An electron accelerated by a potential difference U = 400 V flew into a uniform magnetic field

An electron accelerated by a potential difference U = 400 V flew into a uniform magnetic field with an induction of B = 1.5 mT and described an arc of a circle. Find the radius of this circle R.

The lines of magnetic induction are directed perpendicular to the electron velocity vector.
1.Fl = evB = mv ^ 2 / R, where m = 9.1 * 10 ^ -31 kg is the electron mass,
e = 1.6 * 10 ^ -19 is the electron charge.
2.R = mv / e / B (1)
eU = mv ^ 2/2 – the electric field imparts kinetic energy to the electron.
v ^ 2 = 2eU / m – substitute in (1) -> R = m / (eB) * √ (2eU / m) =
= 1 / B * √ (2Um / e);
3.R = 1 / 1.5 * 10 ^ -3 T * √ (2 * 400 V * 9.1 * 10 ^ -31 kg / 1.6 * 10 ^ -19 C) =
= 4.5 * 10 ^ -2 m = 4.5 cm.
Answer: 4.5 cm.