electromagnestism

"A 30 cm long solenoid with a 2 cm diameter is to produce a 0.2 T magnetic field in its centre. If the maximum current is 8 A, how many turns must the solenoid have? (Assume that each coil is circular rather than helical.)" I know that I have to use the formula B = (μNI)/L, but I don't...

I am stuck on this module 6 electromagnetism question: 'Two charged parallel plates have a potential difference of 35.0 V. How far apart must these two plates be so that a point charge of -4.0*10^-8 C with a mass of 2.5*10^-6 kg will be exactly suspended against gravity?" I dont have a...