The subjects are clearly understood though in your example sentences. Here's an example where the subject is not clearly understood:
Originally Posted by imme9x
Ex: Who completed the Eiffel Tower in1889?Of course, Gustave Eiffel designed the Tower, but he didn't build it. Workers did that.
PassiveWith passive voice, the subject doesn't need to be stated; with active voice, however, the subject always needs to be stated; unless that is it's an imperative structure, a command; e.g., stand up, sit down.
The Eiffel Tower was completed in 1889 (by ?).
Use passive voice if:
1) you don't know who the subject is:
Ex: The Universe was created (by ?). 2) you want to take focus (or blame) off the subject:
Ex: The cake was eaten yesterday.Hope that helps.
(Active: I ate the cake yesterday.)