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Magnets: Attract and Repel

Primary schoolPhysicsCoaxial point-dipole force and linear induced-magnetization energy for fixed-position test specimens

Click either magnet to reverse its poles. Choose a test specimen from the tray, drag the separation or change magnet A strength, then compare force and energy.

3D

Drag the apparatus to adjust it. Drag the background to rotate. Scroll to zoom.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Reverse a magnet, change its strength or replace the second magnet with a material specimen. Measure force across a controlled separation and compare strong permanent-magnet interactions with weak paramagnetic and diamagnetic responses.

Learning goals

  • Predict how pole reversal changes permanent-dipole force.
  • Compare paramagnetic attraction with diamagnetic repulsion.
  • Relate a force-display detection threshold to a nonzero calculated response.

How to use it

  1. 1Measure the default two-magnet force, then reverse one magnet and compare its sign.
  2. 2Change separation and test the inverse-fourth-power force dependence of coaxial dipoles.
  3. 3Replace magnet B with aluminium, copper and the effective iron specimen.
  4. 4Increase magnet A strength and compare exact calculated force with the 0.1 µN display-resolution threshold.

The model

Signed moments determine attraction or repulsion for the coaxial dipoles.
An induced linear response depends on field squared, so reversing the source alone does not reverse its force.

Assumptions

  • Positions are held fixed. Bar lengths and force-arrow lengths are symbolic, and the point-dipole approximation excludes magnet contact.
  • The induced specimen volume is 10 cm³. Effective susceptibilities are illustrative values: iron 1, aluminium 2.2×10⁻⁵, copper −1×10⁻⁵, wood −8×10⁻⁶ and plastic −1×10⁻⁵.
  • Linear induced magnetization omits hysteresis, remanence, demagnetizing geometry and saturation. The effective iron specimen is not a calibrated real iron sample.
  • Forces below 0.1 µN are retained in numerical readouts while marked below the chosen display resolution.

Try this in class

  1. Q1Why does reversing a permanent magnet reverse the force on another permanent magnet?
  2. Q2Why does source reversal leave the induced-material force unchanged?
  3. Q3Can a below-resolution force still be nonzero?

Frequently asked questions

Why do copper, wood and plastic repel weakly?

The selected illustrative susceptibilities are negative, producing a small diamagnetic force away from the stronger field.

Why do the magnets stay in place despite a force?

They are mounted on a virtual fixed-position force bench. Separation is a controlled parameter, so no translational motion is integrated.