Physics unit guide
Nuclear and quantum physics
The photoelectric effect, matter waves, atomic spectra, decay, nuclear reactions and binding energy.
Unit 5 is where the course asks you to hold evidence and model together. Nearly every extended question is really asking which observation forces a particular conclusion - the photoelectric effect is examined because it cannot be explained by waves, and discrete spectra because they cannot be explained by continuous energy levels. Answers that describe the model without naming the evidence lose most of the marks available.
Topics in this unit
What gets examined
- Explaining which features of the photoelectric effect defeat a wave model: the threshold frequency and the instant emission.
- Using E = hf and the photoelectric equation, and reading work function and Planck's constant off a graph.
- Calculating de Broglie wavelength and explaining what electron diffraction demonstrates.
- Relating atomic emission and absorption spectra to discrete energy levels.
- Completing nuclear equations, conserving both nucleon number and proton number.
- Mass defect and binding energy per nucleon, and using the binding energy curve to explain fission and fusion.
How to revise it
Pair every model with its evidence
Photon model and the threshold frequency. Energy levels and discrete spectral lines. Matter waves and electron diffraction. Learning them as pairs answers the question that is actually asked.
Read the photoelectric graph as a straight line
Gradient is Planck's constant, the intercept gives the work function. Recognising it as y = mx + c turns a hard-looking question into a routine one.
Use binding energy per nucleon, not total
The curve peaks near iron, and both fission and fusion move towards that peak. Total binding energy does not explain anything on its own, and answers using it miss the point.
Balance nuclear equations on both numbers
Nucleon number on top, proton number below, both conserved. Checking both is quick and catches nearly every error in this topic.
Where marks get lost
- Saying brighter light gives photoelectrons more energy - it gives more of them, at the same energy.
- Using the work function in joules where the question gives electronvolts, or the reverse.
- Forgetting to convert mass defect from atomic mass units before using E = mc².
- Claiming fusion releases energy because the products are heavier - it is about binding energy per nucleon.
- Writing a decay equation that conserves nucleon number but not charge.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.