

13. Permeability (m)
Permeability is a measure of the ability to concentrate magnetic fields.
Equation: m = B/H
Unit: T A t m × /
teslas per ampere-turn per meter

14. Permeability drops drastically at saturation.
+ Bmax
Slope of
B/H is large
Slope of
B/H is small
- H
+ H
- Bmax
- B

15. Reluctance
Reluctance is the opposition to flux. It is comparable with resistance in an electrical circuit.
Equation: Â = mmf /f
Units: A·t/Wb ampere-turns per weber

16. Permeance
Permeance is comparable with conductance in an electrical circuit (G = 1/R).
Equation: r = 1/Â
Units: Wb/A·t webers per ampere-turn

17. Ohm’s Law for Electrical and Magnetic Circuits
Electrical circuits: I = V/R
Magnetic circuits: f = mmf /Â

18. Increasing reluctance decreases flux.
Adding an air-gap
increases the
reluctance. I
As  increases,  decreases.