NEB Class 11 • Physics • Focused Revision

Nuclear Physics Study Guide: NEB 11 Physics

Build nuclear reasoning in layers: notation, conservation, statistics, mass–energy and application. Retrieve each layer before combining them in exam questions.

Seven-session modern-physics plan
Decay, half-life and binding-energy drills
Graph, calculation and explanation checks

Nuclear notation connects structure, decay and energy.

Revision boundary

Anchor the plan in official scope and reliable physics

Use the CDC Physics Grade 11 page for Nepal’s official context, the paired Nuclear Physics concept guide for the full explanation and the earlier energy study guide for conservation language. Compare with OpenStax nuclear structure and half-life and activity.

Study from questions, not highlights. Every session should end with one empty-page map, at least four independent problems, a written check and a scheduled retest. Keep equations, units and scientific meaning together.

Knowledge map

Separate five reasoning layers before integrating them

Identity

A, Z, N, element, isotope and ion.

Transformation

Alpha, beta and gamma with conservation.

Population

Random events, exponential law, half-life and activity.

Energy

Mass defect, binding energy and Q-value.

Application

Fission, fusion, detection, uses and radiation protection.

When an answer fails, identify its layer. A wrong daughter element is usually notation or conservation; a wrong remaining fraction is exponential reasoning; a wrong MeV value is mass convention or conversion.

Seven sessions

A spaced Nuclear Physics revision sequence

Session	Focus	Exit evidence
1	Nucleus, nuclides and isotopes	20 inventory cards
2	Alpha, beta and gamma	12 balanced equations
3	Decay law, graphs and half-life	8 numerical and 4 graph tasks
4	Activity and units	6 rate/population conversions
5	Mass defect and binding energy	6 calculations with mass checks
6	Fission, fusion, applications and safety	One comparison table and two explanations
7	Timed integrated paper	80% plus no repeated notation/unit error

Begin each session with a two-minute retrieval of previous formulas and meanings. Finish by changing one feature of a solved problem—for example elapsed time, nuclide, decay mode or mass data—and predicting what must change before recalculation.

Notation routine

Read and write a nuclide in six deliberate steps

Read Z and identify the element.
Read A as total nucleons.
Calculate N=A−Z.
State whether an electron charge is shown separately.
Compare Z to identify isotopes or different elements.
Check that every reaction conserves A and electric charge.

Contrast drill

³⁵₁₇Cl and ³⁷₁₇Cl are isotopes: both have 17 protons, but 18 versus 20 neutrons. Cl⁻ changes electron number, not A, Z or N. ³⁷₁₈Ar is a different element despite the same A as chlorine-37.

Create cards with one field hidden: symbol, element, A, Z, N or decay daughter. Retrieve in both directions. Recognition of a printed isotope is easier than producing the correct symbol from prose, so practise both.

Decay and statistics

Combine conservation equations with population thinking

Alpha sequence

After one alpha decay, A decreases by 4 and Z by 2. After two alpha decays, A decreases by 8 and Z by 4. Do not subtract from neutron number independently; derive the daughter from conserved totals.

Beta-minus sequence

Each beta-minus decay keeps A fixed and raises Z by 1 because a neutron changes to a proton while an electron and antineutrino are emitted. Gamma emission afterward changes neither A nor Z.

Half-life table

For N₀=9600 and T_1/2=4 h: at 0, 4, 8, 12 and 16 h, N is 9600, 4800, 2400, 1200 and 600. Plot time horizontally and N vertically; equal time intervals halve, so the curve is exponential rather than a straight line.

Arbitrary time

At 10 h, t/T=2.5 and N/N₀=2^−2.5≈0.177. Linear interpolation between 8 h and 12 h would be physically wrong.

Use the PhET Alpha Decay simulation to compare unpredictable single events with a smooth large-population pattern. Record distributions, not a claim that the simulation predicts one nucleus’s decay time.

Activity and dimensional reasoning

Link half-life, decay constant and rate

Decay constant λ=ln2/T_1/2. Use consistent time units before calculating. Activity A_ct=λN has unit s⁻¹=Bq if λ is in s⁻¹. Activity is not mass, absorbed dose or biological risk by itself.

Activity from population

A nuclide has T_1/2=2.0 h=7200 s and N=5.0×10¹². λ=0.693/7200≈9.63×10⁻⁵ s⁻¹, so activity ≈4.82×10⁸ Bq. The large number reflects many nuclei, not a licence to handle such a source.

If both N and activity refer to the same nuclide, their ratio stays λ. After one half-life, both halve. If a problem gives count rate from a detector, background, geometry and efficiency may matter; do not automatically equate detector counts with total source activity.

Mass–energy routine

Audit the mass convention before multiplying by 931.5

Write the intended separated constituents.
Decide whether data are atomic or nuclear masses.
Check that electron numbers cancel or are corrected.
Compute Δm=m_separated−m_bound.
Convert using 1 u c²≈931.5 MeV.
Divide by A only when binding energy per nucleon is requested.
Interpret positive binding energy as required separation energy.

Binding-energy drill

For a model nucleus with Δm=0.0250 u and A=4, E_b=23.3 MeV and E_b/A=5.82 MeV per nucleon. Comparing total 23.3 MeV with a much larger nucleus’s total binding is not a stability comparison; use per nucleon.

Reaction Q-value

If reactant atomic masses total 5.010 u and product masses total 5.005 u with electrons balanced, Q=(0.005 u)(931.5 MeV/u)=4.66 MeV released. Write where energy can appear: kinetic energy of products and radiation.

Study the OpenStax binding-energy graph. Explain in words why light-nucleus fusion and heavy-nucleus fission can move toward higher binding per nucleon.

Application and safety reasoning

Use claim–mechanism–limit paragraphs

For an application, name the nuclear property, how it produces useful information or energy, and what control is required. Example: a tracer’s detectable radiation can reveal distribution, while choice of nuclide, activity, handling and exposure require trained oversight. Avoid promotional or medical recommendations.

For radiation protection, refer to the IAEA resources. General principles include justification and optimisation, with time, distance and suitable shielding used according to radiation type and energy. Do not suggest touching unknown sources, opening devices or improvising experiments.

For fission and fusion, compare starting nuclei, products, conditions, chain reactions and waste without claiming that either violates conservation. Use the binding-energy-per-nucleon curve as the energy explanation.

Timed transfer test

A forty-minute Nuclear Physics paper

Identify A, Z and N for three nuclides.
Distinguish isotope, ion and isobar with reasons.
Complete alpha, beta-minus and gamma equations.
Explain the antineutrino’s conservation role.
Find remaining nuclei after 3 and 2.5 half-lives.
Calculate λ and activity with unit conversion.
Infer half-life from an exponential graph.
Calculate binding energy and per-nucleon value.
Find a reaction Q-value from mass data.
Compare fission and fusion using the binding curve.
Write a radiation-use paragraph with a safety limit.

Mark identity, conservation, exponential model, mass convention, unit and interpretation separately. Repair the first wrong decision, then solve a contrast question. If the decay arithmetic is right but the daughter element is impossible, the answer is not secure.

Use the OpenStax half-life section and decay-conservation section for institutional practice. Revisit the concept guide after marking.

For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions students ask while studying Nuclear Physics

How should I start Nuclear Physics revision?

Begin with A, Z and N notation, then decay conservation, population decay, mass–energy and applications in that order.

How do I avoid half-life mistakes?

Use repeated halving only for whole intervals; otherwise calculate 2 raised to minus t over the half-life.

Why must time units be converted for activity?

Becquerel is per second, so decay constant must be in reciprocal seconds when activity is required in Bq.

How do I compare nuclear stability?

Use binding energy per nucleon with other stability evidence, not total binding energy alone.

How should I write about radiation safety?

Describe controlled institutional use and appropriate protection; never recommend handling or improvising with a source.

Where can I get Nuclear Physics revision help?

Call 9846662070 for current online or physical NEB tuition schedules and fees.

References and next steps

Sources and related study guides

Return to the concept guide, then retest failed questions after two days. Curriculum scope and linked institutional sources were checked on 2 August 2026; follow current CDC, NEB, school and laboratory instructions if requirements change.

Graph transfer

Read nuclear graphs before reaching for a formula

For an N–t or activity–t graph, identify the initial value and find the time needed to halve it at two different heights. Equal half-times support an exponential model. On a semilog graph, exponential decay becomes a straight line whose slope relates to −λ; this is an extension, not a replacement for the NEB-level half-life method.

For the binding-energy-per-nucleon curve, read the vertical quantity carefully. Moving upward means products are more tightly bound on average. Light-nucleus fusion and heavy-nucleus fission can release energy because their products move toward the high central region. The graph does not say every possible split or fusion pathway occurs spontaneously; reaction barriers and conservation still matter.

Describe each graph in three sentences: observed trend, governing physical model and limitation. Then sketch a deliberately wrong linear decay graph and diagnose why it predicts a finite time at zero, unlike the exponential population model.

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