Physics 001.002.004 Strong Nuclear Force Exposure 1
Alignment
Learning Intentions
By the end of the lesson, students will be able to:
- Describe the strong nuclear force as a short-range attractive force acting between nucleons.
- Identify that the strong nuclear force acts between protons and neutrons.
- Compare the strong nuclear force with electrostatic repulsion in the nucleus.
- Recognise that the strong nuclear force is essential for nuclear stability.
Success Criteria
By the end of the lesson, students have successfully:
- Stated that the strong nuclear force holds the nucleus together.
- Described that the force acts only over very short distances, about the size of a nucleus.
- Explained that the strong nuclear force overcomes proton-proton electrostatic repulsion at short range.
- Used correct terms including proton, neutron, nucleon, nucleus, electrostatic repulsion and strong nuclear force.
Syllabus Reference
- Unit 1: Thermal, Nuclear and Electrical Physics
- Topic 2: Ionising Radiation and Nuclear Reactions
- Nuclear Model and Stability
- Describe the concept of the strong nuclear force.
- Related prior learning: Explain why protons in the nucleus repel each other.
- Related next learning: Explain the stability of a nuclide in terms of the operation of the strong nuclear force over very short distances, electrostatic repulsion, and the relative number of protons and neutrons in the nucleus.
Phenomenon
A helium nucleus contains two protons and two neutrons packed into an extremely small space. Since both protons are positively charged, they repel each other by the electrostatic force.
Yet the helium nucleus does not immediately fly apart.
This raises the central question:
If protons repel each other, why does the nucleus stay together?
Key Idea
The nucleus is held together by the strong nuclear force. This is a very strong attractive force between nucleons, but it only acts over extremely short distances inside the nucleus. It must oppose the electrostatic force observed in experiment otherwise nothing would hold together.
Concept
The strong nuclear force is the force that attracts nucleons to each other.
A nucleon is either:
- a proton
- a neutron
The strong nuclear force acts between:
- proton and proton
- proton and neutron
- neutron and neutron
Inside the nucleus, protons repel each other because they have the same positive charge. This repulsion is due to the electrostatic force.
However, when nucleons are extremely close together, the strong nuclear force is stronger than the electrostatic repulsion. This allows nuclei to exist.
The strong nuclear force:
- is attractive between nucleons
- is much stronger than the electrostatic force at nuclear distances
- acts only over very short distances
- does not act noticeably outside the nucleus
- is responsible for binding protons and neutrons together into elements in the periodic table and isotope nuclides.
Convention
The key conventions associated with the concept and in the branch of established knowledge are below.
- The particles in the nucleus are called nucleons.
- Protons have charge
. - Neutrons have charge
. - The electrostatic force acts between charged particles.
- Like charges repel, so protons repel other protons.
- The strong nuclear force acts between nucleons, not between electrons and the nucleus in this Year 11 model.
- The strong nuclear force is effective only at very short distances, approximately
. - Nuclear stability depends on the balance between strong nuclear attraction and electrostatic repulsion.
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.
- The strong nuclear force is the same as gravity.
- The strong nuclear force acts over long distances like electrostatic force.
- Neutrons are only “space fillers” and do not contribute to nuclear stability.
- Protons do not repel each other inside the nucleus.
- The strong nuclear force acts only between protons.
- A larger nucleus is always more stable because it has more nucleons.
- The strong nuclear force gets stronger without limit as nucleons move further apart.
Further Reading
- QCAA Physics Unit 1: Nuclear Model and Stability
- Nuclear stability and the role of neutrons
- Radioactive decay as a consequence of nuclear instability
- Binding energy and mass defect
Explicit Instruction
The nucleus contains protons and neutrons. Protons are positively charged, so every proton repels every other proton through the electrostatic force.
The electrostatic force between two charged particles can be described qualitatively as:
- repulsive for like charges
- attractive for opposite charges
- able to act over relatively long distances
Therefore, a nucleus with several protons should be unstable if electrostatic force were the only force acting.
However, nuclei exist because another force acts inside the nucleus: the strong nuclear force.
The strong nuclear force is:
- attractive
- very strong at short distances
- effective between nucleons
- limited to nuclear distances
At distances inside the nucleus, the strong nuclear force can overcome electrostatic repulsion between protons.
A useful comparison is:
| Force | Acts on | Attractive or repulsive? | Range | Role in nucleus |
|---|---|---|---|---|
| Electrostatic force | Charged particles | Attractive or repulsive | Long range | Causes protons to repel |
| Strong nuclear force | Nucleons | Attractive at nuclear distances | Very short range | Holds nucleus together |
The strong nuclear force explains why a nucleus can contain multiple positively charged protons without immediately breaking apart.
Worked Examples
Worked Example 1
Question: A helium nucleus contains two protons and two neutrons. Why do the two protons not simply repel each other and leave the nucleus?
Solution: The two protons do repel each other because they both have positive charge. This repulsion is caused by the electrostatic force.
However, the protons and neutrons are extremely close together inside the nucleus. At this very short distance, the strong nuclear force acts between the nucleons.
The strong nuclear force is attractive and stronger than the electrostatic repulsion at nuclear distances. Therefore, the nucleus can remain held together.
Answer: The protons repel each other electrostatically, but the attractive strong nuclear force between nucleons overcomes this repulsion at short nuclear distances.
Worked Example 2
Question: Why does the strong nuclear force not hold electrons close to the nucleus in the same way it holds protons and neutrons together?
Solution: The strong nuclear force acts between nucleons. Nucleons are protons and neutrons.
Electrons are not nucleons. In the nuclear model of the atom, electrons are outside the nucleus and are not held in place by the strong nuclear force.
Electrons are attracted to the positive nucleus by the electrostatic force, not the strong nuclear force.
Answer: The strong nuclear force acts between protons and neutrons in the nucleus, not between the nucleus and electrons. Electrons are held near the atom by electrostatic attraction.
Worked Example 3
Question: A student says, “Neutrons are not important because they have no charge.” Explain why this statement is incorrect.
Solution: Neutrons have no electric charge, so they do not add electrostatic repulsion to the nucleus.
However, neutrons are nucleons, so they experience the strong nuclear force.
This means neutrons can add strong nuclear attraction without adding proton-proton electrostatic repulsion. This helps stabilise many nuclei.
Answer: The statement is incorrect because neutrons contribute to the attractive strong nuclear force. They help hold the nucleus together without increasing electrostatic repulsion.
Check for Understanding
Check 1
Question: What particles does the strong nuclear force act between?
Expected answer: It acts between nucleons: protons and neutrons.
Check 2
Question: Why do protons repel each other inside the nucleus?
Expected answer: Protons are positively charged, so they repel each other due to the electrostatic force.
Check 3
Question: Why is the strong nuclear force important for nuclear stability?
Expected answer: It provides an attractive force between nucleons that can overcome electrostatic repulsion between protons at very short distances.
Investigation (Alternative to Explicit)
Hypothesis
If a nucleus contains multiple protons, then another attractive force must act between nucleons because electrostatic repulsion alone would cause the nucleus to break apart.
Data Collection
Use a model nucleus with:
- small magnets or Velcro balls to represent attractive strong nuclear force
- labelled balls to represent protons and neutrons
- arrows to represent electrostatic repulsion between protons
Students model:
- A nucleus with one proton.
- A nucleus with two protons.
- A nucleus with two protons and two neutrons.
- A larger nucleus with many protons and neutrons.
Students record:
- number of protons
- number of neutrons
- number of proton-proton repulsions
- whether the model appears more stable or less stable
- how neutrons affect the model
Analysis
Students answer:
- What happens to electrostatic repulsion as the number of protons increases?
- Why do neutrons help stabilise the nucleus?
- Why must the strong nuclear force act over very short distances?
- Why does the strong nuclear force not affect electrons far from the nucleus?
Evaluation
Students evaluate the model by identifying:
- one way the model represents the real nucleus well
- one limitation of the model
- one feature of the real strong nuclear force that the model cannot fully show
Possible limitations:
- The model is much larger than a real nucleus.
- Magnets or Velcro do not perfectly represent the strong nuclear force.
- The model does not show quantum effects.
- The model may make forces appear to act over larger distances than they really do.
Problems
The following problems are designed to build conceptual understanding of the strong nuclear force.
-
Define the term nucleon.
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State the two types of particles found in the nucleus.
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State the charge of: a. a proton b. a neutron c. an electron
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Explain why two protons repel each other.
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Describe the strong nuclear force.
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Explain why the strong nuclear force is needed in the nucleus.
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Compare the electrostatic force and the strong nuclear force in terms of: a. particles affected b. range c. effect inside the nucleus
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A carbon nucleus contains six protons and six neutrons. Explain why the protons do not all fly apart.
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Explain why neutrons can help stabilise a nucleus even though they have no charge.
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A student says, “The strong nuclear force must act over long distances because it is very strong.” Identify the misconception and correct it.
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Complete the sentence: The strong nuclear force is strong, but it only acts over __________ distances.
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Explain why electrostatic repulsion becomes more significant in nuclei with many protons.
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Describe why the strong nuclear force is not usually noticed in everyday objects.
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Explain how the strong nuclear force is related to nuclear stability.
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Challenge: Predict why very large nuclei may be less stable than smaller nuclei, even though they contain more nucleons.
Followup
Self-check
Students should be able to answer the following without notes:
- What is the strong nuclear force?
- Which particles does it act between?
- Why do protons repel each other?
- Why does the nucleus not immediately fall apart?
- Why are neutrons important for nuclear stability?
- Why is the short range of the strong nuclear force important?
Next Topic
Explain the stability of a nuclide in terms of:
- the strong nuclear force over very short distances
- electrostatic repulsion between protons
- the relative number of protons and neutrons in the nucleus