Nuclear Physics and Electronics: decay, devices and logic
Nuclear Physics and Electronics: decay, devices and logic
NEB Class 12 Nuclear Physics and Electronics covers radioactive decay, half-life, nuclear reactions and mass defect, then semiconductors, diodes, transistors and logic gates. Numerical work includes activity after a time and energy from mass defect; electronics questions test biasing and truth tables.
Core rules
- Decay law: N = N₀e^(−λt); activity A = λN; half-life T₁/₂ = ln2/λ = 0.693/λ.
- After n half-lives, remaining fraction = (1/2)^n; mean life τ = 1/λ.
- Mass defect Δm gives energy E = Δmc²; 1 u ≈ 931.5 MeV.
- Fission splits a heavy nucleus; fusion joins light nuclei; both can release energy when binding energy per nucleon increases.
- Diode: forward bias lowers the depletion layer and allows current; reverse bias widens it. NAND and NOR are universal gates.
Worked example
A radioactive sample has initial activity 6400 Bq and half-life 8 days. Find its activity after 24 days. Number of half-lives n = 24/8 = 3. Remaining fraction = (1/2)^3 = 1/8. A = 6400/8 = 800 Bq. The activity falls exponentially; after each half-life it halves, not by a constant amount.
Traps
Confusing half-life with mean life; using days in λ without matching time units; treating activity as linear; forgetting to convert mass defect to kg or atomic mass units consistently; reversing diode bias; assuming all gates are universal; misreading a truth table with three inputs.