Topic 11: Nuclear Radiation — Edexcel A-Level Physics
Test yourself on Topic 11: Nuclear Radiation with PEARSON EDEXCEL A-Level practice questions.
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Topic 11: Nuclear Radiation explained
The atomic mass unit is a mass defined so that one atom of carbon-12 has a mass of exactly 12 u.
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Its value is 1 u = 1.66 × 10⁻²⁷ kg, so it lets you quote nuclear and particle masses as convenient small numbers instead of tiny kilogram values. To convert u to kg, multiply by 1.66 × 10⁻²⁷; to convert kg to u, divide by 1.66 × 10⁻²⁷. For example, a proton of mass 1.007 u is 1.007 × 1.66 × 10⁻²⁷ kg ≈ 1.67 × 10⁻²⁷ kg. A mass defect of 0.030 u becomes 0.030 × 1.66 × 10⁻²⁷ kg ≈ 5.0 × 10⁻²⁹ kg, ready for use in E = mc². Always check the direction of the conversion and keep the power of ten consistent.
Your focus
- State the value of the atomic mass unit in kilograms.
- Convert a mass given in u into kg and a mass in kg into u.
- Apply the conversion to a nuclear mass defect before an energy calculation.
Topic 11: Nuclear Radiation exam tips
Marking Points
- States that 1 u = 1.66 × 10⁻²⁷ kg (the unified atomic mass unit).
- Converts u to kg by multiplying the number of u by 1.66 × 10⁻²⁷.
- Converts kg to u by dividing the mass in kg by 1.66 × 10⁻²⁷.
- Applies the conversion to a nuclear or particle mass, e.g. 0.030 u ≈ 5.0 × 10⁻²⁹ kg.
- Keeps the correct power of ten and checks the direction of the conversion.
Examiner Tips
- 💡Write the conversion factor 1 u = 1.66 × 10⁻²⁷ kg at the start of any calculation so the direction is clear.
- 💡Estimate the answer's order of magnitude to catch a misplaced power of ten.
- 💡Show the conversion step explicitly before substituting into E = mc².
Common Mistakes
- Dividing instead of multiplying when converting u to kg: the correction is to multiply by 1.66 × 10⁻²⁷ to go from u to kg.
- Using the wrong power of ten, such as 10⁻³¹ instead of 10⁻²⁷: the correction is to use 1 u = 1.66 × 10⁻²⁷ kg.
- Forgetting to convert a mass defect in u to kg before using E = mc²: the correction is to convert to kg first so the energy comes out in joules.