Compass & Magnetic North

Students drag the compass or the bar magnet across the table, drag the white handle to turn the magnet and click it to flip its poles. The needle swings toward the sum of Earth's field and the magnet's field. Sliders set Earth field strength and magnet strength, and toggles show field lines, field arrows and Earth's field. Readouts give needle heading, deviation from true north, total field and the magnet's part. It shows how a compass works and why magnets can fool it.

• Bring the magnet close to the compass and watch the needle leave north.
• Flip the magnet by clicking it and see the needle turn to the opposite side.
• Drag the white handle to rotate the magnet and watch the field lines turn.
• Move the magnet far away until the needle settles back to Needle heading near 0 degrees.
• Turn Earth's field on and off to see what the needle does without it.
• Raise Magnet strength and compare the Off true north readout at the same distance.

1. 1Explain that a compass needle lines up with the strongest magnetic field nearby.
2. 2Describe how unlike poles attract and like poles repel using the needle and magnet.
3. 3Predict how distance affects the pull of a magnet on a compass.
4. 4Interpret field lines and arrows as showing the field direction.

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