GCSE PHYSICS - MAGNETISM AND ELECTROMAG - LESSON 8 - demagnetising

Demagnetizing a Magnet: Methods and Concepts

Understanding Magnetism and Domains

• The lesson focuses on methods to demagnetize a magnet, specifically how to remove its magnetism.

• Permanent magnets are made from ferromagnetic materials where each atom acts like a tiny magnet, aligning their magnetic fields in the same direction due to magnetic coupling.

• This alignment creates individual magnetic domains within the material, which is crucial for understanding how to demagnetize.

Method 1: Heating Above Curie Temperature

• To demagnetize a magnet, one effective method is heating it. As temperature increases, iron atoms vibrate more vigorously.

• When reaching the Curie temperature (named after Pierre Curie), the alignment of magnetic fields becomes disorganized, leading to loss of magnetism.

• Different ferromagnetic materials have varying Curie temperatures; for example:

• Iron and cobalt: over 1000°C

• Nickel: lower than iron

• Commercial ferrite magnets: about 720°C

• Neodymium: approximately 600°C

Practical Demonstration of Heating

• In practical demonstrations, heating neodymium magnets above their Curie temperature shows immediate loss of magnetism as they drop from an iron support when exceeding 600°C.

• This illustrates the strong nature of these magnets and how heat can effectively disrupt their magnetic properties.

Method 2: Using Alternating Current

• Another method for demagnetization involves placing the magnet inside a solenoid and passing alternating current (AC).

• The AC causes the solenoid's magnetic field to change direction rapidly (e.g., north-south-north), which disorients the magnetic domains in the permanent magnet.

Summary of Demagnetization Techniques

• Two primary methods for demagnetizing a permanent magnet include:

• Heating above its specific Curie temperature (e.g., >1070°C for iron or >600°C for neodymium).