Why do both fission and fusion release energy?

Both move towards nuclei with higher binding energy per nucleon, and the mass difference leaves as energy.

Does the missing mass disappear?

No, it becomes energy by E = mc squared, so total mass-energy is conserved.

S6 Physics Nuclear Energy, Paper 1: mass-energy and reactor sums

2026-11-02

Ten graded items from E = mc squared through mass defect, fission energy, chain reaction and reactor power and efficiency.

Focus　Always start from E = mc squared or energy per fission times the count, and watch the u to MeV and eV to J conversions.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S6 Physics: Nuclear Energy
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationHow much energy is released when 1 kg of mass is converted completely, with c as 3.0 times 10 to the eighth metres per second?
FoundationA nuclear reaction has a mass defect of 0.001 kg. Find the energy released.
FoundationOne fission has a mass defect of 0.2 u. Find the energy released, taking 1 u as 931 MeV.
FoundationEach fission releases 3 neutrons. How many fissions have occurred after 3 generations?
IntermediateFission of 1 kg of uranium gives 9.0 x 10^16 J while 1 kg of coal gives 3.0 x 10^7 J. How many times greater is the uranium?
IntermediateA reactor has a thermal power of 3.0 x 10^9 W at 33 per cent efficiency. Find the electrical output.
IntermediateEach fission gives 200 MeV and 1 eV is 1.6 x 10^-19 J. How many fissions per second does a 1 GW reactor need?
ChallengeA uranium-235 nucleus has 235 nucleons and 7.6 MeV of binding energy per nucleon. Find the total binding energy.
ChallengeA reactor delivers 8.64 x 10^13 J in a day and each fission gives 3.2 x 10^-11 J. Find the number of fissions needed.
Error redoA student uses E = mc instead of E = mc squared and gets 3.0 x 10^8 J for 1 kg. What is the correct value and the ratio?

03　Answers and one-line notes

#	Answer	One-line note
1	9.0 x 10^16 J	Mass times the square of the speed of light gives 9.0 times ten to the sixteenth joules.
2	9.0 x 10^13 J	Energy is the mass defect times the speed of light squared, so 0.001 kg gives 9.0 times ten to the thirteenth joules.
3	186.2 MeV	Each atomic mass unit stands for 931 MeV, so a defect of 0.2 u gives 186.2 MeV.
4	27 fissions	Each generation multiplies the neutrons by three, so three generations give twenty-seven fissions.
5	3.0 x 10^9 times	Dividing the two figures gives 3.0 times ten to the ninth, which shows how dense nuclear fuel is in energy.
6	9.9 x 10^8 W	Output power is thermal power times efficiency, so 33 per cent of three gigawatts is about 0.99 GW.
7	3.125 x 10^19 per second	Each fission gives about 3.2 times ten to the minus eleventh joules, so a gigawatt needs roughly 3.1 times ten to the nineteenth per second.
8	1786 MeV	Binding energy per nucleon times the nucleon number gives 1786 MeV for the whole nucleus.
9	2.7 x 10^24 fissions	Daily energy divided by the energy per fission gives 2.7 times ten to the twenty-fourth fissions.
10	9.0 x 10^16 J, a factor of 3.0 x 10^8 larger	He forgot to square the speed of light, so the true value is 9.0 times ten to the sixteenth joules, larger by a factor of c.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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