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 Physics Paper 2, Nov/Dec. 2013
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength

Question 15

Question   15

1.   Explain why it is possible for a slow moving neutron to penetrate a nucleus while an alpha particle  needs a larger amount of energy to penetrate the same nucleus.
2. List four properties of cathode rays.
3. In a photoelectric emission, a faint blue light of wavelength 5.0 x 10-7 m and a bright red light of wavelength 7.0 x 10-7m are incident on a metal plate.  Which of the radiations will release:
4.  greater number of electrons;
5. electrons of  higher maximum kinetic energy ?  Give reasons for your answers
6.  Caesium  metal of work function 2.0 eV is illuminated with radiations of wavelength 5.0 x 10-7m.  Calculate the:
7.  threshold wavelength of the metal;
8.  maximum energy of the liberal electrons.

[h = 6.6 x 10-34 Js; IeV = 1.6 x 10-19 J; c = 3.0 x 108 ms-1]

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Part(a)  Most of the candidates did not attempt this question, this could be due to lack of understanding of the topic as most teaches sometimes do not teach this aspect of the syllabus.  Most of the responding candidates did not know that it was the repulsive force of the alpha particles when approaching the nucleus that caused the extra energy.
Part (b)  Most candidates answered this part correctly.
Part (c)  Candidates were able to identify the radiations appropriately but failed to give the reasons for their choice.
Part (d)  Most candidates have no problem in tackling this aspect. Performance was    fair.
(a)       The nucleus is positively charged .  The neutron carries no charge while the alpha particle is positively charged.
The alpha particle will experience a repulsive force as it approaches the nucleus.  Hence extra amount of energy will be needed to penetrate it, while the neutron does not experience any repulsive force
(b)     Cathode rays
e.g. Cathode rays
-  are deflected by electric/magnetic field
-  are negatively charged
-  ionize gases (at certain pressures)
-   possess momentum
-   cause certain materials to glow or fluoresce
-   travel in straight path (in field free space)
-   are fast moving particles.

(c) (i)   Bright red light will release greater number of electrons because it possesses higher intensity.
(ii)   Faint blue light  will release electrons with greater maximum kinetic energy because it possess higher energy/frequency

(d)  (i)       Wo   =  hfo  =       OR   𝛌o   =
=
=    6.2  x  10-7m

(ii)                      K.Emax  =     -  Wo

=      - 2 x 1.6 x 10 -19

=    7.6  x  J