An electron is at P at t=0 and moves anticlockwise with constant angular speed ω on the shown circular path of radius R. Point Q lies on the diameter line with CQ=2R. At which of the following times is the magnetic field at Q zero?

Question

G1 Physics · Accepted Answer

The field of the moving electron at Q is proportional to v⃗×r⃗, where r⃗ is from the electron to Q. It is zero when v⃗ is parallel or antiparallel to this line. From the circle geometry these positions occur at t=π/(3ω), 5π/(3ω), and 7π/(3ω) among the listed times. Hence A,C,D.