It makes sense that if there is no prior knowledge, then there is nothing to activate. Students often study topics they know very little about (that is why, after all, that they attend school!) and therefore teachers must think of ways of providing the necessary background knowledge quickly and efficiently.
A popular activity as a warm-up or for early finishers is to find all the words using the letters of a long word. The letters can be mixed up. I would not recommend this activity unless it’s something to fill in time at the end of the term or year.
From my decades of being a teacher and teacher educator, I have learned many things. However, one thing that resonates very strongly is the importance of getting students engaged in learning from the opening minutes of a lesson.