State two methods by which a piece of magnetised steel can be demagnetised.
Answers
Demagnetize a Magnet by Heating or Hammering
If you heat a magnet past the temperature called the Curie point, the energy will free the magnetic dipoles from their ordered orientation. The long-range order is destroyed and the material will have little to no magnetization. The temperature required to achieve the effect is a physical property of the particular material.
You can get the same effect by repeatedly hammering a magnet, applying pressure, or dropping it on a hard surface. The physical disruption and vibration shake the order out of the material, demagnetizing it.
Self Demagnetization
Over time, most magnets naturally lose strength as long range ordering is reduced. Some magnets don't last very long, while natural demagnetization is an extremely slow process for others. If you store a bunch of magnets together or randomly rub magnets against each other, each will affect the other, changing the orientation of the magnetic dipoles and lessening the net magnetic field strength. A strong magnet can be used to demagnetize a weaker that has a lower coercive field.
Demagnetization processes include heating past the Curie point, applying a strong magnetic field, applying alternating current, or hammering the metal.
Demagnetization occurs naturally over time. The speed of the process depends on the material, the temperature, and other factors.
While demagnetization may occur by accident, it is often performed intentionally when metal parts become magnetized or in order to destroy magnetic-encoded data.