Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field

4.1 Simple Phenomena of Magnetism

Plotting Magnetic Field Lines 🔭

Magnetic field lines are invisible highways that show the direction and relative strength of a magnetic field. They always start at the north pole of a magnet and curve around to end at the south pole.

  1. Place a small bar magnet on a piece of paper.

  2. Sprinkle iron filings over the paper. The filings align along the field lines, revealing the pattern.

  3. Notice that the filings are denser near the poles – this indicates a stronger field.

  4. Use a compass: place it near the magnet and watch the needle align with the field lines.

Think of the field lines like invisible roads that a compass needle would naturally follow. The closer the filings or the needle is to the magnet, the steeper the “road” becomes.

Mathematically, the force on a moving charge in a magnetic field is given by the cross‑product:

\$\,\vec{F}=q\,\vec{v}\times\vec{B}\,\$

where \$q\$ is the charge, \$v\$ its velocity, and \$B\$ the magnetic field.

Using a Compass to Determine Magnetic Field Direction 🧭

A compass needle is a tiny bar magnet that freely rotates. It aligns itself with the local magnetic field, pointing towards magnetic north.

  1. Hold the compass flat and steady.

  2. Move it slowly around a magnet or a current‑carrying wire.

  3. Observe the direction the needle points. This direction is the direction of the magnetic field at that point.

  4. Mark the needle’s orientation with a pencil or a small flag for later reference.

Example: Place a straight wire carrying current upward. The compass will rotate to point perpendicular to the wire, following the right‑hand rule.

Block formula for the magnetic field around a long straight wire:

\$B=\frac{\mu_0 I}{2\pi r}\$

Here \$I\$ is the current, \$r\$ the distance from the wire, and \$\mu_0\$ the permeability of free space.

Magnetic Field DirectionCompass Needle Orientation
North → South (around a bar magnet)Needle points from its south pole to the magnet’s north pole
Perpendicular to a current‑carrying wire (right‑hand rule)Needle rotates to align with the field, pointing to the right of the wire if current flows upward

Exam Tip 📚

  • When asked to sketch magnetic field lines, remember they never cross and always form closed loops.

  • Use a compass to verify the direction of the field in your sketch.

  • Show the relationship between the direction of current and the magnetic field using the right‑hand rule.

  • Include a brief explanation of why the filings are denser near the poles.

  • Practice drawing field lines for different configurations (bar magnet, solenoid, current loop) to build confidence.