Fundamental Electrical and Electronic Principles, Third Edition

132 Fundamental Electrical and Electronic PrinciplesTable 4.1 Comparison of QuantitiesElectrical Magnetic ElectrostaticQuantity Symbol Unit Quantity Symbol Unit Quantity Symbol Unitemf E V mmf F At emf E Vcurrent I A fl ux Φ Wb fl ux Q Cresistance R Ω reluctance S At/Wb resistance R Ωresistivity ρ Ωm permeability μ H/m permittivity ε F/mpotentialgradientcurrentdensity— V/m fi eldstrengthH At/m fi eldstrengthJ A/m 2 fl ux density B T fl ux density D C/m 2Also listed below are some comparable equationsE IR volt; F ΦS ampere turnJ I A amp/metre2 ; B Φ A tesla; D Q A coulomb/metre2R R 1 R 2 … ohm; S S 1 S 2 … ampere turn/weberEV/mpotential gradient E d volt/metre; H F lampere turn/metre;E V d volt/metreB μ 0 μ r H tesla; D ε 0 ε r E coulomb/metre 2μ rBB21D2; ε r orD1CC21μ r 1 for all non-magnetic materals; ε r 1 air onlyμ 0 4π 10 7 henry/metre; ε 0 8.854 10 12 farad/metreNote: Although the concept of current density has not been coveredpreviously, it may be seen from Table 4.1 that it is simply the value ofcurrent flowing through a conductor divided by the csa of the conductor.4.14 Magnetic HysteresisHysteresis comes from a Greek word meaning ‘ to lag behind ’ . It isfound that when the magnetic field strength in a magnetic material isvaried, the resulting flux density displays a lagging effect.Consider such a specimen of magnetic material that initially iscompletely unmagnetised. If no current flows through the magnetisingcoil then both H and B will initially be zero. The value of H isnow increased by increasing the coil current in discrete steps. Thecorresponding flux density is then noted at each step. If these values