Question

12(a)(i). State the function of each of the following parts of a modern X-ray tube: heater; high tension source; cooling fins.
12(b). State one design feature each for: the glass envelope being highly evacuated; the target being a metal of very high melting point; the cooling fins being located outside the glass envelope.
12(c). In a nuclear fission reaction, a nuclide ²³⁵₉₂U is bombarded with a neutron to produce ⁹³₃₆Kr and ¹⁴¹₅₆Ba with additional neutrons; the energy involved in the process is Q. (i) Write the balanced nuclear reaction equation. (ii) State with reason whether Q is absorbed or released. (iii) Calculate the value of Q in joules.
12(d). State three differences between nuclear fusion and nuclear fission.

Accepted Answer

(a)(i) Heater: heats up the filament to release thermoelectrons. High tension source: accelerates the liberated thermoelectrons towards the target. Cooling fins: reduce/dissipate the intense heat produced at the target by radiation.
(b) The glass envelope is highly evacuated to prevent gaseous ionization in the tube (which could interfere with the process). The target is a metal of very high melting point to withstand the high energy generated from collisions of thermoelectrons with the target, to prevent it from melting. The cooling fins are located outside the glass envelope to prevent a possible breakup/crack of the glass due to heat generated, and to increase the rate of cooling.
(c)(i) Balanced equation: ²³⁵₉₂U + ¹₀n → ⁹³₃₆Kr + ¹⁴¹₅₆Ba + 2¹₀n + Q.
(ii) Q is released, because the sum of the masses of the reactants (235.044+1.009=236.053u) is greater than the sum of the masses of the products (91.898+140.914+2(1.009)=234.830u); the mass defect is converted to energy which is released.
(iii) Δm=236.053u-234.830u=1.223u=1.223×1.66×10⁻²⁷kg. Q=Δmc²=1.223×1.66×10⁻²⁷×(3×10⁸)²=1.83×10⁻¹⁰ J.
(d) Differences between nuclear fission and nuclear fusion: In fission, a heavy nucleus splits into lighter nuclei; in fusion, lighter nuclei combine to form a heavy nucleus. Fission produces harmful by-products; fusion produces harmless by-products. Fission produces less energy than fusion, which produces greater energy. Fission has a lower initiation temperature requirement; fusion has a higher initiation temperature requirement. Fission uses expensive/less abundant raw materials; fusion uses cheaper and more abundant raw materials.

WAEC Physics 2023 Theory — Question 12

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