Particle accelerators and detectorsEdexcel A-Level Physics: Subtopic test

1. 1
   In a particle accelerator, electrons are produced by a heated metal filament (the cathode) in an evacuated tube. They are then accelerated towards an anode through a p.d. of 2.0 kV. The charge on an electron is 1.60 × 10⁻¹⁹ C.

   (a)

   Which process releases electrons from the heated filament?

   [1 mark]

   • APhotoelectric emission
   • BIonisation of the gas
   • CThermionic emission
   • DElectromagnetic induction

   (b)

   What is the kinetic energy gained by an electron accelerated from rest through the 2.0 kV p.d.?

   [1 mark]

   • A1.6 × 10⁻¹⁹ J
   • B2.0 × 10³ J
   • C8.0 × 10⁻¹⁷ J
   • D3.2 × 10⁻¹⁶ J

   (c)

   Explain why the accelerator tube must be evacuated.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cyclotron consists of two hollow D-shaped electrodes (dees) in an evacuated chamber, placed in a uniform magnetic field of flux density 1.2 T perpendicular to the plane of the dees. An alternating p.d. is applied between the dees, and protons are injected near the centre. The charge on a proton is 1.60 × 10⁻¹⁹ C and its mass is 1.67 × 10⁻²⁷ kg.

   (a)

   What is the purpose of the magnetic field in the cyclotron?

   [1 mark]

   • ATo make the protons move in a circular path inside the dees
   • BTo accelerate the protons across the gap between the dees
   • CTo increase the speed of the protons inside the dees
   • DTo change the sign of the charge on the protons

   (b)

   Why does the p.d. between the dees have to alternate?

   [1 mark]

   • ATo make the protons move in circles
   • BSo that the electric field in the gap accelerates the protons each time they cross it
   • CTo stop the protons hitting the walls
   • DTo reverse the direction of the magnetic field

   (c)

   A proton is moving at 5.0 × 10⁶ m s⁻¹ inside one of the dees. Calculate the radius of its circular path.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A linear accelerator (linac) consists of a line of hollow cylindrical drift tubes in an evacuated chamber, connected alternately to the two terminals of an alternating supply of frequency 25 MHz. A beam of protons travels along the axis of the tubes.

   (a)

   Explain why the protons travel at constant speed inside each tube, where they are accelerated, and why the polarity of the supply must alternate.

   [3 marks]

   (b)

   A proton enters a drift tube at 1.0 × 10⁷ m s⁻¹ and spends half a cycle of the supply inside it. Calculate the length of this tube. Explain how the length of the next tube compares.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In a particle detector a uniform magnetic field of flux density 0.80 T acts perpendicular to the plane in which charged particles travel. The particles ionise the gas along their paths, leaving visible tracks. A proton leaves a circular track of radius 0.45 m. The charge on a proton is 1.60 × 10⁻¹⁹ C and its mass is 1.67 × 10⁻²⁷ kg.

   (a)

   Explain how tracks like these in a detector can be used to obtain information about the particles that produced them.

   [6 marks]

   (b)

   Show that the radius of the circular path of a particle of momentum p and charge Q is given by r = p/BQ. Use it to calculate the momentum, the speed and the kinetic energy of the proton in the detector.

   [6 marks]

   Total for question 4: 12 marks

End of questions