Physics 001.002.014 Energy

Alignment

Learning Intentions

By the end of the lesson, students will be able to:

• Describe energy as a quantity that can be measured in joules, , or electron volts, .
• Explain why electron volts are useful for atomic, nuclear and particle-scale energies.
• Convert energy values between , , and .
• Compare the scale of everyday energies and nuclear energies using appropriate units.

Success Criteria

By the end of the lesson, students have successfully:

• Stated that the joule, , is the SI unit of energy.
• Stated that is the energy gained by one electron when it moves through a potential difference of .
• Used to convert between electron volts and joules.
• Used prefixes such as and correctly.
• Selected an appropriate energy unit for everyday, atomic and nuclear contexts.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 2: Ionising Radiation and Nuclear Reactions
• Energy and Mass Defect: Describe energy in terms of electron volts, , and joules, .

Phenomenon

A single alpha particle emitted during radioactive decay can have an energy of about .

Written in joules, this is approximately .

This number is extremely small on the everyday scale, but very large on the nuclear scale. The same energy can therefore look small or large depending on the unit used and the scale being described.

Key Idea

Energy is measured in joules, , in the SI system. However, for very small particles such as electrons, alpha particles, beta particles and gamma photons, the joule is often too large to be convenient. The electron volt, , is a smaller unit of energy that is useful for atomic and nuclear physics.

Concept

The concept and thought that best describes the cause of the phenomenon is below.

Energy can be transferred to a charged particle when it moves through an electric potential difference. One electron volt is defined as the energy gained by a particle with the elementary charge, , when it moves through a potential difference of .

The elementary charge is:

Since :

Therefore:

and

Convention

The key conventions associated with the concept and in the branch of established knowledge is below.

• The joule, , is the SI unit of energy.
• The electron volt, , is commonly used for atomic, nuclear and particle-scale energies.
• The electron volt is a unit of energy, not a unit of voltage.
• Common prefixes include:
• In nuclear physics, energies are often written in because nuclear changes usually involve millions of electron volts.
• When converting from to , multiply by .
• When converting from to , divide by .

Misconceptions

Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.

• Students may think is a unit of voltage because it contains the word “volt”.
• Students may think is the energy of only an electron, rather than the energy gained by any particle with charge moving through .
• Students may forget that means , not .
• Students may multiply or divide by in the wrong direction.
• Students may not recognise that a small value in joules can still be large on the nuclear scale.

Further Reading

• QCAA Physics Unit 1: Ionising Radiation and Nuclear Reactions
• Topic links: nuclear reactions, mass defect, binding energy, mass-energy equivalence
• Related electrical concept: potential difference and energy transfer to charge

Explicit Instruction

Energy is the capacity to cause change or transfer energy between systems. In physics, energy may be measured using different units depending on the scale.

The joule, , is useful for everyday and macroscopic energy values. For example, the energy transferred when lifting a book, heating water, or powering an electrical appliance is usually measured in joules.

The electron volt, , is useful for microscopic energy values. It is commonly used when describing electrons, photons, ionising radiation, nuclear decay, nuclear fission and nuclear fusion.

The defining conversion is:

This means is a very small amount of energy compared with .

To convert from electron volts to joules:

To convert from joules to electron volts:

To convert from mega-electron volts to joules:

Worked Examples

Worked Example 1

Convert to joules.

Worked Example 2

Convert to electron volts.

Since :

Worked Example 3

An alpha particle has an energy of . Convert this energy to joules.

This is a tiny amount of energy in joules, but it is a large amount of energy for a single nuclear particle.

Check for Understanding

Check 1

Which unit is the SI unit of energy?

Answer: The joule, .

Check 2

Why is the electron volt useful in nuclear physics?

Answer: Nuclear and atomic energies are often extremely small in joules, so using , or gives more convenient numbers.

Check 3

A gamma photon has an energy of . Is this equal to , , or ?

Answer:

Investigation (Alternative to Explicit)

Hypothesis

If energy values are very small in joules, then converting them to electron volts will make the numbers easier to interpret in atomic and nuclear contexts.

Data Collection

Students are given a table of particle or photon energies in joules and electron volts.

Phenomenon	Energy in joules	Energy in electron volts
Visible light photon
Ultraviolet photon
X-ray photon
Alpha particle

Students convert each joule value into electron volts using:

Analysis

Students compare the converted values and decide whether , or is the most appropriate unit for each phenomenon.

Example:

A visible light photon has an energy of approximately , so is an appropriate unit.

Evaluation

Students evaluate why scientists use different energy units at different scales.

Prompt questions:

• Why is useful for everyday energy values?
• Why is useful for atomic and nuclear values?
• Why might be more useful than for nuclear decay?
• What errors could occur if a student forgets the meaning of , or ?

Problems

The following problems are designed to practise describing and converting energy in terms of joules and electron volts.

1. Define one electron volt in words.

2. State the conversion factor between and .

3. Convert to joules.

4. Convert to joules.

5. Convert to electron volts.

6. Convert to joules.

7. Convert to electron volts.

8. Convert to joules.

9. Convert to electron volts.

10. Convert to electron volts and then to mega-electron volts.

11. A beta particle has an energy of . Convert this to joules.

12. A gamma photon has an energy of . Convert this to .

13. Explain why is a small energy in everyday life but a large energy for a single nuclear particle.

14. Choose the most appropriate unit for each value: , , or .

    • energy used by a phone battery
    • energy of a visible light photon
    • energy of an X-ray photon
    • energy released by one nuclear decay particle

15. A student writes: “An electron volt is a type of voltage.” Explain why this statement is incorrect.

Followup

Self-check

Students should be able to answer the following without notes:

• What is the SI unit of energy?
• What is one electron volt?
• Is a unit of energy or voltage?
• What is the value of in joules?
• How do you convert from to ?
• How do you convert from to ?
• Why are and commonly used in nuclear physics?

Next Topic

The next topic is artificial transmutation. Students will use their understanding of nuclear-scale energy units when describing nuclear reactions, fission, fusion, mass defect and binding energy.