Atomic, Nuclear, and Astronomical Units
A non-SI unit used with SI is a defined multiple of an SI unit for the same quantity; conversion preserves the quantity while replacing the specialised scale by its exact or stated SI equivalent.
Why this shows up in the exam
Atomic energy calculations · Nuclear mass bookkeeping · Astronomical distance reporting
Learn the idea
Specialised units improve scale readability but must still reduce consistently to SI dimensions. Electron-volts, ångströms, light-years, and atomic mass units are convenient scale choices, not new physical dimensions.
🧠 Memory hook: Special scale, ordinary dimension.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- 1 eV = 1.602176634 x 10⁻¹⁹ J — electron-volt is an energy unit
- 1 u = 1.66053906660 x 10⁻²⁷ kg — unified atomic mass unit
- 1 light-year = c x 1 year — astronomical distance defined by light travel
How to approach it
- 1Name the represented quantity
- 2Write the accepted SI conversion
- 3Carry dimensions and significant digits consistently
Common slip-ups that cost marks
- •Treating electron-volt as electric potential
- •Calling a light-year a time unit
- •Mixing approximate and exact conversion factors without notice
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A quantity Q is defined as Q = force^1 / speed^2. Which dimensional formula represents Q?
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