DP Physics (first assessment 2025)

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Question 22N.1A.SL.TZ0.25

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Date November 2022 Marks available [Maximum mark: 1] Reference code 22N.1A.SL.TZ0.25
Level SL Paper 1A Time zone TZ0
Command term Question number 25 Adapted from N/A
25.
[Maximum mark: 1]
22N.1A.SL.TZ0.25

A fusion reaction of one nucleus of hydrogen-2 and one nucleus of hydrogen-3 converts 0.019 u to energy. A fission reaction of one nucleus of uranium-235 converts a mass of 0.190 u to energy.

What is the ratio specific energy of this fusion of hydrogenspecific energy of this fission of uranium?

 

A.  0.1

B.  0.2

C.  5

D.  10

[1]

Markscheme

C

Examiners report

The most frequently chosen alternative was (incorrect) A. This alternative represents the ratio of energies released in both reactions, but since the masses of the nuclides are so much different in fission and fusion, the ratio of specific energies cannot be equal to 0.1. Just remembering that fusion generally releases more energy than fission per unit mass of fuel should be sufficient to eliminate alternatives A and B.

The fusion reaction involves two isotopes of hydrogen of different masses. In this case, specific energy refers to the energy available from unit mass of a fuel containing both isotopes in the same mass proportion as in the fusion reaction. The simplest way to think about it is that the specific energy is the energy released in the reaction divided by the combined mass of the reactants, regardless of whether they are the same or different nuclides.

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