K326 / K327 / 2027
Magnetism and electromagnetism overview

Topic 4 of 7

Magnetic field patterns

A field pattern combines shape and direction. Identify every pole first, then draw smooth lines with arrows that agree with the compass convention.

The north-seeking end of a settled compass points along the local field. Outside a magnet, field lines leave N and enter S.

Follow the exterior field from N to S

Teal arrows show field direction. These idealised patterns neglect background fields. Internal return paths through the magnets are omitted.

One bar: exterior field from N to S

One bar: exterior field from N to SOne horizontal bar has N on its left and S on its right. Three curves above and their three reflected counterparts below leave the north region, curve around outside the bar and enter the south region. Teal arrowheads are tangent to those curves and point from N to S outside the magnet. The opaque bar hides the unshown interior return paths. The drawing represents a symmetric qualitative field, with background fields neglected; it has no numerical field-strength scale.NS

Outside the bar, arrows leave its north region and enter its south region. A compass's N end aligns with the local tangent.

Unlike facing poles: field crosses the gap

Unlike facing poles: field crosses the gapTwo horizontal bars both have S on the left and N on the right. The left bar therefore presents N to the gap and the right bar presents S. Five inner field curves, including the central straight line, run from the facing N towards the facing S. Other curves return to the outer south poles, with arrows following the same exterior N-to-S rule. In particular, a large outer curve runs from the far-right N to the far-left S. No field curves cross. These field arrows are distinct from the two forces of attraction.SNSN

The gap's arrows run from the facing N to S. Outer curves obey the same rule, following the other labelled poles.

Like facing poles: lines bend away from the gap

Like facing poles: lines bend away from the gapThe left bar is S-left and N-right; the right bar is N-left and S-right, so N faces N. Field curves leave each facing north region, bend away from the central gap and return to the outer south region on that side. The left and right patterns are mirror images. No curve joins N to N, and no curves cross. The identical symmetric model has cancellation exactly at the midpoint between the facing poles; no finite direction arrow is drawn there. Background fields are neglected. The field-line pattern is not a picture of lines physically pushing one another.SNNS

The curves leave both facing north regions and bend away from the gap. They do not connect one N pole to the other.

Field lines form continuous loops and do not cross. Line spacing is qualitative here; counting the drawn curves in different panels does not measure or compare field strength.

The panels show a single bar, unlike facing poles and like facing poles. Arrows describe the magnetic field outside the magnets, rather than forces on the complete magnets.

One bar magnet

Draw lines that leave the north pole, curve through the space around the bar and enter the south pole. The compass needle lies tangent to the local line, with its N end following the arrow.

Magnetic field lines continue through the magnet to form loops. Inside the bar their return direction is S to N. Keep the word outside with the familiar N-to-S rule.

Unlike poles facing

With N on the left of the gap and S on the right, lines connect across the air gap with arrows left to right, from N to S. Lines farther from the axis curve around the magnets.

The magnets attract, but that force observation does not change the field-arrow convention. A force on a whole magnet and the local field direction are different things to label.

Like poles facing

For N facing N, lines leave both facing north poles and bend away from the central gap region. They do not join N directly to N. For the corresponding S-facing-S arrangement, reverse the arrows so that lines enter both south poles.

For two identical magnets arranged symmetrically with like poles facing, their fields can cancel at the exact midpoint if background fields are neglected. There is no nonzero field-direction arrow to draw at that cancellation point. A real compass there can still respond to Earth's field or another disturbance.

Check the representation

  • Lines do not cross. A crossing would assign two different field directions to one position.
  • Arrows agree along a line. They follow a compass N end; outside magnets they leave N and enter S.
  • Closer spacing indicates a stronger field qualitatively within one consistently drawn pattern. An illustrator's arbitrary line count does not give an exact field strength.
  • The lines represent the field. They are not physical strings pushing one another apart, nor paths that every moving magnet must follow.

Repair a drawing

The gap shape is right, but the arrows run S to N

For unlike facing poles, keep the connecting curves and reverse the arrows so that they run N to S across the gap. The correction follows the compass convention.

If one magnet is then turned around, identify the new facing poles before drawing again. The gap pattern may need to change as well as its arrows.

Optional check Two bar magnets have N facing S across an air gap, with N on the left of the gap. A drawing joins the facing poles with smooth non-crossing lines but points the arrows from right to left. What should be corrected?
Two bar magnets have N facing S across an air gap, with N on the left of the gap. A drawing joins the facing poles with smooth non-crossing lines but points the arrows from right to left. What should be corrected?