The photograph shows the track of a positron in a particle detector. There is a magnetic field
perpendicular to the plane of the track.
Which of the following is not a correct statement?
A The energy of the positron is decreasing.
B The magnetic field acts into the page.
C The positron is gaining mass.
D The speed of the positron is decreasing
The only correct answer is C
A is not correct as the statement is true. The inward spiral is due to a decrease in momentum,
since r = p/BQ and B and Q are unchanged, so this must be linked to a decrease in energy.
B is not correct as the statement is true. Using knowledge of particle tracks we know that the
radius of the track is decreasing with time, so the particle enters the picture from the left.
As it is positively charged, this is the direction of the current. The direction of the spiral shows
us the direction of the force acting on the particle. Using Fleming’s left hand rule, the field must
be acting into the page.
D is not correct as the statement is true since we know from the inward spiral that the momentum is
decreasing
