Permanent Magnet and Electromagnet

Electromagnet

• Soft iron has a large permeability ($\mu_r$ greater than 1000) and retains only a small amount of magnetization. Both properties are exactly what an electromagnet needs, so soft iron — or another soft ferromagnetic material — is used for the core.
• Construction and diagram: a soft iron rod is placed as the core of a solenoid. Draw the solenoid winding round the rod, the current direction marked, and the field lines running along the axis of the rod and closing outside it.
• On passing current through the solenoid, the magnetic field associated with the solenoid increases about a thousandfold, because $B = \mu_0\mu_r nI$ with $\mu_r > 1000$.
• When the current through the solenoid is switched off, the associated magnetic field effectively becomes zero, since the soft core retains almost nothing.
• Why soft iron and not steel: soft iron has low coercivity and low retentivity, so the magnetisation follows the current faithfully and switches off cleanly, and its narrow hysteresis loop means a small energy loss per cycle.
• Applications: electric bells, loudspeakers, circuit breakers, and research laboratories. Giant electromagnets on cranes lift heavy iron loads. Superconducting magnets produce very high fields of the order of a few tesla and are used in NMR (nuclear magnetic resonance) spectroscopy.
• One application explained fully — the electric bell: the current through the coil magnetises the soft iron core, which attracts the soft iron armature so that the hammer strikes the gong; the movement breaks the circuit at the contact, the core loses its magnetism at once because soft iron has negligible retentivity, the armature springs back, the circuit is remade, and the cycle repeats. The bell rings continuously only because the core demagnetises the instant the current stops.

Permanent magnet

• Permanent magnets are prepared in the same way as electromagnets but with a hard ferromagnetic rod in place of the soft one.
• When the current is switched on, the magnetic field of the solenoid magnetises the rod. Because a hard ferromagnetic material has the property of retaining magnetization to a large extent, the material remains magnetised even after the current through the solenoid is switched off.
• What a permanent magnet requires: high retentivity, so that it remains strongly magnetised when the magnetising field is removed, and high coercivity, so that a stray opposing field or rough handling does not demagnetise it.
• The wide hysteresis loop of a hard material, which is a disadvantage in a transformer core, is exactly what makes it suitable here.

Comparison

Standard results

• Soft iron: $\mu_r > 1000$; low retentivity, low coercivity; used for electromagnets and transformer cores.
• Hard ferromagnetic material: high retentivity, high coercivity; used for permanent magnets.
• Field in a cored solenoid: $B = \mu H = \mu_0\mu_r nI$.