K326 / K327 / 2027
Magnetism and electromagnetism overview

Lesson 3 of 7

Finding field direction with a compass

  • I can describe how a bar magnet or compass determines magnetic field direction.Syllabus

    Syllabus K326 / K327, 15(d). Describe how a bar magnet or compass determines magnetic field direction.

Make a guess. You are not marked.

A small compass is placed close to the north pole of a bar magnet. Which way does the north end of the compass needle point?

  1. Towards the magnet's north pole, because the needle points north.
  2. Away from the magnet's north pole, along the field line there.
  3. Towards geographic north, because a compass is not affected by magnets.
Show the answer

Away from the magnet's north pole, along the field line there.

The needle's north end is pushed by like poles and points along the field, which leaves a north pole. So it points away from the magnet's north pole.

A small compass is a bar magnet free to turn. Its north-seeking end gives the magnetic field direction at the compass's position.

Magnetic forces act without contact. A magnetic field describes the magnetic effect around a source; the field can turn a small magnet until it aligns with the local direction.

Read the north-seeking end

Identify the compass's marked N end and allow the needle to settle. The direction in which that end points is the field direction. Its S end points the other way.

The compass responds to the field where it is placed. It need not point towards the centre of a nearby bar magnet or towards geographic north. The field direction describes the needle's alignment; it is not a rule for the direction in which the centre of the whole compass must move.

Read the north-seeking end at each position

These are four separate settled-compass observations around one bar. The magnet's field is assumed to dominate the background field.

Four compass directions around an N-left, S-right bar magnetA horizontal bar at the centre has N on its left and S on its right. A small compass on the axis to the left of N has its marked N end pointing left, away from N. A compass above the centre points its N end right. A compass on the axis to the right of S points its N end left, towards S. A compass below the centre points its N end right. Each teal half-needle ends at the side labelled N. Its direction is tangent to the ideal local field, not necessarily geographic north or the direction towards the bar's centre. No field-direction arrow is being used as a force on the whole compass.NSLeft of NNAbove centreNRight of SNBelow centreN

The marked N end indicates the local field direction. Reversing the bar reverses these four ideal directions.

This bar has N on the left and S on the right. With other fields neglected, the compass N end points left on the axis beyond either end of the bar, and right above and below its centre.

Read four local directions

The same bar, four observation points

  • Left of the N pole: the compass N end points left, away from that pole.
  • Above the centre: the N end points right.
  • Right of the S pole: the N end points left, towards that pole.
  • Below the centre: the N end points right.

These are local directions around one source. If the magnet is reversed while the positions stay fixed, the ideal directions reverse too.

Map a field using a plotting compass

  1. Place a bar magnet on paper on a non-magnetic surface. Draw its outline and label its poles. Keep the magnet and paper fixed.
  2. Check the marked N end and that the compass turns freely. Place it at a starting point near the magnet and wait for it to settle.
  3. Mark the positions of the needle ends, identifying N. Draw a short arrow from its S-end mark towards its N-end mark.
  4. Move the compass so its S end occupies the previous N-end mark. Let it settle and mark the new N-end position.
  5. Repeat in small steps, then join the marks with a smooth curve and add arrows in the recorded direction.
  6. Start again at several other positions to build the field pattern.

A small compass better approximates the direction at one position. Record a settled needle rather than a moving or stuck one, and keep nearby magnets and magnetic objects away from the map.

The needle responds to the resultant field, including Earth's field. An ideal bar-magnet diagram neglects that background. Make observations where the bar's effect is strong enough to dominate; far from it, the background can change the observed direction.

What iron filings show

Iron filings on a sheet near a magnet become magnetised and align, revealing the rough shape of the pattern. The visible chains do not identify an arrow direction by themselves. Use the compass N-end convention to add the arrows.

Field lines are drawn representations. The filings help reveal their shape; they are not threads that already existed around the magnet.

Check your understanding

A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?

  1. Towards the centre of the bar, regardless of the needle direction.
  2. Right, along the north-seeking end of the settled compass.
  3. Left, along the south-seeking end.
Show the answer

Right, along the north-seeking end of the settled compass.

The field-direction convention follows the compass N end. It describes the local field, which need not point towards geographic north or towards the centre of the nearby magnet.