English: An parallel plate capacitor. Using an imaginary pillbox, it's posible to use Gauss's law to explain the relationship between electric displacement and free charge.
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|Description={{en|1=An parallel plate capacitor. Using an imaginary pillbox, it's posible to use Gauss's law to explain the relationship between electric displacement and free charge.}}
|Source=Own work by uploader
|Author=[[User:老陳|老
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