Electron in Helical Motion in a Magnetic Field

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Magnetism Intermediate Electric Particles in Magnetic Field

Source: High school physics (Chinese)

Problem

An electron moves along a helical path of radius $R = 20$ cm and pitch $h = 5.0$ cm in a uniform magnetic field of magnitude $B = 2 \times 10^{-3}$ T. The electron's charge-to-mass ratio is $e/m = 1.76 \times 10^{11}$ C/kg.

Find the electron's speed.
$v_\perp \approx 7.04 \times 10^{7}$ m/s; $v_\parallel \approx 2.80 \times 10^{6}$ m/s; $v \approx 7.05 \times 10^{7}$ m/s.

Decompose the velocity into a component perpendicular to $\vec{B}$ (which gives circular motion of radius $R$) and a component parallel to $\vec{B}$ (which advances the helix).

From $R = \dfrac{m v_\perp}{eB}$:

$$v_\perp = \dfrac{e}{m} B R = (1.76 \times 10^{11})(2 \times 10^{-3})(0.20) = 7.04 \times 10^{7} \text{ m/s}.$$

The pitch equals the parallel distance traveled in one cyclotron period $T = \dfrac{2\pi m}{eB}$:

$$v_\parallel = \dfrac{h}{T} = \dfrac{h \cdot (e/m) \cdot B}{2\pi} = \dfrac{(0.05)(1.76 \times 10^{11})(2 \times 10^{-3})}{2\pi} \approx 2.80 \times 10^{6} \text{ m/s}.$$

Total speed: $v = \sqrt{v_\perp^2 + v_\parallel^2} \approx 7.05 \times 10^{7}$ m/s.