AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Mass and energy (A-level only): Conversion of units; 1 u = 931.5 MeV.
The conversion 1 u = 931.5 MeV links the atomic mass unit to energy via Einstein’s mass–energy equivalence, E = mc².
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The conversion 1 u = 931.5 MeV links the atomic mass unit to energy via Einstein’s mass–energy equivalence, E = mc². This allows nuclear masses expressed in u to be converted directly into energy in MeV, which is a convenient unit for nuclear processes. The value arises because 1 u = 1.661 × 10⁻²⁷ kg, and multiplying by c² (where c = 3.00 × 10⁸ m s⁻¹) gives approximately 1.492 × 10⁻¹⁰ J, which is equivalent to 931.5 MeV. In calculations, if a mass defect is given in u, the energy released can be found by multiplying the mass defect by 931.5 MeV u⁻¹.
For example: A mass defect of 0.020 u corresponds to 0.020 × 931.5 = 18.63 MeV.
What earns the marks
- State the conversion factor 1 u = 931.5 MeV.
- Convert mass defect in u to energy in MeV by multiplying by 931.5.
- Convert energy in MeV to mass in u by dividing by 931.5.
- Use the conversion in calculations involving nuclear binding energy or reaction energy.
- Explain the origin of the conversion factor from E = mc².
Common mistakes and how to put them right
Using 1 u = 931.5 MeV as a mass conversion without recognising it is an energy equivalent.
Put it right: It converts mass in u to energy in MeV.
Forgetting to multiply by the number of particles when calculating total energy release.
Put it right: Ensure the mass defect used corresponds to the total number of nuclei involved.
Mixing units, such as using MeV with kg without conversion.
Put it right: Consistently use either u and MeV or kg and J.
Exam tips
- Memorise the conversion factor 1 u = 931.5 MeV for quick calculations.
- When using E = mc², ensure mass is in kg and energy in J, or use the direct u to MeV conversion.
- Check that your final answer has the correct unit and magnitude.
Try the lesson
The start of the Mass and energy (A-level only): Conversion of units; 1 u = 931.5 MeV. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following is the energy equivalent of a mass of 1 u?
Listen
The conversion 1 u = 931.5 MeV links the atomic mass unit to energy via Einstein’s mass–energy equivalence, E = mc². This allows nuclear masses expressed in u to be converted directly into energy in MeV, which is a convenient unit for nuclear processes. The value arises because 1 u = 1.661 × 10⁻²⁷ kg, and multiplying by c² (where c = 3.00 × 10⁸ m s⁻¹) gives approximately 1.492 × 10⁻¹⁰ J, which is equivalent to 931.5 MeV. In calculations, if a mass defect is given in u, the energy released can be found by multiplying the mass defect by 931.5 MeV u⁻¹. For example, a mass defect of 0.020 u corresponds to 0.020 × 931.5 = 18.63 MeV. A strong answer earns marks for these points. State the conversion factor 1 u = 931.5 MeV. Convert mass defect in u to energy in MeV by multiplying by 931.5. Convert energy in MeV to mass in u by dividing by 931.5. Use the conversion in calculations involving nuclear binding energy or reaction energy. A common slip: Using 1 u = 931.5 MeV as a mass conversion without recognising it is an energy equivalent. Instead, it converts mass in u to energy in MeV. Exam tip: Memorise the conversion factor 1 u = 931.5 MeV for quick calculations.
Key words
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conversion factor
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mass defect
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MeV
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E = mc²
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See every definition
- conversion factor
- The value 1 u = 931.5 MeV, which links the atomic mass unit to energy via Einstein's mass–energy equivalence, E = mc².
- mass defect
- A quantity given in u that, when multiplied by 931.5 MeV u⁻¹, gives the energy released.
- MeV
- A convenient unit for nuclear processes, to which nuclear masses expressed in u can be converted directly.
- E = mc²
- Einstein's mass–energy equivalence, through which the conversion 1 u = 931.5 MeV links the atomic mass unit to energy.
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