Connected Classrooms uses Google+ Hangouts On Air to take students on virtual field trips to museums and zoos. In these Google+ Hangouts On Air students may have the opportunity to ask questions of the museum and zoo experts that are leading the virtual field trips. You can find a complete schedule of virtual field trips on the Connected Classrooms website. On the same site you can find links to the recordings of past Connected Classrooms field trips. Watching the recordings of the Connected Classrooms field trips probably won't be an engaging experience for your students, but it will give you a sense of what to expect when your class joins a live session.
Applications for Education
If you're looking for your students to learn first-hand from experts on a particular subject, the Connected Classrooms Hangouts could be useful to you.