The introduction purpose is to tell the reader what the paper topic is about and, more importantly, to justify to the reader why the paper is significant. If a "previous work" section is included here or elsewhere, its goal should be to show how your paper is an advance or improvement on previous work. A section on "previous work" that just shows that the author did his or her time in the library will be skipped or will make the reader quit reading entirely.
After justifying interest in the topic, don't be afraid to use a sentence that begins "This paper uses/examines/reports . . .", because telling the reader what the paper is about is also one of the main goals of the introduction. This sentence may end with a clause that begins "in order to . . ." or "as a contribution to . . .", so that you also tell the reader what your goal is with respect to refining or increasing scientific knowledge. Such a sentence can, from the reader's perspective, be one of the most important in the whole paper, because the reader then knows what to expect as she or he proceeds. It may be an important sentence for the writer as well, because you're forced to decide what the focus of your paper really will be, and you may realize that you're covering either too many things or not enough things, or not using those things to help refine or increase scientific knowledge. The introduction summarizes the relevant literature so that the reader will understand why you were interested in the question you asked. One to fo ur paragraphs should be enough. End with a sentence explaining the specific question you asked in this experiment.
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