Conventions have changed. Fifty or a hundread years ago much of scientific literature, especially in the humanities, was written in an impersonal (often passive) or generalizing ("one", "we") style, implying professional detachment and an objective view.
Today, both the relativity of knowledge (that "things might be different for different people or at different times") and the role of the scholar (effects of unavoidable preconceptions on results) have been painstakingly studied and led to the necessity that scientists not ignore or smooth over these effects that are always there, but clearly explain how they dealt with them. This more honest and self-reflective scientific approach is reflected in a writing style that echos the responsibility of the scholar.
Practically this means, that at least in the natural sciences the agent(s) of the study and their exact procedures are clearly named, which leads to unambiguous self-reference – discussed in this related question – and
We invited participants and asked them to do this or that.
I found that so and so.
But conventions being what they are, you should always ask your teachers for the conventions in your field, and adhere to them.
The APA Publication Manual (2009, p. 77) recommends using the active voice:
Verbs are vigorous, direct communicators. Use the active rather than the passive voice, and select tense and mood carefully.
Prefer the active voice.
We conducted the survey in a controlled setting.
The survey was conducted in a controlled setting.
The passive voice is acceptable in expository writing and when you want to focus on the object or recipient of the action rather than on the actor. For example, "The speakers were attached to either side of the chair" emphasizes the placement of the speakers, not who placed them—the more appropriate focus in the Method section. "The president was shot" emphasized the importance of the person shot.
The MLA Handbook (in chapter 3) also recommends the use of the active voice, because it is usually briefer, clearer, and more emphatic than the passive voice. But if the actor is unknown or unimportant or the recipient of the action should receive the emphasis, a passive construction is better.