A very long solenoid of circular cross section with radius R = 4.0 cm, is centered on, and has its length parallel to, the z axis. It has 85.0 turns per centimeter. A proton is sitting just outside the solenoid on the x axis, at x = +4.50cm. The current in the solenoid is turned on and ramped up at a rate of 43.0 A/s, so that the magnetic field points in the +z direction. The +x axis points to the right, the +y axis points towards the top, and the +z axis points out of the page.
What direction is the force on the proton? The magnitude is 1.31*10^(-21) N. The answer is in the -y direction, but I am wondering why!