Hysteresis, Soft Iron & Steel
Ferromagnets remember their history.
Lesson at a glanceShow the key points
- Magnetization depends on history: hysteresis.
- Retentivity: I left at H = 0; coercive force: reverse H that removes it.
- Loop area = heat per unit volume per cycle.
- Soft iron (thin loop) for cores and electromagnets; steel (fat loop) for permanent magnets.
A ferromagnet remembers. Its magnetization depends not just on the present field but on what has been done to it before, and that memory is what makes permanent magnets possible.
Measuring a ferromagnet’s response
Make a ring of the material and wind a toroid on it with n turns per length. A current i gives H = ni (the field from the winding alone is B₀ = μ₀ni). The ring’s magnetization adds to this, and the total field B in the ring can be measured (with a Rowland ring). Then
The hysteresis loop
- Start unmagnetized (O): H = 0, I = 0. Raise the current: I rises along OA until it nearly saturates at H = H₀.
- Lower the current to zero. I falls, but not back along AO. At H = 0 some magnetization remains (point C): the retentivity. The domains lined up earlier don’t fully randomize.
- Reverse the current. A reverse intensity, the coercive force (OD), is needed to force I to zero.
- Continue to −H₀: the material magnetizes the other way (E). Bring H back to zero (F) and up to H₀ again via G to A.
Repeat the cycle and the closed curve ACDEFGA is traced again and again: the hysteresis loop. Hysteresis means the magnetization lags behind H and doesn’t return to its old value when H does. The area of the loop equals the energy turned into heat per unit volume in each cycle.
A core material’s loop area represents 250 J/m³ per cycle. A core of volume 1.0 × 10⁻³ m³ is cycled 50 times a second. What power is lost as heat?
Soft iron versus steel
Narrow loop: small retentivity, small coercive force, little energy lost per cycle. Easily magnetized, keeps little magnetism. Ideal for electromagnets, transformer cores and galvanometer cores.
Wide loop: large retentivity and coercive force. Hard to magnetize, but keeps its magnetism and resists stray reverse fields. Ideal for permanent magnets.
- 1Ferromagnetic magnetization depends on history: hysteresis.
- 2Retentivity: I left at H = 0. Coercive force: reverse H needed to make I = 0.
- 3Loop area = heat produced per unit volume per cycle.
- 4Soft iron: thin loop (cores, electromagnets). Steel: fat loop (permanent magnets).
Check yourself
Keep exploring
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