Particle accelerators and detectorsEdexcel International A Level Physics: Subtopic test

1. 1
   An electron gun in an evacuated tube has a heated tungsten filament. Electrons released from the filament are accelerated through a potential difference of 1500 V between the filament and a metal anode.

   (a)

   The process by which electrons are released from the heated filament is

   [1 mark]

   • Aphotoelectric emission
   • Bionisation of gas in the tube
   • Cthermionic emission
   • Delectromagnetic induction

   (b)

   Assuming the electrons are released with negligible kinetic energy, their speed on reaching the anode is about

   [1 mark]

   • A1.6 × 10⁷ m s⁻¹
   • B5.3 × 10¹⁴ m s⁻¹
   • C4.6 × 10⁷ m s⁻¹
   • D2.3 × 10⁷ m s⁻¹

   (c)

   Explain why the electron gun must be evacuated.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cyclotron accelerates protons using two hollow D-shaped metal electrodes (dees) separated by a small gap. A uniform magnetic field of flux density 1.2 T acts perpendicular to the plane of the dees, and an alternating potential difference is applied across the gap.

   (a)

   The role of the magnetic field in the cyclotron is to

   [1 mark]

   • Amake the protons move in semicircles inside the dees
   • Bincrease the kinetic energy of the protons inside the dees
   • Caccelerate the protons across the gap between the dees
   • Dmake the protons move in straight lines inside the dees

   (b)

   The frequency of the alternating potential difference needed for protons of mass 1.67 × 10⁻²⁷ kg is about

   [1 mark]

   • A1.1 × 10⁸ Hz
   • B1.8 × 10⁷ Hz
   • C3.7 × 10⁷ Hz
   • D2.9 × 10⁶ Hz

   (c)

   Explain why the radius of the path increases as the protons gain speed, but the time taken for each semicircle stays the same.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A proton moves in a uniform magnetic field of flux density 1.5 T that is perpendicular to its velocity, in a bubble chamber. It leaves a circular track of radius 0.085 m. The proton charge is 1.60 × 10⁻¹⁹ C.

   (a)

   Calculate the momentum of the proton.

   [3 marks]

   (b)

   Derive the equation r = p/BQ for a particle of charge Q and momentum p moving perpendicular to a uniform field B. A helium nucleus (charge +2e) with the same momentum as the proton enters the same field in the same way. Calculate the radius of its path.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research group uses a linear accelerator (linac) to accelerate protons. The protons are then directed into a detector placed in a uniform magnetic field, which is used to study the tracks left by charged particles.

   (a)

   Describe how a linear accelerator accelerates charged particles, and explain why the drift tubes get longer along the accelerator.

   [6 marks]

   (b)

   Explain how the tracks left by charged particles in the detector can be used to find the sign of the charge and the momentum of a particle, and why a track may spiral inwards.

   [6 marks]

   Total for question 4: 12 marks

End of questions