Establishing Purpose and Goals

The basic goals of presenting a one time public lecture are many and can include:

To bring physics (and science in general) concepts and demos to populations who for normally would not have exposure to these
To excite people about physics
To encourage people to become more interested in physics
To stimulate support for your department, university, you, and physics in general in the form of financial gifts, legislative support
To create a media “buzz” around physics and you and your department specifically (see Public Relations)
To recruit students to study physics
To showcase physicists in the community

Of course, you have your own goals for doing a public lecture. Examples of scenarios that might prompt you doing a public lectures include:

there is major issue in the news, for example, global warming. Your research is related to this. You can give a lecture about global warming, while incorporating your research/knowledge on the subject. The goal would be to better inform the public about the subject of global warming and demonstrate how physics and physicists are involved in the research and solution.
there is a major physics discovery/research result/event, for example, the start of the Large Hadron Collider (LHC) at CERN. People are hearing about this in the news, but they don’t know what the heck it is. You can give a lecture about this experiment and how it has the potential to impact everyone on the globe (and not in a bad way with miniscule black holes swallowing us up). You can clear up any fears people have about those crazy black holes and explain its significance in a fun, easy-to-understand manner. The audience will appreciate understanding the facts behind the news they are inundated with. This recently took place at the University of Arizona and the speaker, a physicist who worked on the LHC, spoke to a full house (several hundred people).

A word about Public Speaking

Another goal for doing a public lecture can include you getting experience in public speaking. But take note: this is somewhat of a Catch 22. You want to get experience in public speaking so you do a public lecture, but a public lecture (especially one that is heavily publicized) should not be given by a novice speaker without experience giving a public lecture. So how do you get the experience if you don’t do the experience? A public lecture is recommended for people who do have experience speaking before the public, ie people who are not physicists. But if you have not done this before, but you have given talks at, say, an APS meeting, and you are comfortable in front of a large audience, choosing to do a public lecture for the experience can still be a good idea. The key elements that will help you with this will be to:

really know the subject
figure out a way to translate the subject into “public speak” without patronizing the audience
practice, practice, practice: in front of others who are of the same education level your audience might consist of
get to know the room you will be speaking in and practice in there too
seek the help of a colleague or mentor/advisor who has done this before