OCR Specification focus:
‘Magnetic fields arise from moving charges or permanent magnets.’
Magnetic fields are fundamental features of physical systems, emerging whenever electric charges move or when materials possess aligned magnetic domains, shaping interactions across natural, technological, and experimental contexts.
Sources of Magnetic Fields
Magnetic fields permeate many physical environments, from simple laboratory apparatus to planetary-scale systems. The OCR specification emphasises that magnetic fields arise from moving charges or permanent magnets, and this forms the conceptual foundation for understanding electromagnetism. This section explores how each of these sources generates magnetic influence and why these mechanisms are central to A-Level Physics.
Moving Electric Charges as Sources of Magnetic Fields
A moving electric charge produces a magnetic field that surrounds its path. This principle is essential for explaining why electrical currents in wires generate magnetic effects and why beams of charged particles, such as electrons, create magnetic fields in accelerators or cathode-ray tubes. When introducing this term, it is useful to formalise it with a definition.
Moving Electric Charge: A charged particle in motion that produces a surrounding magnetic field whose strength and direction depend on the particle’s velocity and charge.
The existence of magnetic fields around moving charges underpins much of electromagnetism. A single moving charge produces a field pattern consisting of concentric circles centred on the direction of motion.
Iron filings reveal the circular magnetic field surrounding a current-carrying wire, illustrating how moving charges create a magnetic field with circular symmetry around the conductor. Source.
In practice, these circular patterns become especially important when many charges move together, such as in an electric current. Unlike electric fields, which begin and end on charges, magnetic fields form continuous closed loops and thus operate differently within space.
When large numbers of charges move collectively in a conductor, the magnetic field can be substantial. The strength of this field increases with higher current, showing that magnetic effects are scalable with the motion of charge. This principle is used extensively in devices such as solenoids, electromagnets, and motors. Although these applications belong to later subsubtopics, the underlying reason for their behaviour originates here.
Furthermore, moving charges in space, not just in conductors, generate magnetic fields. For example:
Charged particles in the solar wind produce magnetic interactions with Earth’s magnetosphere.
Electrons circulating in particle accelerators generate measurable magnetic fields.
Ionic currents in biological systems also create extremely small magnetic fields.
These examples reinforce that the creation of magnetic fields by moving charges is not limited to metal conductors but is universal across physical systems.
Permanent Magnets as Sources of Magnetic Fields
The OCR specification notes that magnetic fields also arise from permanent magnets, which are materials capable of producing persistent magnetic effects without ongoing electrical input. The behaviour of permanent magnets originates from microscale magnetic domains—regions within a material where atomic magnetic moments align.
Permanent Magnet: A material in which internal magnetic domains remain aligned without an external field, producing a continuous and stable magnetic field.
A normal sentence must separate definition blocks, and here it is important to recognise the structural reason why permanent magnets generate magnetic fields. In many materials, atomic magnetic moments are randomly oriented, cancelling each other. In ferromagnetic materials such as iron, cobalt, and nickel, these atomic moments can align collectively to form magnetic domains. When these domains are forced into alignment by an external magnetic field and retained after the field is removed, the material becomes magnetised.
Diagram showing magnetic domains aligning to form a net magnetisation in a ferromagnet. The hysteresis loop provides additional information about magnetic retention, which exceeds the syllabus requirement but supports understanding of permanent magnet formation. Source.
Permanent magnets create magnetic fields that follow predictable patterns.
A bar magnet produces closed magnetic field lines emerging from the north pole and curving back into the south pole, illustrating the characteristic dipole field pattern of permanent magnets. Source.
Permanent magnets remain essential in many practical applications because their magnetic fields are stable, long-lasting, and require no power supply. Examples include:
Magnetic compasses.
Door latches and small mechanical switches.
Loudspeakers and microphones.
Magnetic storage media.
These uses highlight the importance of understanding how permanent magnets function as reliable sources of magnetic fields.
Comparing Moving Charges and Permanent Magnets
Although moving charges and permanent magnets appear to be very different sources, they are linked by the same underlying physics. At a microscopic level, the magnetism of permanent magnets originates from the motion and spin of electrons inside atoms. Even though the electrons are bound within the material, their intrinsic magnetic moments—arising from quantum motion—create the magnetic domains that lead to large-scale magnetisation. Thus, all magnetic fields ultimately originate from moving charges, whether the motion is macroscopic (currents) or microscopic (electron spin and orbital motion).
Permanent magnets generate stable fields from built-in electron motion.
Conductors produce controllable fields from externally driven electron flow.
Charged particles in motion create dynamic magnetic fields in free space.
This perspective unifies the OCR specification point by showing that both primary sources—moving charges and permanent magnets—fit into a broader electromagnetic framework.
Situations Where Magnetic Fields Naturally Occur
Understanding sources of magnetic fields allows students to recognise their presence in familiar and scientific contexts. Typical situations include:
Currents in wires producing magnetic fields used in electromagnets.
Planetary fields, such as Earth’s magnetic field arising from electric currents in its liquid outer core.
Astrophysical plasma flows, where moving charged particles generate large-scale magnetic structures.
Everyday magnets, where aligned domains produce convenient, stable magnetic effects.
These examples illustrate how the concepts required by the OCR specification apply widely across physics and beyond.
FAQ
The link between motion of charge and magnetism was established by observing that a compass needle deflects when placed near a current-carrying wire. This deflection only occurs when current flows, showing that moving charges produce a magnetic effect.
Further confirmation comes from:
• Particles in cathode-ray tubes following curved paths in magnetic fields
• Measurements of forces on charged particle beams in accelerators
• Magnetic fields detected around biological ionic currents using sensitive instruments
These observations consistently show that stationary charges do not create magnetic fields.
Individual charged particles do produce magnetic fields, but these are typically extremely weak and difficult to measure directly.
However, in high-energy physics experiments, beams of charged particles such as electrons or protons create detectable magnetic fields because:
• Large numbers of particles move together
• Each particle travels at high speed
• Advanced detectors can measure tiny magnetic effects
In everyday contexts, only collective motion of many charges (such as in electrical currents) produces fields large enough to observe.
Magnetic field lines always form closed loops because magnetism has no isolated “north” or “south” magnetic charges (monopoles) that act like sources or sinks.
When charges move:
• Their magnetic field loops circle around the direction of motion
• These loops do not begin or end at any point
• The continuity of field lines reflects the mathematical property that magnetic flux has zero divergence
This fundamental behaviour holds for both currents and permanent magnets.
The strength depends on several properties of the moving charge or current:
• Size of the charge — larger charges produce stronger fields
• Speed of motion — faster motion increases magnetic field strength
• Distance from the charge — field strength decreases with distance
• Geometry of motion — straight-line motion, circular motion, or confined current paths produce different field distributions
In electrical circuits, current size and wire geometry are the primary factors controlling field strength.
Domains align when an external magnetic field causes atomic magnetic moments to rotate and match its direction.
This occurs through:
• Rotation of individual atomic moments
• Growth of domains already aligned with the external field
• Shrinking or elimination of domains pointing in other directions
If the material has high magnetic “hardness” (resistance to demagnetisation), many domains remain aligned once the external field is removed, creating a permanent magnet.
Practice Questions
Question 1 (2 marks)
State two distinct sources of magnetic fields and briefly describe how each source produces its magnetic field.
Mark scheme:
Award 1 mark for each correct source plus brief description.
• Moving electric charges (1) — motion of charged particles or electric current generates a surrounding magnetic field (1).
• Permanent magnets (1) — aligned magnetic domains in certain materials create a stable, continuous magnetic field (1).
Question 2 (5 marks)
A student states that all magnetic fields, whether from electric currents or permanent magnets, ultimately come from moving charges.
(a) Explain how a current in a wire produces a magnetic field.
(b) Describe how permanent magnets generate magnetic fields, referring to the behaviour of atoms or domains within the material.
(c) Explain why both mechanisms can be understood in terms of moving charges.
Mark scheme:
(a) Current in a wire (2 marks)
• Current is the flow of charged particles/electrons (1).
• Moving charges create a magnetic field that forms concentric circles around the wire (1).
(b) Permanent magnets (2 marks)
• Permanent magnets contain aligned atomic magnetic moments or magnetic domains (1).
• These aligned domains produce a strong, stable magnetic field without the need for an external current (1).
(c) Link to moving charges (1 mark)
• Atomic magnetism arises from electron motion (spin or orbital motion), so both current-generated fields and permanent magnets are due to moving charges (1).
