[填空题]
A mass spectrometer is a tool that uses a magnetic field to determine the mass of a moving charged particle.
An alpha particle with a speed of $2.0\times10^7\,ms^{−1}$ enters a region of magnetic field directed into the plane of the paper.
(1)Suggest why the alpha particle undergoes circular motion inside the magnetic field.
(2)Explain why the centripetal force does no work on the alpha particle.
2.(1)On the diagram, draw an arrow to represent the path of the alpha particle inside the magnetic field.
(2)State the charge on the alpha particle.
(3)The radius of the path of the alpha particle is determined to be $15\,cm$, and the magnetic field strength is $2.77\,T$.
Calculate the mass of the alpha particle, expressing your answer to the appropriate number of significant figures. m = $\times10^{-27}\,kg$
