There is a toroid coil on the iron core the iron core has a circumference of 100cm and a cross sectional area of 10 cm² the number of turns is 1000 and the value of Ur is 500 find the inductance (L = ?). How long time does it take to increase the primary coil uniformaly from 0 to 20 Ampers, if the induced voltage on the coil was 2.5Vdc?