Describe the use of plotting compasses to show the shape and direction of…

The idea

A plotting compass is a small magnet that is free to turn, so its needle lines up with the magnetic field at that point. To map a bar magnet's field, place the magnet on paper, put the compass near one pole and mark where the needle points. Move the compass so its tail sits on the last mark, mark the new needle direction, and repeat to trace a field line. Joining the marks gives the shape of the field, and the needle's north-seeking end shows its direction, running from the magnet's north pole round to its south pole. Repeating from different starting points builds a full pattern. The same method maps the Earth's field: away from the magnet, the compass needle points roughly north, showing the Earth's field direction at that place.

What earns the marks

• States that a plotting compass is a small magnet free to rotate so it aligns with the local magnetic field.
• Describes marking the needle direction, then moving the compass so its tail meets the previous mark to trace a field line.
• Explains that joining successive marks reveals the shape of the field around the magnet.
• Identifies that the north-seeking end of the needle indicates the field direction, from north pole to south pole outside the magnet.
• Recognises that the same plotting method shows the direction of the Earth's magnetic field away from the magnet.

Common mistakes and how to put them right

• Assumes the compass needle points towards the magnet's south pole.

  Put it right by noting the north-seeking end points along the field away from the north pole.

• Moves the compass randomly rather than placing its tail on the previous mark.

  Put it right by chaining marks to trace one continuous field line.

• Treats the compass as showing the field only at one point.

  Put it right by repeating from several starting points to build the whole pattern.

• Confuses the Earth's field with the magnet's field when plotting.

  Put it right by plotting the Earth's field away from the magnet.

Exam tips

• Link each step to why it works: the needle is a free magnet aligning with the field.
• Use the phrase 'north-seeking end' when stating field direction.
• For the Earth's field, state that the compass points roughly north away from any magnet.