DP Physics (last assessment 2024)

Question 20N.2.SL.TZ0.6

Date	November 2020	Marks available	[Maximum mark: 11]	Reference code	20N.2.SL.TZ0.6
Level	SL	Paper	2	Time zone	TZ0
Command term	Calculate, Determine, Estimate, Outline, Show that, State, Write down	Question number	6	Adapted from	N/A

[Maximum mark: 11]

20N.2.SL.TZ0.6

One possible fission reaction of uranium-235 (U-235) is

${}_{92}^{235}U + {}_{0}^{1}n \to {}_{54}^{140}{Xe} + {}_{38}^{94}{Sr} + 2 {}_{0}^{1}n$

Mass of one atom of U-235 $= 235 u$
Binding energy per nucleon for U-235 $= 7.59 MeV$
Binding energy per nucleon for Xe-140 $= 8.29 MeV$
Binding energy per nucleon for Sr-94 $= 8.59 MeV$

(a(i))

State what is meant by binding energy of a nucleus.

[1]

Markscheme

energy required to «completely» separate the nucleons
OR
energy released when a nucleus is formed from its constituent nucleons ✓

Allow protons AND neutrons.

(a(ii))

Outline why quantities such as atomic mass and nuclear binding energy are often expressed in non-SI units.

[1]

Markscheme

the values «in SI units» would be very small ✓

(a(iii))

Show that the energy released in the reaction is about $180 MeV$ .

[1]

Markscheme

$140 \times 8.29 + 94 \times 8.59 - 235 \times 7.59$ OR $184 « MeV »$ ✓

A nuclear power station uses U-235 as fuel. Assume that every fission reaction of U-235 gives rise to $180 MeV$ of energy.

(b(i))

Estimate, in $J {kg}^{- 1}$ , the specific energy of U-235.

[2]

Markscheme

see $« energy = » 180 \times 10^{6} \times 1.60 \times 10^{- 19}$ AND $« mass = » 235 \times 1.66 \times 10^{- 27}$ ✓

$7.4 \times 10^{13} « J {kg}^{- 1} »$ ✓

(b(ii))

The power station has a useful power output of $1.2 GW$ and an efficiency of $36 %$ . Determine the mass of U-235 that undergoes fission in one day.

[2]

Markscheme

energy produced in one day $= \frac{1.2 \times 10^{9} \times 24 \times 3600}{0.36} = 2.9 \times 10^{14} « J »$ ✓

mass $= \frac{2.9 \times 10^{14}}{7.4 \times 10^{13}} = 3.9 « kg »$ ✓

A sample of waste produced by the reactor contains $1.0 kg$ of strontium-94 (Sr-94). Sr-94 is radioactive and undergoes beta-minus ( $β^{-}$ ) decay into a daughter nuclide X. The reaction for this decay is

${}_{38}^{94}{Sr} \to X + {\bar{v}}_{e} + e$ .