En anglais

Magnetisation and magnetic history

Science · Electricity & Magnetism · ages 20-21

Name ______________________ Date ____________

A local-moment paramagnet is warmed while the same nonuniform applied field is maintained. What generally happens to its attraction?
- Attraction grows because heat strengthens alignment with the field
- Attraction stays unchanged because every moment remains aligned
- Attraction weakens as thermal motion reduces net alignment
A sample develops a small magnetisation opposite to an applied field. In a nonuniform field it moves toward weaker-field regions. Which response is dominant?
- A diamagnetic response
- A local-moment paramagnetic response
- A strongly reinforcing ferromagnetic response
At the same coil current, a copper core increases a solenoid's field as strongly as an iron core.
Circle one: True False
Iron is heated above its Curie temperature. What happens to its spontaneous magnetic order?
- It becomes paramagnetic and loses spontaneous magnetisation
- It remains ferromagnetic, with domains ordered in the same way as before
- It becomes diamagnetic because heating removes every atomic magnetic moment
An iron core greatly increases a solenoid's field while the coil current stays fixed. What explains the increase?
- Iron's electrical conductivity supplies extra current even when the coil current is held fixed
- The field favours aligned domains, increasing the core's net magnetisation
- Iron removes the magnetic field outside the coil and transfers all of it into the coil's interior
You stroke an iron nail in one direction with the same magnet pole. Why lift the magnet before returning to the starting end?
- To increase the nail's temperature so its domains remain aligned
- To use the opposite pole on the return stroke and strengthen alignment
- To avoid an opposite return stroke that can reduce alignment
Two equal-volume cores run at the same cycle frequency and peak B. Their slowly measured B-H loop areas are X: 18 J/m^3 and Y: 42 J/m^3. Using these areas to compare hysteresis only, which loses less energy per second?
- Core Y, because a larger loop area releases less heat per cycle
- Both cores, because equal volume and frequency give equal hysteresis loss
- Core X, because its hysteresis loss per cycle is smaller
A symmetric B-H loop is measured slowly, with negligible eddy-current loss. It crosses B = 0 at H = -30 A/m and +30 A/m. Its area is 12 J/m^3. What do these readings mean?
- B-coercivity: 60 A/m; loss: 12 J/m^3 per complete cycle
- B-coercivity: 30 A/m; loss: 12 J/m^3 per complete cycle
- B-coercivity: 30 A/m; loss: 12 J for the whole sample per complete cycle

Answer key

For grown-ups. Fold this page away before handing over the rest.

Magnetisation and magnetic history W1-mt_1apyQ1_1gg-s1

Attraction weakens as thermal motion reduces net alignment · At fixed applied field, warming generally reduces the partial alignment of permanent moments in a local-moment paramagnet.
A diamagnetic response · An induced magnetisation opposite to the applied field is diamagnetic. Motion toward weaker-field regions is consistent with that response.
False · Iron can greatly increase the field through its domain response. Copper contributes a small opposing diamagnetic response instead.
It becomes paramagnetic and loses spontaneous magnetisation · Iron loses spontaneous ferromagnetic order above its Curie temperature. It becomes paramagnetic and can still respond to an applied field.
The field favours aligned domains, increasing the core's net magnetisation · Domain growth and changes in direction increase the core's net magnetisation. This adds a magnetic field without increasing the specified coil current.
To avoid an opposite return stroke that can reduce alignment · Returning through the air avoids an opposite contact stroke, which can reduce the net alignment produced by forward passes.
Core X, because its hysteresis loss per cycle is smaller · Core X loses less energy per volume per cycle. Equal volume and cycle frequency mean it also has less hysteresis energy loss per second.
B-coercivity: 30 A/m; loss: 12 J/m^3 per complete cycle · B-coercivity is the magnitude of either zero crossing, 30 A/m. The specified loop area gives 12 J/m^3 of hysteresis loss per complete cycle.