PHYS1008 Permanent Magnets

Nijmegen High Field Magnet Laboratory

Permanent Magnets Objectives: by the end of this you will be able to Understand the origin of permanent magnetic fields. Understand the difference between paramagnetism, diamagnetism and ferromagnetism. fly a frog!

Permanent Magnets

How about permanent magnets?

• Ampere argued that if there were no magnetic poles, there would have to be current loops in atoms

• so we have many of these which would cancel out inside the material but add up on the surface

• However, these could add up to produce a much larger loop.

• and this would produce a field exactly like a solenoid, but there is no current that can be measured.

• What produces the atomic currents (we now know that it is electrons spinning in atoms)
• Why are some materials strongly magnetic (like iron) but most are not.
• Why can iron be magnetized?

• Why is even non-magnetic iron attracted to a magnet?

• In practice materials divide into 3:
• 1. Diamagnetic
  2. Paramagnetic
  3. Ferromagnetic

Diamagnetism

• Diamagnetic: oppose an external magnetic field

• Changing field induces current of electrons inside atoms which opposes BA

Paramagnetism

Paramagnetic: increases external field
Elementary dipoles (electrons in orbits) are aligned with field

Ferromagnetism

Ferromagnets: field exists in absence of external field
Electron spins are aligned

• Why can iron be magnetized? Normally iron has random domains, in which all spins point in same direction

• Applying external field will liine up all the magnets

• Why is even non-magnetic iron attracted to a magnet?

• Flying frogs

• A little frog levitates inside a Bitter solenoid in a magnetic field of about 16 Tesla at the Nijmegen High Field Magnet Laboratory.

• To finish of electromagnetism, we look at electro-magnetic waves.