β Unit 4: Revolutions in modern physics
Topic 3: The standard model
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
A focused answer to the QCE Physics Unit 4 dot point on the four fundamental forces. Strong, electromagnetic, weak, gravitational; their mediating bosons, relative strengths and ranges, and roles in atomic structure, nuclear stability, beta decay and gravitation.
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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 m). (the reference strength; about 100 times stronger than electromagnetism at this distance).
Range. About 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 m). . About 100 times weaker than the strong force at nuclear distances.
Range. Infinite. The Coulomb force falls off as 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, Z bosons. Massive (80-91 GeV/c). Charged or neutral.
Acts on. All quarks and leptons (including neutrinos, which feel only the weak and gravitational forces).
Strength (at m). to . Much weaker than the strong or electromagnetic forces.
Range. About 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 m). . Extraordinarily weak compared to the others at small scales.
Range. Infinite. Falls off as (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 m) | Range |
|---|---|---|
| Strong | 1 | IMATH_21 m |
| Electromagnetic | IMATH_22 | Infinite |
| Weak | IMATH_23 to IMATH_24 | IMATH_25 m |
| Gravitational | IMATH_26 | 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:
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.
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, m range). Weak is responsible for beta decay (W/Z mediators, 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 m differ from strengths at atomic or molecular scales because of how the forces fall off with distance.
In one sentence
The four fundamental forces are: the strong nuclear force (gluon-mediated, binds quarks into hadrons and hadrons into nuclei, 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, m range), and the gravitational force (hypothetical graviton, infinite range, dominant only at astronomical scales due to its extreme weakness at the particle level).
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2024 QCAA-style4 marksDescribe 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 β
(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 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.
2023 QCAA-style3 marks(a) Identify the gauge boson(s) of the weak force. (b) State one important process that the weak force mediates.Show worked answer β
(a) Weak force bosons. W, W (charged) and Z (neutral). All three are massive (80 GeV/c for W and 91 GeV/c for Z), which is why the weak force has very short range ( m).
(b) Important process. Beta decay. A neutron transforms into a proton through a W exchange: . 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.
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