Skip to content

Salam Academy

Sign in Create account

LearnAQA7408First teaching 2015Paper 2 · Paper 2Nuclear physics

Paper 2 · Module

Nuclear physics

4 subtopics≈ 155 min in total

Start the module: Mass defect and binding energy Open graphing calculator

Mass defect and binding energy
Mass and energy are equivalent: $E=mc^2$ . A nucleus has less mass than its separate protons and neutrons; this mass defect $\Delta m$ corresponds to the binding energy $c^2\Delta m$ , the energy needed to separate the nucleus into its nucleons. Binding energy per nucleon peaks near iron-56. Splitting heavy nuclei (fission) or joining light ones (fusion) moves towards the peak, releasing energy.
≈ 40 min
Atoms, nuclei and radiation
The α-scattering experiment showed that an atom's mass and positive charge are concentrated in a tiny nucleus. A nuclide is written $^A_Z\text{X}$ ( $A$ nucleons, $Z$ protons). Unstable nuclei emit α (a helium nucleus), β⁻ (an electron plus an antineutrino), β⁺ (a positron plus a neutrino) or γ (a photon). Nucleon number and charge are conserved in every decay.
≈ 35 min
Radioactive decay
Radioactive decay is random (you can't predict which nucleus will decay next) and spontaneous (unaffected by temperature, pressure or chemistry). The activity is proportional to the number of undecayed nuclei: $A=\lambda N$ , where $\lambda$ is the decay constant. So $N$ , $A$ and the count rate all fall exponentially: $x=x_0e^{-\lambda t}$ . The half-life is $t_{1/2}=\dfrac{\ln2}\lambda=\dfrac{0.693}\lambda$ .
≈ 40 min
Nuclear radius and fission reactors
The closest approach of an α particle gives an upper limit on nuclear size; electron diffraction gives better values. Nuclear radius follows $R=R_0A^{1/3}$ with $R_0\approx1.2$ fm, so all nuclei have about the same density, $\sim10^{17}$ kg m⁻³. In a thermal reactor, slow neutrons sustain a chain reaction in uranium-235, controlled by a moderator and control rods.
≈ 40 min