IGCSE Physics 0625 – Simple Phenomena of Magnetism
4.1 Simple Phenomena of Magnetism
Objective
Know that the relative strength of a magnetic field is represented by the spacing of the magnetic field lines.
Key Concepts
Magnetic field lines are a visual tool used to illustrate the direction and relative strength of a magnetic field (B). The following points summarise their meaning:
Lines emerge from the north pole and enter the south pole of a magnet.
The tangent to a field line at any point gives the direction of the magnetic field vector B at that point.
The density (spacing) of the lines indicates the relative magnitude of B:
Closer (denser) lines → stronger magnetic field.
Wider spaced lines → weaker magnetic field.
Field lines never cross each other.
Why Spacing Represents Strength
The magnetic field exerts a force on moving charges and magnetic materials. The force is proportional to the field strength:
\$F = q\,\mathbf{v}\times\mathbf{B}\qquad\text{or}\qquad F = m\,\mathbf{B}\$
When the field is stronger, the same test charge or magnetic needle experiences a larger force, causing the field lines to be drawn closer together in diagrams.
Examples of Field Line Spacing
Near the poles of a bar magnet – lines are very close together, indicating a strong field.
Mid‑way between the poles – lines are more spread out, indicating a weaker field.
Inside a solenoid – lines are parallel and densely packed, showing a uniform strong field.
Outside a solenoid – lines spread out, showing the field weakens with distance.
Comparative Table
Region
Field‑line spacing
Relative field strength
Typical observation
Near north pole of bar magnet
Very close (dense)
Strong
Compass needle aligns quickly
Mid‑point between poles
Moderate spacing
Medium
Compass needle shows weaker deflection
Far from magnet
Widely spaced
Weak
Compass needle barely moves
Inside a long solenoid
Uniformly close
Strong and uniform
Uniform magnetic force on a moving charge
Suggested Diagram
Suggested diagram: Sketch of a bar magnet showing dense field lines near the poles and sparse lines away from the poles.
Common Misconceptions
Thinking that the number of lines drawn is a physical quantity – it is only a convention to illustrate relative strength.
Assuming field lines exist as physical objects – they are a representation, not a material.
Believing that field strength is the same everywhere around a magnet – spacing clearly shows it varies with position.
Quick Check Questions
If the field lines are drawn twice as close together in a region, how does the magnetic field strength compare to a region where the lines are spaced normally? (Answer: Approximately twice as strong.)
Describe what would happen to a compass needle placed in a region where the field lines are widely spaced.
Explain why the magnetic field inside a solenoid appears uniform.