Q.With reference to the hysteresis loop of a ferromagnetic material, define retentivity and coercivity, stating the SI unit of each and the physical condition of the sample at which each is measured.

Magnetise a ferromagnetic sample to saturation, then reduce the magnetising field $H$ back to exactly zero. Retentivity (remanence) is the flux density $B$ (or magnetisation $M$) that remains inside the sample at this point, $H=0$ -- SI unit tesla, T (or A/m if quoted as $M$). It measures how strongly the sample stays magnetised after the field is removed. …

State each definition against the same reference state (a saturated sample), and quote the correct unit for each ($B$' …

• Swapping the two definitions.
• Giving retentivity the unit A/m when it is asked as a …