In simple terms
A friendly intro before the formal notes — no formulas yet.
Concept of a magnetic field
Cambridge 9702 Paper 4 — Concept of a magnetic field (20.1). Senpai Corner diagram-backed pilot with premium structure and live visuals.
- 1
Direction: Field lines point from North to South poles outside a magnet.
- 2
Strength: The density of the lines (how close they are) represents the field strength. Closer lines mean a stronger field.
- 3
No Crossing: Magnetic field lines never cross each other, as the field has a unique direction at any given point.
- 4
Continuity: They form continuous closed loops, passing through the magnet from South to North internally to complete the circuit.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 20.1.1
Understand that a magnetic field is an example of a field of force produced either by moving charges or by permanent magnets
- 20.1.2
Represent a magnetic field by field lines
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Step 1
Direction: Field lines point from North to South poles outside a magnet.
12 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
12 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- 3JCN PhysicsStart here · 19702 20.1 · IB D.2
Magnetic Field Bar Magnet Mapping
Map the field lines around a bar magnet with a probe
Why this one: Map the field around a bar magnet point by point and see the lines close from N to S.
Try this
- Map the field lines around the bar magnet with the probe.
- Compare the line density near the poles and far away.
Look for Field lines run from north to south outside the magnet and are densest at the poles.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsStart here · 29702 20.1 · IB D.2
Magnetic Field U-shape Magnet Mapping
Map the field between poles of a U-shaped magnet
Why this one: Probe between the poles of a U-magnet and find the near-uniform field used in F = BIL.
Try this
- Map the field between the poles of the U magnet.
- Compare with the field outside the gap.
Look for Between the poles the field is nearly uniform and runs from north to south.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsStart here · 39702 20.4 · 9702 20.1 · IB D.2
Orsted's Compass
Switch on a current and see a compass deflect, as Oersted did
Why this one: Switch on the current and watch the compass swing — a current makes its own magnetic field.
Try this
- Switch on the current and watch the compass.
- Reverse the current and compare.
Look for A current produces a magnetic field, and reversing the current reverses the compass deflection.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- PhETStart here · 49702 20.1 · 20.4
Magnets and Electromagnets
Compare a bar magnet with a coil carrying current; change loops and current and probe B.
Why this one: Compare a bar magnet with a current-carrying coil and see they produce the same field pattern.
Try this
- On “Electromagnet”, set 1 loop and show the field; raise to 4 loops.
- Reverse the current — the poles swap.
- Use the field meter inside the coil, then outside — where is B strongest?
Look for A current loop makes the same field shape as a bar magnet; more loops or current → stronger B.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
More simulations8 more on this topic — core ones first
- The Physics ClassroomCore9702 20.1 · IB D.2
Magnetic Fields
Drag a compass needle through the space around a bar magnet and observe the magnetic field
Try this
- Drag the compass round the magnet.
- Trace a field line from pole to pole.
Look for The compass points along the field, from north pole to south pole outside the magnet.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
- The Physics ClassroomCore9702 20.1 · IB D.2
Bar Magnets
Drag, flip and orient six bar magnets and observe their attractions, repulsions and the surrounding field
Try this
- Bring two north poles together.
- Flip one magnet and compare.
- Line up several magnets and watch the field.
Look for Like poles repel and unlike poles attract.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
- SimuPhysicsCore9702 20.4 · 9702 20.1 · IB D.2
Compass Above and Below Wire
Compasses sit above and below a current-carrying wire; they point in opposite directions because the field wraps round the wire
Try this
- Switch the current on and watch both compasses.
- Reverse the current.
- Compare the two compass directions.
Look for The field circles the wire, so compasses above and below point in opposite directions.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- 3JCN PhysicsCore9702 20.1 · IB D.2
Compass in Magnetic Field
Move a compass through a field and see it align with field lines
Try this
- Move the compass through the field and watch it align.
- Trace the path the needle points along.
Look for The compass needle lies along the field line at each point.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsCore9702 20.4 · 9702 20.1 · IB D.2
Axial Magnetic Field inside Solenoid
Probe the axial field inside a solenoid; vary turns and current
Try this
- Probe the field along the axis and read B.
- Double the current and compare.
- Double the turns and compare.
Look for Inside a long solenoid B is uniform and proportional to current and turns per unit length.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsCore9702 20.4 · 9702 20.1 · IB D.2
Magnetic Field Solenoid Mapping
Map the field inside and around a solenoid
Try this
- Map the field inside the solenoid.
- Map it outside and compare.
Look for Inside the field is strong and uniform; outside it resembles a bar magnet's.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Tap a symbol — great for exam definitions
Tap a symbol — great for exam definitions
Tap a symbol — great for exam definitions
Tap a symbol — great for exam definitions
Full topic notes
Formal explanation with the rigour you need for the exam.
Understanding Magnetic Fields
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. It is a region of space where a magnetic force can be detected. These fields arise either from the intrinsic magnetic moment of elementary particles (as in permanent magnets) or from the movement of electric charges (as in electromagnets).
Magnetic Field Lines: Visualising the Invisible
To help us understand and describe magnetic fields, we use magnetic field lines. These lines are a fantastic visual tool that represent the direction and strength of the field. They have several key properties:
Direction: Field lines point from North to South poles outside a magnet.
Strength: The density of the lines (how close they are) represents the field strength. Closer lines mean a stronger field.
No Crossing: Magnetic field lines never cross each other, as the field has a unique direction at any given point.
Continuity: They form continuous closed loops, passing through the magnet from South to North internally to complete the circuit.
Magnetic Flux Density (B): Measuring Strength
When we talk about the 'strength' of a magnetic field, we're usually referring to its Magnetic Flux Density, denoted by 'B'. It's sometimes just called 'magnetic field strength'. This quantity is formally defined as the force experienced per unit current per unit length on a conductor placed perpendicular to the field. The SI unit for magnetic flux density is the Tesla (T).
(for a conductor perpendicular to the field)
For the more general case where the conductor is at an angle θ to the magnetic field, the formula for the force becomes:
Here, θ is the angle between the direction of the current (L) and the direction of the magnetic field (B). The force is maximum when the conductor is perpendicular to the field (sin(90°) = 1) and zero when it is parallel (sin(0°) = 0).
Magnetic Fields from Electric Currents
One of the most exciting aspects of magnetism is that moving electric charges – in other words, an electric current – can create magnetic fields! This principle is fundamental to motors, generators, and countless electronic devices. The relationship between electricity and magnetism is a cornerstone of physics. Let's break down the magnetic fields produced by common current configurations:
A solenoid acts like a bar magnet because the circular fields from each individual loop of wire add up inside the coil to create a strong, uniform field pointing along the axis of the solenoid. Outside the solenoid, the field is much weaker and loops around from the North pole to the South pole.
(for a long straight wire)
In this formula, 'B' is the magnetic flux density, 'I' is the current, 'r' is the perpendicular distance from the wire, and 'μ₀' (mu-nought) is the permeability of free space, a fundamental constant with the value 4π × 10⁻⁷ T m A⁻¹.
Straight Wire: Field lines form concentric circles around the wire in a plane perpendicular to the wire.
Right-Hand Grip Rule: To find the field direction, point your right thumb in the direction of the conventional current. Your fingers will curl in the direction of the magnetic field lines.
Wire Loop: The field is concentrated and strongest at the centre of the loop, with all field lines passing through the loop in the same direction.
Solenoid: A long coil of wire creates a strong, uniform magnetic field inside it, much like a bar magnet. Outside, the field is much weaker.
Solenoid Poles: Use the Right-Hand Grip Rule on the coil. If your fingers follow the current direction, your thumb points towards the North pole.
Iron Core: Adding a ferromagnetic core (like soft iron) inside a solenoid drastically increases its magnetic field strength.
Magnetic Flux (Φ): Total Field Through an Area
While magnetic flux density (B) tells us the strength at a point, Magnetic Flux (Φ) quantifies the total amount of magnetic field passing through a specific area. Think of it as counting all the field lines that pierce through a surface. The SI unit for magnetic flux is the Weber (Wb).
(for area perpendicular to the field)
If the area is not perpendicular to the field, we must consider the component of the magnetic field that is perpendicular to the area. The general formula is:
In this formula, θ is the angle between the magnetic field lines and the normal (a line perpendicular) to the area A. Flux is maximum when the area is perpendicular to the field (θ = 0°, cos(0°) = 1) and zero when the area is parallel to the field (θ = 90°, cos(90°) = 0).
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A straight wire of length 0.25 m carries a current of 3.0 A. It experiences a force of 1.5 N when placed perpendicular to a uniform magnetic field. Calculate the magnetic flux density (B) of the field.
- 1
Identify the given values:
A rectangular coil of wire with dimensions 10 cm by 5 cm is placed in a uniform magnetic field of flux density 1.2 T. The field lines are initially perpendicular to the plane of the coil. Calculate the magnetic flux (Φ) through the coil. The coil is then rotated by 30° about an axis in its plane. What is the new magnetic flux?
- 1
Identify given values and convert units:
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- Magnetic Flux Density
When we talk about the 'strength' of a magnetic field, we're usually referring to its Magnetic Flux Density, denoted by 'B'. It's sometimes just called 'magnetic field strength'.
- permeability of free space
In this formula, 'B' is the magnetic flux density, 'I' is the current, 'r' is the perpendicular distance from the wire, and 'μ₀' (mu-nought) is the permeability of free space, a fundamental constant with the value 4π × 10⁻⁷ T m A⁻¹.
- Magnetic field
A region in space where magnetic forces are exerted on other magnets, magnetic materials, or current-carrying wires.
- Magnetic Flux Density (B)
It's the force per unit current per unit length exerted on a conductor placed perpendicular to the magnetic field. Its unit is the Tesla (T).
- SI unit for Magnetic
The Tesla (T).
- Magnetic Flux (Φ)
The product of the magnetic flux density and the area normal to the field lines. It represents the total amount of magnetic field passing through a given area.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Direction: Field lines point from North to South poles outside a magnet.
Strength: The density of the lines (how close they are) represents the field strength. Closer lines mean a stronger field.
No Crossing: Magnetic field lines never cross each other, as the field has a unique direction at any given point.
Continuity: They form continuous closed loops, passing through the magnet from South to North internally to complete the circuit.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Explain why the two wires exert a magnetic force on each other.
Define magnetic flux density.
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9702/41 · Q7(c)(i) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
Discuss Concept of a magnetic field
Ask, share and discuss with other Physics students