1. The problem statement, all variables and given/known data The magnetic flux density in the vicinity of a large air-cored electromagnet is determined by measuring the induced flow of charge in a small coil as the current in the electromagnet is switched on -- see the diagram below. Calculate the magnetic flux density at the position of the small coil, due to the electromagnet from the following data: radius of small coil = 10 mm, resistance per unit length of wire of small coil = 0.10 Ω m-1, total flow of charge through the small coil due to switching on the electromagnet = 0.01 C. Answer: 0.2 T. 2. The attempt at a solution Q = B A N / R → B = Q R / A N. Q = 0.01, R = 0.1 Ω, A = ?, N = 1 turn (assumed). A = π r2 = π * (10 / 10 / 100)2 = 3.14 * 10-4 m2. B = 0.01 * 0.1 / 3.14 * 10-4 * 1 = 3.18 T. What's wrong?