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The four fundamental forces (QCE Physics Unit 4)

Syllabus dot point

“Describe the four fundamental forces (gravitational, electromagnetic, strong nuclear, weak nuclear), their gauge boson mediators (in the Standard Model), their relative strengths and effective ranges, and applications in nuclear and particle physics”

10 min read

Quick answer

The four fundamental forces are: the strong nuclear force (gluon-mediated, binds quarks into hadrons and hadrons into nuclei, 10−1510^{-15} m range), the electromagnetic force (photon-mediated, infinite range, binds atoms and molecules), the weak nuclear force (W and Z boson-mediated, responsible for beta decay and pion / kaon decays, 10−1810^{-18} m range), and the gravitational force (hypothetical graviton, infinite range, dominant only at astronomical scales due to its extreme weakness at the particle level).

Jump to a section
  1. What this dot point is asking
  2. The four forces
  3. Relative strengths summary
  4. Why each force matters
  5. Beta decay as a worked example
  6. Examples in context
  7. Try this
  8. Exam-style questions

What this dot point is asking

QCAA wants you to describe the four fundamental forces, identify their gauge boson mediators, state their relative strengths and ranges, and apply this framework to nuclear and particle physics. The four forces are: gravitational, electromagnetic, strong nuclear, weak nuclear.

The four forces

Strong nuclear force

Mediator
Gluon (8 types). Massless. Carry colour charge.
Acts on
Quarks (and hadrons by extension).
Strength (at 10−1510^{-15} m)
∼1\sim 1 (the reference strength; about 100 times stronger than electromagnetism at this distance).
Range
About 10−1510^{-15} m (one femtometre, nuclear size). Effectively confining for quarks; never observed in isolation.
Role
Binds quarks into hadrons. The residual strong force (sometimes called the nuclear force) binds protons and neutrons into nuclei despite Coulomb repulsion between protons.

The strong force has an unusual property called asymptotic freedom: it is weaker at very short distances (inside hadrons, quarks behave almost freely) and stronger at larger distances (which is what confines quarks). This is the opposite of most other forces and is responsible for the experimental difficulty of isolating quarks.

Electromagnetic force

Mediator
Photon. Massless. No charge.
Acts on
Electrically charged particles. (Quarks, charged leptons, W bosons.)
Strength (at 10−1510^{-15} m)
∼10−2\sim 10^{-2}. About 100 times weaker than the strong force at nuclear distances.
Range
Infinite. The Coulomb force falls off as 1/r21/r^2 but never vanishes.
Role
Binds electrons to nuclei (atomic structure). Holds atoms together into molecules. Responsible for all of chemistry, optics, and ordinary matter at scales above nuclei. Coulomb repulsion between protons in heavy nuclei is the reason nuclei larger than uranium are unstable.

Weak nuclear force

Mediator
W+^+, W−^-, Z0^0 bosons. Massive (80-91 GeV/c2^2). Charged or neutral.
Acts on
All quarks and leptons (including neutrinos, which feel only the weak and gravitational forces).
Strength (at 10−1510^{-15} m)
∼10−6\sim 10^{-6} to 10−710^{-7}. Much weaker than the strong or electromagnetic forces.
Range
About 10−1810^{-18} m (one thousandth of a nuclear diameter). Extremely short range because of the heavy mediator bosons.
Role
Beta decay (neutron to proton plus electron plus antineutrino, mediated by W−^-). Pion and kaon decays. Most processes involving neutrinos. The weak force is the reason free neutrons are unstable (half-life about 10 minutes) and the reason the sun's hydrogen fusion proceeds slowly enough for stars to last billions of years.

In modern physics the electromagnetic and weak forces are unified as the electroweak force above approximately 100 GeV; below this energy they appear distinct because the W and Z bosons are massive while the photon is massless.

Gravitational force

Mediator
Graviton (hypothesised, not observed). Would be massless, spin 2.
Acts on
All particles with mass-energy (i.e., everything).
Strength (at 10−1510^{-15} m)
∼10−38\sim 10^{-38}. Extraordinarily weak compared to the others at small scales.
Range
Infinite. Falls off as 1/r21/r^2 (Newton's law).
Role
Dominant on astronomical scales because (a) it has infinite range and (b) all mass-energy is positive, so gravity always attracts and accumulates over cosmic distances. Negligible at atomic scales because of its weakness. Not yet successfully quantised; the graviton remains hypothetical.

Gravity is described by Einstein's general relativity (1915), not by the Standard Model. Unifying general relativity with quantum mechanics is the major unsolved problem of theoretical physics.

Relative strengths summary

Force Strength (relative, at 10−1510^{-15} m) Range
Strong 1 ∼10−15\sim 10^{-15} m
Electromagnetic 10−210^{-2} Infinite
Weak ∼10−6\sim 10^{-6} to 10−710^{-7} ∼10−18\sim 10^{-18} m
Gravitational 10−3810^{-38} Infinite

These strengths are dramatically different. The strong force dominates inside nuclei; electromagnetism dominates at atomic and chemical scales; gravity dominates at astronomical scales.

Why each force matters

Strong
Without it, no atomic nuclei beyond hydrogen would be stable (Coulomb repulsion would tear them apart). No stars, no elements heavier than hydrogen, no life.
Electromagnetic
Without it, no atoms (electrons would not bind to nuclei). No chemistry, no light, no electromagnetism. Most of everyday physics.
Weak
Without it, no beta decay; the cosmic distribution of elements would be very different. The hydrogen fusion in the sun begins with a weak-force process (proton-proton fusion via a W boson), so without the weak force, the sun would not shine. Most natural radioactive decays involve the weak force.
Gravitational
Without it, no large-scale structure (stars, planets, galaxies). Negligible at scales below planets, but cumulatively decisive at astronomical scales.

Beta decay as a worked example

A free neutron decays via the weak force into a proton, electron, and electron antineutrino:

n→p+e−+νˉen \to p + e^- + \bar{\nu}_e

At the quark level: a down quark in the neutron emits a virtual W−^- boson, becoming an up quark. The W−^- then decays into an electron and electron antineutrino.

d→u+W−→u+e−+νˉed \to u + W^- \to u + e^- + \bar{\nu}_e

The process takes about 10 minutes for a free neutron (half-life). Bound in stable nuclei, neutrons can be stable.

Beta-plus decay (positron emission) is the equivalent process for an up quark to down quark, emitting a W+^+. The W+^+ decays into a positron and electron neutrino.

Common errors
Confusing strong force with weak force
Strong is what binds nuclei (gluon mediator, 10−1510^{-15} m range). Weak is responsible for beta decay (W/Z mediators, 10−1810^{-18} m range, much weaker).
Treating the photon as carrying only visible light
The photon mediates all electromagnetic interactions, including the static Coulomb force between charges. Virtual photons are exchanged between charged particles even when no light is being emitted.
Forgetting that gravity is in a different theoretical framework
Standard Model: strong, electromagnetic, weak (Quantum Field Theory). General Relativity: gravity (classical, geometric). Unification is unsolved.
Confusing mediator type with force
The photon mediates electromagnetism but is not itself the electromagnetic force. A force is an interaction; the mediator is the particle exchanged.
Comparing force strengths at the wrong scale
Strengths quoted at 10−1510^{-15} m differ from strengths at atomic or molecular scales because of how the forces fall off with distance.

Examples in context

Example 1. ANSTO Mt Cotton's neutron-activation analysis of mineral samples exploits the strong nuclear force to bind neutrons into target nuclei. The strong force is mediated by gluons over ∼10−15 m\sim 10^{-15} \text{ m}, dominating inside the nucleus despite electromagnetic repulsion of protons. QCAA Unit 4 dot-point comparison of force strengths and ranges is exactly this analysis.

Example 2. A Gladstone industrial transformer relies on the electromagnetic force (mediated by photons, infinite range) to induce EMF via Faraday's law. The same force at the atomic scale binds electrons to nuclei. Beta decay in cobalt-6060 sources operates via the weak force (W-boson mediated, range ∼10−18 m\sim 10^{-18} \text{ m}). All four fundamental forces appear in the QCAA Unit 4 EA Paper 2 comparative-magnitudes question.

Try this

Q1. List the four fundamental forces in order of decreasing relative strength. [2 marks]

  • Cue. Strong, electromagnetic, weak, gravitational.

Q2. Identify the gauge bosons that mediate the strong and electromagnetic forces, and state the range of each. [3 marks]

  • Cue. Strong: gluons, ∼10−15 m\sim 10^{-15} \text{ m}; electromagnetic: photons, infinite range.

Q3. A cobalt-6060 source undergoes beta-minus decay. (a) Identify the fundamental force responsible and the mediating boson. (b) Justify why this force is weaker than electromagnetism. (c) Compare with the gravitational force binding the satellite tracked by Mt Cotton at 500 km500 \text{ km} altitude. [3+2+3 marks; ISMG: Knowledge and conceptual understanding, Evaluation]

  • Cue. (a) Weak force, W-boson; (b) very short range (10−18 m10^{-18} \text{ m}); (c) gravity is ∼10−39\sim 10^{-39} relative to strong, but dominates at large scale due to infinite range.

Exam-style questions

Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Practice4 marks
Describe the role of the strong nuclear force, including (a) which particles experience it, (b) its mediator, (c) its approximate range, and (d) why it is necessary in atomic nuclei.
Show worked answer →
(a) Which particles
The strong force acts on quarks (and on hadrons made of quarks, including protons and neutrons). Leptons do not experience the strong force.
(b) Mediator
The gluon. There are 8 distinct gluons in the Standard Model, all massless.
(c) Range
Effectively about 10−1510^{-15} m (1 femtometre, comparable to the size of a nucleus). The strong force is confining: quarks cannot be isolated, and the force between hadrons (the residual strong force, often called the nuclear force) falls off rapidly beyond nuclear size.
(d) Why necessary
Protons in a nucleus are mutually repulsive due to their positive electric charge (Coulomb repulsion). Without the strong force, no nucleus heavier than hydrogen could be stable. The strong force, although effective only over short range, is roughly 100 times stronger than electromagnetism at nuclear distances and binds the protons and neutrons together. The competition between strong (binding) and electromagnetic (repulsive) forces explains why nuclei larger than uranium are unstable.

Markers reward identification of the gluon mediator, the femtometre range, and the binding-vs-Coulomb-repulsion competition.

Practice3 marks
(a) Identify the gauge boson(s) of the weak force. (b) State one important process that the weak force mediates.
Show worked answer →

(a) Weak force bosons. W+^+, W−^- (charged) and Z0^0 (neutral). All three are massive (80 GeV/c2^2 for W and 91 GeV/c2^2 for Z), which is why the weak force has very short range (∼10−18\sim 10^{-18} m).

(b) Important process. Beta decay. A neutron transforms into a proton through a W−^- exchange: n→p+e−+νˉen \to p + e^- + \bar{\nu}_e. This is the basis of radioactive beta-minus decay (carbon-14 dating, tritium decay, etc.). At the quark level, a down quark in the neutron transforms into an up quark, emitting a W−^- that decays into an electron and antineutrino.

Other examples: pion decay, muon decay, kaon decay.

Markers reward identification of W and Z, and a specific weak-mediated process.

Practise this

Sources & how we know this

Based on the official QCAA syllabus and published QCAA past papers and marking guidelines. Not yet individually reviewed by a person; always check QCAA and your school's materials for current rules.

ExamExplained