HALF QUESTION Typical of Question 12 in Leaving Cert. Exam
Q3 Answer all of the following parts (a), (b) (c) and (d).
(a) What is thermionic emission? (6)
An electron is emitted from the cathode and accelerated through a potential difference of 5 kV in a cathode ray tube (CRT) as shown in Fig 21.11.
Copy the diagram and indicate the path you would expect the electron to follow in the magnetic field. (6)
Low voltage 5 kV
Magnetic flux density into page
The electron gains 8 1016 J of energy. Assuming the speed of the electron leaving the cathode is negligible, what is the speed of the electron when at the anode? (8)
Fig 21.11
The magnetic flux density of the field is 0.4 T. Calculate the force on the electron due to the field. (b) Who won the first Nobel Prize in Physics in 1901 for the discovery of X-rays? Explain with the aid of a labelled diagram how X-rays are produced.
Calculate the kinetic energy gained by an electron when it is accelerated through a potential difference of 60 kV in an X-ray tube.
Calculate the minimum wavelength of an X-ray emitted from the anode.
(c) What is the difference between photoelectric emission and thermionic emission? (6)
Fig 21.12 shows a photocell connected in series with a sensitive galvanometer and a battery.
(i) Name the parts labelled A and B. (6) (ii) What happens at A when light falls on it? (6)
(iii) What would happen to the current if the terminals of the battery were reversed? Explain, with reference to the parts labelled A and B. (10)
(d) The threshold frequency of iron is 1.1 1015 Hz.A piece of iron is illuminated with ultraviolet light of wavelength 220 nm. Calculate:
(i) the work function of iron. (ii) the maximum kinetic energy of an emitted electron.
(Planck’s constant 6.6 1034 J s; speed of light 3.0 108 m s1; 1 eV 1.6 1019 J.) (iii) Would you still expect photoemission if visible light was shone on the metal? Explain.
