Magnetic Field Concept
A magnetic field is an invisible region of space surrounding a magnet or a moving electric charge, within which magnetic forces can be detected. Just as a gravitational field tells us where gravity will act on a mass, a magnetic field tells us where a magnetic force will act on another magnet or on a moving charged particle. The field exists whether or not anything is placed in it — it is a property of the space itself, shaped by whatever source is creating it. We represent magnetic fields using imaginary lines called field lines (or lines of flux). These lines run from the north pole to the south pole outside a magnet, forming closed loops that continue through the magnet's interior from south back to north. The direction of a field line shows the direction a free north pole would move if placed there. The spacing of the lines encodes strength: closely packed lines mean a strong field, widely spread lines mean a weak one. This visual language makes an invisible quantity tangible and easy to reason about. Magnetic fields are produced by two fundamental sources: permanent magnets (due to the aligned magnetic moments of electrons) and electric currents (moving charges). Understanding the field concept is the gateway to explaining compasses, electric motors, MRI scanners, and particle accelerators — essentially every device that converts electrical energy to mechanical energy or vice versa.
- The Field as a Physical Reality
- Field Lines and Their Rules
- Measuring Field Strength: Magnetic Flux Density
- Direction Convention: Into and Out of the Page
- The Force on a Moving Charge
The Field as a Physical Reality
A magnetic field B is a vector quantity — it has both magnitude (strength) and direction at every point in space. Think of it like wind: wind has a speed and a direction at every location, even when no leaf is present to show it. The magnetic field exists in that region regardless of whether a test object is there to experience it.
Field Lines and Their Rules
Field lines are a map of the magnetic field. Four key rules govern them: (1) Lines always point from north to south outside the magnet. (2) Lines never cross — if they did, the field would have two directions at one point, which is impossible. (3) Lines form closed, continuous loops (unlike electric field lines, which start and end on charges). (4) The density of lines is proportional to field strength. Near the poles of a bar magnet the lines are dense and the field is strong; far away they spread and the field weakens.
Measuring Field Strength: Magnetic Flux Density
The strength of a magnetic field is measured by its magnetic flux density, symbol B, in units of tesla (T). One tesla is a very strong field — a typical fridge magnet is about 0.005 T, the Earth's field is around 50 µT (0.00005 T), and an MRI machine operates at 1–3 T. The tesla is defined via the force the field exerts on a current-carrying wire: 1 T produces a force of 1 newton per ampere of current per metre of wire.
Direction Convention: Into and Out of the Page
When drawing 2-D diagrams, physicists use two symbols to show a field perpendicular to the page. A dot (•) represents the field coming out of the page towards you — like the tip of an arrow flying at you. A cross (×) represents the field going into the page away from you — like the tail feathers of an arrow flying away. These conventions are universal in textbooks and exam papers, so recognising them instantly is an essential skill.
The Force on a Moving Charge
A magnetic field reveals itself through force. A charged particle moving through a magnetic field experiences the Lorentz magnetic force F = qvB sin θ, where θ is the angle between the velocity and the field direction. Crucially, the force is always perpendicular to both the velocity and the field — it changes the particle's direction but never its speed. This is why charged particles spiral in magnetic fields rather than accelerating along straight lines.
The Key Equation
- F — Magnetic force (Newton (N)): The force experienced by the charged particle due to the magnetic field; always perpendicular to both v and B.
- q — Electric charge (Coulomb (C)): The charge of the particle. A negative charge reverses the direction of the force.
- v — Speed of particle (Metres per second (m s⁻¹)): The magnitude of the particle's velocity. A stationary charge experiences no magnetic force.
- B — Magnetic flux density (Tesla (T)): The strength of the magnetic field at the particle's location.
- \theta — Angle (Degrees or radians): The angle between the velocity vector and the magnetic field vector. Force is maximum when θ = 90° and zero when the particle moves parallel to the field.
Worked Example
Problem. A proton (charge q = 1.60 × 10⁻¹⁹ C) travels at 3.0 × 10⁶ m s⁻¹ perpendicular to a uniform magnetic field of flux density 0.25 T. Calculate the magnetic force acting on the proton.
Answer. The magnetic force on the proton is 1.20 × 10⁻¹³ N, directed perpendicular to both the velocity and the magnetic field.
Origins · 1 minMichael Faraday and how this was discovered
Michael Faraday invented the concept of the magnetic field and field lines — one of the most powerful ideas in all of physics. Before Faraday, physicists described magnetic and electric interactions as forces acting instantly at a distance between objects. Faraday, largely self-taught and unable to follow the advanced mathematics of his contemporaries, developed an intuitive but physically profound picture: the region of space around a magnet is modified into a field, and that field is the true physical agent carrying the force. He drew field lines to visualise magnetic flux and demonstrated electromagnetic induction — the basis of every electrical generator and transformer. His field concept was later given rigorous mathematical form by James Clerk Maxwell. Faraday came from an extremely poor family and received almost no formal education, yet became one of the greatest experimental scientists in history. He taught himself science by reading books while working as a bookbinder's apprentice.
Related Concepts
- Magnetic Field Lines — Field lines are the primary tool for visualising and interpreting magnetic fields; understanding them is the next step after grasping the field concept.
- Magnetic Flux Density — Magnetic flux density B is the quantitative measure of field strength central to all calculations involving magnetic fields.
- Force on a Current-Carrying Conductor — A current-carrying wire in a magnetic field experiences a force — a direct application of the field concept that underlies electric motors.
- Permanent Magnets and Magnetic Materials — Permanent magnets are one of the two primary sources of magnetic fields and provide the physical context for the field concept.