Magnetism and Moving Charges Practice Test

In a lab, a straight wire carries $I=8.0,\text{A}$ upward. Moving charges in the wire create circular magnetic field lines around the wire; use the right-hand rule (thumb along current, curled fingers give $\vec B$ direction). A point $r=0.040,\text{m}$ east of the wire lies in air ($\mu_0=4\pi\times10^{-7},\text{T·m/A}$). Refer to the scenario above. Calculate the magnetic field strength at that point.

A proton ($q=+e$, $m=1.67\times10^{-27},\text{kg}$) enters a uniform magnetic field $B=0.30,\text{T}$ with velocity perpendicular to $\vec B$. Moving charges create magnetic fields, but here the external field causes a Lorentz force $F=qvB$ that bends the path without changing speed. The proton speed is $v=2.4\times10^{6},\text{m/s}$. Refer to the scenario above. Explain why the charged particle follows a circular path in the magnetic field.