EXAMPLE 1.1 Figure E1.1 represents the magnetic circuit of a primitive relay. The coil has 500 turns and the mean core path is le = 360 mm. When the air gap lengths are 1.5 mm each, a flux density of 0.8 tesla is required to actuate the relay. The core is cast steel. (a) Find the current in the coil. (b) Compute the values of permeability and relative permeability of the core. (c) If the air gap is zero, find the current in the coil for the same flux density (0.8 T) in the core. Core Mean path m Flux density, B (tesla) 1.3 14 -Silicon+ sheet steel 1.2 1.1 Cast steel 1.0 0.9 0.8 0.7 0.6 0.5 Cast iron 0.4 B* 0.3 0.2 0.1 0 200 400 600 Field intensity, H (At/m) 800 1000