Bar Magnet and Magnetic Field Lines: Properties Explained

A bar magnet is a permanent magnet with two poles, north and south, where its magnetic effect is strongest. Around it in space lies its magnetic field, pictured by magnetic field lines that show the direction a free north pole would move and how strong the field is at each place.

Field lines are more than a drawing aid. They explain why compasses point north, why iron filings form patterns, and why magnets attract or repel without touching. Learning their rules lets you predict the field of any magnet arrangement quickly.

Key Concepts

North pole: end that points toward Earth's geographic north when freely suspended. Field line: curve whose tangent gives field direction; outside the magnet it runs from north to south, inside from south to north, forming closed loops. Field strength: shown by crowding — lines close together mean a strong field, measured in tesla (T). Neutral point: place where a magnet's field exactly cancels Earth's field, net field zero. Unlike poles attract, like poles repel: unlike charges, magnetic poles always exist in pairs — cutting a magnet gives two smaller magnets, never an isolated pole.

How It Works

1. Place a small compass at a point near the magnet; its north points along the field there.
2. Move the compass step by step in that direction, marking dots, then join the dots to trace one field line.
3. Sprinkle iron filings on paper over the magnet and tap gently; filings align along lines, revealing the whole pattern at once.
4. Observe: lines leave the north pole, curve through space, enter the south pole, and continue inside the magnet back to the north.
5. Where another magnet or Earth's field is present, the observed pattern is the vector sum of both fields, and neutral points appear where they cancel.

The field is strongest near the poles because lines are most crowded there; midway between equal poles of opposite type the field is weaker and nearly uniform over a small region.

Worked Example

A bar magnet lies with its north pole pointing toward geographic north. Earth's horizontal field is 0.36 × 10⁻⁴ T. On the magnet's axial line the field falls as 1/d³, and at 20 cm from the centre it equals Earth's field in the opposite direction above the magnet. What is the neutral point type and where does a compass rest there?

1. On the axial side of the north pole, the magnet's field points away from the magnet, same direction as Earth's, so they add — no neutral point there.
2. On the equatorial line (perpendicular bisector), the magnet's field points opposite to its moment, against Earth's field.
3. At the distance where magnitudes match (given as 20 cm on axis; equatorial field is half the axial value at the same distance, so cancellation occurs closer, at about 20/∛2 ≈ 16 cm), the net field is zero.

Neutral points appear on the equatorial line in this orientation, and a compass placed exactly there shows no definite direction.

Common Misconceptions

• Field lines start and end at poles: They are continuous closed loops passing through the magnet's interior.
• Cutting a magnet isolates a north pole: Each piece immediately becomes a complete magnet with both poles.
• Lines crossing means a stronger field: Field lines never cross, because the field has one unique direction at each point.

Real-World Applications

• Compasses, navigation and magnetic surveying using Earth's field.
• Magnetic separators, door latches, motors and generators using permanent magnets.
• Credit-card stripes, magnetic storage and sensors that read field patterns.

A bar magnet surrounds itself with closed field lines from north to south outside and south to north inside; their direction and crowding describe the field everywhere and govern every magnetic interaction.