1. The problem statement, all variables and given/known data An electron in a cathode-ray tube is accelerated through a potential difference of 10 kV, then passes through the d = 2.4-cm-wide region of uniform magnetic field in the figure. 2. Relevant equations - Am I taking the entirely wrong approach in the solution? 3. The attempt at a solution Since this is circular motion, we can draw a circle which intersects the square in the picture at where the electron exits the cathode ray tubes and the place it exits the square. Geometry then gives the angle between the tangent to the latter point and the hypotenuse, and we find that the radius is 10.669 cm. We could then use mv^2/R = qvB, and we know that 1/2mv^2 = q*10000 from the cathode ray tube. Solving for B gives B = 4.1 * 10^-7 T, which is incorrect.