- magnetic fields have flux lines that eminate from north to south
- they decrease in intensity s they get further away from the magnetic source
- flux density B is a property of magnetic fields that is a measure of the number of flux lines passing through one square unit of space and is measured in teslas T
- flux lines or flux linkage Φ have units weibers Wb
- B= Φ/A
- magnetic circuits can be compared to real circuits, where emf is like mmf (magnetomotive), current is like flux and resistance is the property reluctance R
- thus V=IR becomes Fm=ΦR
- Fm is also defined by NI, number of coils times current, giving it units ampere-turn At
- Reluctance is defined thus by At/Wb
- permeability is a property unique to a medium in a magnetic circuit, µ
- relative permeability µr is a ratio between material permeability and permeability of free space, µo
- there are three levels of magnetization properties in materials; strong ferromagnetic with µr >100, mild paramagnetic µr>=1, and repulsive diamagnetic with µr
- in a symmetrically shaped core, the reluctance of each side is the same provided the depth is uniform
- in an asymmetrical shape, the reluctances vary for each side
- either case forms a basic series magnetic circuit with reluctance and mmf
- reluctance of side ab is Rab= lab/µr*µo*Aab where A is area
- gaps in magnetic cores are common and usually small so flux can cross it
- flux is lost through fringing effect when crossing a gap, but a small gap makes this loss negligible
- air gaps behave as another reluctance as well as the core
- parallel magnetic circuits work much the same as basic parallel circuits, with equivalent kerchoff laws for flux and mmf
- when finding the reluctance of an air gap, µr=1 and the cross-sectional area of the gap is 5% greater than the physical gap
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