Magnet & Compass

About

A bar magnet and a compass sit on a table. Move either one and the compass needle swings round, overshoots a little, and settles. Its red end always comes to rest pointing along the magnetic field at the spot where the compass sits. Past the north end it points away from the magnet. Past the south end it points toward it. Sitting on top of the magnet, it points from north to south, the same way a real compass does.

The magnet is modelled as two magnetic poles set slightly in from each end, and the compass rests just above them, so it always reads the field outside the magnet. The swing is tuned so the needle still moves visibly when the magnet is far away; where it settles is unchanged. Earth’s magnetic field is left out, so the needle follows the magnet anywhere on the table, even at the far edge. Everything works by keyboard and screen reader too: a live readout gives the needle’s direction, the field strength and where the compass is relative to the magnet.

Learning Goals

• Describe the direction of a magnet’s field at different places around it, using a compass as the probe
• Explain that a compass needle is itself a small magnet that lines up with the field it sits in
• Recognise that outside a bar magnet the field runs from the north end around to the south end
• Predict, before testing, how reversing the magnet changes the needle at every position
• Compare how strongly and how quickly the needle responds near the magnet and far from it

How to Use

• On a touch screen, tap Magnet or Compass, then tap the table where you want it to go. Dragging with a finger works too
• With a mouse, drag the magnet or the compass, or click Magnet or Compass and then click the table
• With a keyboard, focus the table and use the arrow keys to move the selected item. Hold Shift for small steps; Space switches between magnet and compass
• Walk the compass slowly in a circle around the magnet and watch how the needle turns
• Press Flip magnet and predict what the needle will do before it moves
• Use the Think about it questions beside the table as a short guided activity