Using active instead of passive voice simplifies and clarifies sentences – and saves words, which is often important in academic writing. For example:
Those shoes were destroyed by your cat!
Terrible injuries can result from bicycle accidents.
Anchovies are what penguins like to eat.
are better as:
Your cat destroyed those shoes!
Bicycle accidents can cause terrible injuries.
Penguins like to eat anchovies.
In active voice the subject of the sentence is the actor (‘cat destroyed’), but in passive voice the subject is acted upon (‘by your cat’).
Annoyingly, it’s not always possible to use active voice in scientific writing and theses as it’s the convention to avoid the first person. This means ‘I conducted an experiment’ has to be rendered as ‘an experiment was conducted’. Otherwise, active voice is the go!