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.