Describe the pattern and direction of magnetic field lines around a bar magnet

What are Magnetic Field Lines?

Magnetic field lines are a visual tool that shows the direction and strength of a magnetic field. • They always form closed loops. • They never cross each other. • The density of lines indicates field strength: the closer the lines, the stronger the field.

Pattern Around a Bar Magnet

A bar magnet has a north (N) and a south (S) pole. The field lines emerge from the N pole, curve around, and re‑enter at the S pole. The lines are denser near the poles, showing a stronger field there.

• From N to S outside the magnet.
• From S back to N inside the magnet.

Direction of Field Lines

The direction of a field line is the direction a north pole of a test magnet would move if placed in the field.
Mathematically, the field vector $\mathbf{B}$ points along the line: $$\mathbf{B} = B \,\hat{t}$$ where $\hat{t}$ is the unit tangent to the line.

Analogy: Water Flow Around a Rock

Imagine a river flowing around a rock. • The water (field lines) flows from one side (north) to the other (south). • Where the rock is close, the water speeds up (lines are closer). • The water never splits or crosses itself, just like magnetic lines.

🌀 The “rock” is the magnet, and the “water” is the magnetic field.

Exam Tips for 4.1

1. Remember: field lines start at N and end at S.
2. Use the arrow convention – arrows point in the direction a north pole would move.
3. When sketching, show denser lines near the poles to indicate stronger field.
4. Explain that the field inside the magnet runs from S to N.
5. Use the water‑flow analogy to describe why lines never cross.