Problem 4: Here's a terrible model for Earth's magnetic field: Imagine that Earth's core is a circular current loop carrying current I, with radius R equal to the radius of the core. In this model, what does the current have to be to make the field we observe at Earth's north magnetic pole? Be as imprecise as you like. This is just an order-of-magnitude problem! Give a numerical answer in Amperes. Extra credit: Find a combination of B, R, and /o that have units of energy. Compare the energy in the magnetic field to (say) the gravitational binding energy of the Earth. Technically you will be more accurate if you use the B-field value in the core, not at the surface