Page 516 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
9 The proportions of different isotopes in rocks can be used to date the rocks. The half-life of uranium-238 is 4.9 × 109 years. A sample has 99.2% of the proportion of this isotope compared with newly formed rock.
a Calculate the decay constant for this isotope of uranium. [2]
b Calculate the age of the rock. [3]
10 The table shows the received count rate when a sample of the isotope vanadium-52 decays. 012345678
187 159 134 110 85 70 60 56 40
a i Draw a graph of the count rate against the time. [5] ii Comment on the scatter of the points. [1]
b From the graph, deduce the half-life of the isotope. [1]
c Describe the changes to the graph that you would expect if you were given a larger sample of
the isotope. [2]
11 The graph in Figure 31.8 shows how randomness affects count rate. State and explain what
happens if the experiment is performed using the same amount of radioactive material but at a
higher temperature. [2]
12 This question is about the nucleus of uranium-235 (235U), which has a mass of 3.89 × 10−25 kg. 92
a State the number of protons and neutrons in this nucleus. [1]
b The radius r of a nucleus is given by the equation:
r = 1.41 × 10−15 A1/3
where A is the nucleon number of the nucleus.
Determine the density of the 235U nucleus. [3]
c Explain why the total mass of the nucleons is different from the mass of the 235U nucleus. [2] 92
d Without calculations, explain how you can determine the binding energy per nucleon for the
uranium-235 nucleus from its mass and the masses of a proton and a neutron. [4]
Time / min
Count rate / Bq
92
13 a
b The main reactions which fuel the Sun are the fusion of hydrogen nuclides to form helium nuclides.
Explain what is meant by nuclear fusion and explain why it only occurs at very high temperatures. [3]
However, other reactions do occur. In one such reaction, known as the triple alpha process, three helium nuclei collide and fuse to form a carbon-12 nucleus.
i Explain why temperatures higher than those required for the fusion of hydrogen are needed for
the triple alpha process. [1]
ii Calculate the energy released in the triple alpha process. [3]
(Mass of a helium (42He) nucleus = 4.001506u, mass of a carbon (126C) nucleus = 12.000000u, 1u=1.660×10−27kg.)
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