Moving charges in a magnetic fieldAQA A-Level Physics: Mind map

Exam questions on Moving charges in a magnetic field

1. A beam of protons travels in a vacuum at a speed of 2.0 × 10⁶ m s⁻¹ and enters a region of uniform magnetic field of flux density 0.15 T. The velocity of the protons is at right angles to the field. The charge of a proton is 1.60 × 10⁻¹⁹ C.
   Explain why the magnetic force does not change the kinetic energy of a proton.2 marks
2. In a vacuum chamber, a beam of protons and then a beam of electrons are fired in turn, each at a speed of 4.0 × 10⁶ m s⁻¹, horizontally due east into a region of uniform magnetic field of flux density 2.0 mT directed vertically upwards. The mass of an electron is 9.11 × 10⁻³¹ kg, the mass of a proton is 1.67 × 10⁻²⁷ kg and the magnitude of the charge on each is 1.60 × 10⁻¹⁹ C.
   Calculate the radius of the circular path of the electrons.2 marks
3. A cyclotron accelerates protons. They move in semicircles inside two hollow, D-shaped metal chambers that lie in a uniform magnetic field of flux density 0.80 T, perpendicular to the plane of the semicircles. Each time a proton crosses the gap between the two chambers it is accelerated by an alternating potential difference. The protons spiral outwards and leave the cyclotron at an outer radius of 0.30 m. Proton mass = 1.67 × 10⁻²⁷ kg and charge = 1.60 × 10⁻¹⁹ C.
   Explain why the protons move in semicircles inside the chambers, and why the frequency of the alternating potential difference does not need to change as the protons gain speed.3 marks