field density的用法和樣例：
例句
- A unit of magnetic flux density equals to 1 maxwell per square centimeter.
一種磁通量密度單位，等於每平方厘米麥克斯韋。 - In particular, approximate expressions for the air gap flux and the airgap flux density are derived.
特別是，近似表達式氣隙磁通和氣隙磁通密度的計算公式。 - Considering the inconvenience of using the magnetic vector potential as state variable, magnetic flux density is used as the state variable in the design optimization.
針對以往用磁位作為狀態變數不方便的情況，文中直接採用磁通密度為狀態變數。
