Question

Consider a proton in an MRI machine with an angular momentum aligned with the Z axis. The MRI machine applies a torque about the +X axis.

After one quarter of the precession period, along which axis does the angular momentum point ?

A. Along the +x axis

B. Along the –x axis

C. Along the +y axis

D. Along the –y axis

E. Along the +z axis

Answer 1

when a proton is kept in the presence of only magnetic field then it aligns itself with the magnetic field. But there is an angular momentum of the spin of the proton which causes it to wobble or precess in some direction. This is similar to a gyroscope. Now the precession frequency is given by the Larmor equation.

we know that the direction of the torque is perpendicular to both angular momentum and Magnetic field B applied. Thus, if the torque is in the x-direction and angular momentum is in the z-direction, then Magnetic field must be in the y-direction.

Hence the angular momentum must change along the + y-axis.